# 16th European Solar Physics Meeting

Europe/Rome
Online

#### Online

Description

The European Solar Physics Meetings (ESPM) are organised every three years by the board of the European Solar Physics Division (ESPD), a joint division of the European Physical Society (EPS) and the European Astronomical Society (EAS). The meetings bring together a large number of from Europe and beyond, who are active in the theoretical and observational study of solar phenomena.

The 16th European Solar Physics Meeting (ESPM-16), originally planned for 2020,  took place as a virtual online meeting in 6-10 September 2021, with free registration for all participants.

You can access the posters and presentation materials uploaded by the authors in the meeting timetable.

### Confirmed invited speakers

• Axel Brandenburg (SE)
• Sanja Danilovic (SE)
• Louise Harra (CH)
• Andrew Hillier (UK)
• Natasha Jeffrey (UK)
• Laurene Jouve (FR)
• Larisa Kashapova (RU)
• Ioannis Kontogiannis (DE)
• Simone Landi (IT)
• Eric Priest (UK, ESPD senior prize winner 2020)
• Marco Velli (US)

### Important dates

• 1 April 2021. Registration and abstract submission open.
• 1 June 2021. Deadline for abstract submission.
• 1 August 2021. Deadline for registration.
• 6-10 September 2021. Meeting.
Participants
• Aabha Monga
• Aalaa Samy
• Aaron Peat
• Aarti Fulara
• Abdulaziz Alharbi
• ABDULRAHMAN ALBIDAH
• Abhishek Rajhans
• Agata Chuchra
• Agnieszka Ruminska
• Akie Moritsuka
• Alberto Vasquez
• Alejandro J. P. Aparicio
• Alejandro Miguel Moreno Vacas
• Alejandro Reina Conde
• Alena Zemanova
• Alexander Getling
• Alexander Hillaris
• Alexander James
• Alexander Kutsenko
• Alexander Nindos
• Alexander Pietrow
• Alexander Stepanov
• Alexander Warmuth
• Alexandr Afanasiev
• Alexandros Koukras
• Amanda Stricklan
• Ana Belén Griñón Marín
• Anamaría Navarro Noguera
• Ananya Rawat
• Anastasiya Zhukova
• Andrea Diercke
• Andrea Francesco Battaglia
• Andrea Perdomo
• Andreas Wagner
• Andrei Afanasev
• Andrei Chelpanov
• Andrei Plotnikov
• Andrei Zhukov
• Andrew Hillier
• Andrzej Fludra
• Andrés Asensio Ramos
• Andy S.H. To
• Anett Elek
• Angela Yulieth Bautista Sierra
• Anik De Groof
• Anna Khlystova
• Anna Kępa
• Anthony Yeates
• Antoine Dolliou
• Antonino Petralia
• Antonio Esteban Niemela
• Anwesha Maharana
• Arpit Shrivastav
• Arturs Vrublevskis
• Arun Awasthi
• Astrid Veronig
• Atul Mohan
• Axel Brandenburg
• Azaymi Siu
• Barbara Perri
• Barbara Sylwester
• Bartosz Dabrowski
• Beatrice Popescu Braileanu
• Ben Alterman
• Ben Snow
• Bernhard Fleck
• Bernhard Hofer
• Bibhuti Kumar Jha
• Bidya Binay Karak
• Bin Zhuang
• BISWAJIT MONDAL
• Björn Löptien
• Brandon Panos
• Brigitte Schmieder
• Błażej Kuźma
• Callan Noble
• Camilla Scolini
• Camille Lorfing
• Carlos Jose Diaz Baso
• Carlos Larrodera
• Carolina Robustini
• Catherine Fischer
• Catia Grimani
• Cecilia Mac Cormack
• Chandrashekhar Kalugodu
• Chen Xing
• Christian Baumgartner
• Christian Vocks
• Christoph Kuckein
• Christopher Prior
• Ciara Maguire
• Cis Verbeeck
• Clara Froment
• Clementina Sasso
• Cosima Breu
• Costas Alissandrakis
• Costas Bouratzis
• Cristina Mandrini
• D. Shaun Bloomfield
• Dan Kiselman
• Dana-Camelia Talpeanu
• Daniel Clarkson
• Daniel Müller
• Daniel Nóbrega-Siverio
• Daniel Price
• Daniel Seaton
• Daniele Calchetti
• Daniele Telloni
• Dario Del Moro
• David Berghmans
• David Brooks
• David Long
• David MacTaggart
• David Martínez Gómez
• David Millar
• David Orozco Suarez
• David Pontin
• David Stansby
• Deborah Baker
• Dennis Tilipman
• Devojyoti Kansabanik
• DIBYA KIRTI MISHRA
• Diego Lloveras
• Dipali Burud
• Dipankar Banerjee
• Divya Oberoi
• Dmitrii Kolotkov
• Dorcas Oseni
• Dorota Przepiórka
• Eduard Kontar
• Eino Valtonen
• Ekaterina Dineva
• Eleanna Asvestari
• Elena Dzifčáková
• Elena García Broock
• Elena Khomenko
• Elena Kupriyanova
• Elena Orlando
• Eleni Lavasa
• Elke D'Huys
• Emanuele Papini
• Emilia Kilpua
• Eric Buchlin
• Eric Priest
• Etienne Pariat
• Evangelia Samara
• Evgeniia Volchok
• Ewan Dickson
• Fabiana Ferrente
• Fabio Lepreti
• Fabio Reale
• Fabio Riva
• Fan Zhang
• Federica Chiappetta
• Federica Frassati
• Federico Nuevo
• Fedor Vybornov
• Fernando Carcaboso
• Fernando Moreno-Insertis
• Fisal Asiri
• Flavio Calvo
• Florentino Sánchez Bajo
• Florian Koller
• Francesca Zuccarello
• Francesco Berrilli
• Francesco Pecora
• francesco pegoraro
• Francesco Pegoraro
• Frank Schulz
• Frederic Schuller
• Frederik Clemmensen
• Frédéric Auchère
• Gabriel Pelouze
• Gaetano Zimbardo
• Galina Chikunova
• Galina Motorina
• Georgia Tsiropoula
• Georgina Graham
• Georgios Chintzoglou
• GERRY DOYLE
• Gherardo Valori
• Gianluca Napoletano
• Giorgio Viavattene
• Giovanni Lapenta
• Girjesh Gupta
• Giulia Murtas
• Giuliana Russano
• Giulio Del Zanna
• Giuseppe Emanuele Capuano
• Giuseppe Nisticò
• Giuseppina Carnevale
• Giuseppina Nigro
• Gordon Julian Koehn
• GUILLEM CASTELLÓ
• haitang l
• Hamish Reid
• Hana Meszarosova
• Hardi Peter
• Hariharan Krishnan
• Harry Callingham
• Hechao Chen
• Helen Mason
• Hema Kharayat
• Henrik Eklund
• Hugh Hudson
• Hui Tian
• Iain Hannah
• Ilaria Ermolli
• Immanuel Christopher Jebaraj
• Ineke De Moortel
• Ioannis Dakanalis
• Ioannis Kontogiannis
• Irene Tovar Hernández
• Irina Bakunina
• Irina Bilenko
• Istvan Ballai
• Ivan Dorotovič
• Ivan Yakovkin
• Ivica Skokić
• Iñigo Arregui
• Jack Jenkins
• Jack Reid
• Jacques Gustin
• Jaime de la Cruz Rodriguez
• Jamie Gorman
• Jan Benáček
• Jan Gieseler
• Jan Janssens
• Jan Jurcak
• Jan Rybak
• Jana Kasparova
• Janusz Sylwester
• Jaroslav Dudík
• Jaroslav Urbář
• Jasmina Magdalenic
• Jayant Joshi
• Jean-Marie Malherbe
• Jenny Marcela Rodríguez Gómez
• Jens Pomoell
• Jiajia Liu
• Jie Hong
• Jie Jiang
• Jinge Zhang
• Joe Scalisi
• Johannes Tschernitz
• John Leibacher
• Jonas Saqri
• Jonas Sinjan
• Jonas Zbinden
• Jorge Amaya
• Joris Hermans
• Jorrit Leenaarts
• Jose Carlos del Toro Iniesta
• Jose Luis Ballester
• José Manuel Vaquero
• José Roberto Canivete Cuissa
• João M. da Silva Santos
• Juan Camilo Guevara Gomez
• Juan Manuel Borrero
• Juie Shetye
• Julia Thalmann
• Julius Koza
• Juraj Loricik
• Jörn Warnecke
• Kalevi Mursula
• Kamil Bicz
• Kamlesh Bora
• Karin Dissauer
• Karl-Ludwig Klein
• Katarzyna Mikuła
• KD Leka
• Ke Yu
• Kevin Reardon
• Kilian Krikova
• Konstantinos Karampelas
• Korsos Marianna
• Kostas Moraitis
• Kostas Tziotziou
• Kristopher Cooper
• Krzysztof Barczynski
• Lakshitha Nama
• Larisa Kashapova
• Larisza Krista
• Laura Bercic
• Laura Hayes
• Laura Rodríguez-García
• Laurene Jouve
• Laurent Dolla
• Lekshmi Biji
• Li Feng
• Liam Edwards
• Lianne Fyfe
• Lidia van Driel-Gesztelyi
• lijia Liu
• Lingfang Wang
• Lisa-Marie Zessner
• Louise Harra
• Loukas Vlahos
• Luc Rouppe van der Voort
• Luca Giovannelli
• Lucia Abbo
• Lucia Kleint
• Luciano Rodriguez
• Lucie Green
• Luis Eduardo Vieira
• Maciej Zapiór
• Mahir Pirguliyev
• Malcolm Druett
• Malte Bröse
• manjunath hegde
• Manolis Georgoulis
• Manuel Luna
• Manuela Temmer
• Marco Marongiu
• Marco Romoli
• Marco Stangalini
• Marco Velli
• Marek Stęślicki
• Margit Haberreiter
• Maria Kazachenko
• Maria Loukitcheva
• Mariana Cécere
• Mariano Poisson
• Mariarita Murabito
• Marie Dominique
• Marilena Mierla
• Marina Battaglia
• Markus Roth
• Marta García Rivas
• Martin Reiss
• Maryam Saberi
• Mateja Dumbovic
• Matheus Kriginsky
• Mats Carlsson
• Mats Ola Sand
• Matteo Cantoresi
• Matthew Lennard
• Max McMurdo
• Małgorzata Pietras
• Michalina Litwicka
• Michaël Geeraerts
• Michele Fabi
• michele piana
• Mihalis Mathioudakis
• Mijie Shi
• Mikhail Modestov
• Mikolaj Szydlarski
• Mirko Piersanti
• Moa Skan
• Mohamed Nedal
• Momchil Dechev
• Morgan Stores
• Nancy Narang
• Nasrin Talebpour Sheshvan
• Nataliia Meshalkina
• Nataliia Shchukina
• Natasha Jeffrey
• Nicola Plutino
• Nicolas Fuller
• Nicolas Labrosse
• Nicolas Poirier
• Nicole VILMER
• NIkola Veselinović
• Nikola Vitas
• Nikolay Topchilo
• Nikolina Milanovic
• Nina Dresing
• Nitin Vashishtha
• Nived Vilangot Nhalil
• Noemi Kinga Zsamberger
• Norbert Magyar
• Olga Andreeva
• Olga Golubchina
• Olga Kutsenko
• Olga Panasenco
• Olga Sheiner
• Oliver Rice
• Oskar Steiner
• Paolo Massa
• Paolo Pagano
• Paolo Romano
• Patrick Antolin
• Patrick Ondratschek
• Paul Geyer
• Paul Rajaguru
• Paulo Simões
• Pavol Schwartz
• PAWAN KUMAR
• Pearse Murphy
• Peter Duchlev
• Peter Hunana
• Peter Wyper
• Petr Heinzel
• Petr Jelínek
• Philipp Löschl
• Philippe-A. Bourdin
• Prantika Bhowmik
• Prateek Mayank
• Priyank Srivastava
• Quentin Noraz
• Raffaele Reda
• Raffaello Foldes
• Rajab Ismayilli
• Rajees Uthayasegaram
• Rami Vainio
• Ramon Oliver
• Raveena Khan
• Ravindra Desai
• Rebecca Robinson
• Reetika Joshi
• Regina Biktimirova
• Renzo Ramelli
• Ricardo Gafeira
• Richard Morton
• Rita Ventura
• Ritesh Patel
• Rob Rutten
• Robbe Vansintjan
• Robert Cameron
• Robert Jarolim
• Robert Kavanagh
• Robert Sych
• Robert Walsh
• Roberta Morosin
• Roberto Bruno
• Roberto Soler
• Robertus Erdelyi
• Roger Dufresne
• Rohit Sharma
• Rolf Schlichenmaier
• Roman Brajša
• Rony Keppens
• Rositsa Miteva
• Rossana De Marco
• Rui Liu
• Rui Pinto
• Runbin Luo
• Russell Howard
• Ryan Campbell
• Ryan French
• Ryan Milligan
• Sabrina Bechet
• Sabrina Guastavino
• Sahel Dey
• Saida Milena Diaz Castillo
• Salvatore Luigi Guglielmino
• Salvatore Mancuso
• Sami Solanki
• Samrat Sen
• Samriddhi Sankar Maity
• Sanchita Pal
• Sanja Danilovic
• Sanjay Kumar
• Santiago Vargas Domínguez
• Sara Mulas
• Sarah Paterson
• Sargam Mulay
• SATABDWA MAJUMDAR
• SATYAM AGARWAL
• sean bruinsma
• Sebastián Castellanos Durán
• Seray Sahin
• Serena Maria Lezzi
• Sergei Shestov
• Sergey Anfinogentov
• Sergio Dasso
• Sergio Díaz Suárez
• Sergio Javier González Manrique
• Sergio Toledo-Redondo
• Shantanu Jain
• Shanwlee Sow Mondal
• Shaonwita Pal
• Shaun McLaughlin
• Shilpi Bhunia
• Siegfried Gonzi
• Silja Pohjolainen
• Simeon Asenovski
• Simon Cloutier
• Simon Good
• Simone Landi
• Smitha Narayanamurthy
• Sneha Pandit
• Sofia Carla Spago
• Sofya Belov
• Sondre Vik Furuseth
• Sonja Jejčič
• Sophie Masson
• Sophie Musset
• Sotiris Stamkos
• Soumyaranjan Dash
• Souvik Bose
• Sowmya Krishnamurthy
• SPIROS PATSOURAKOS
• Spyros Armatas
• Stanislav Gunár
• Stavro L. Ivanovski
• Stefaan Poedts
• Stefan Hofmeister
• Stefan Purkhart
• Stephan G. Heinemann
• Stephane Regnier
• Stephanie Yardley
• Sudip Mandal
• Supriya Hebbur Dayananda
• Surajit Mondal
• Susan Samwel
• Susanna Parenti
• Sushree S Nayak
• Suzana S. A. Silva
• Sven Wedemeyer
• Szabolcs Soós
• Talwinder Singh
• Tatyana Kaltman
• Teimuraz Zaqarashvili
• Teodora Mihailescu
• Thanassis Katsiyannis
• Theodosios Chatzistergos
• Thomas Chen
• Thomas Hackman
• Thomas Howson
• Thomas Neukirch
• Thomas Prevenslik
• Thomas Rees-Crockford
• Thore Espedal Moe
• Théo Pellegrin
• Tiago Pereira
• Tim Duckenfield
• Timo Laitinen
• Tinatin Baratashvili
• Tingyu Gou
• Tishtrya Mehta
• Tobias Felipe
• Tomasz Mrozek
• Tomin James
• Upasna Baweja
• Urszula Bak-Steslicka
• Valentina Abramenko
• Valentina Penza
• Valeria Quintero Ortega
• Valeria Sieyra
• Valeriia Liakh
• Valery NAGNIBEDA
• Valery Nakariakov
• Vartika Pandey
• Vasco Manuel de Jorge Henriques
• Vasileios Archontis
• Veronika Jercic
• Veronique Delouille
• Victor Melnikov
• Victor Réville
• Victoria Smirnova
• Viktor Fedun
• Vincenzo Andretta
• Vindya Vashishth
• Vishal Upendran
• Vsevolod Lozitsky
• Vyacheslav S. Lukin
• Véronique Bommier
• Víctor Carrasco
• wen wang
• Xiaohong Li
• Xiaozhou Zhao
• Xin Cheng
• Yajie Chen
• Yamini K Rao
• Yannis Zouganelis
• Yara De Leo
• Yasir Aljohani
• Ye Qiu
• Ying Li
• Yu Zhong
• Yuankun Kou
• Yuhang Gao
• Yulei Wang
• Yulia Kartavykh
• Yulia Kopylova
• Yuriy Fursyak
• Yuriy Tsap
• Zebin Zhang
• zhenjun zhou
• Zhenyong Hou
• Zhuofei Li
• Zi-Fan Wang
• Zihao Yang
• Ziwen Huang
• Zoran Simic
• Zurab Vashalomidze
• Åke Nordlund
• Żaneta Szaforz
• Monday, September 6
• Plenary 1
Convener: Eduard Kontar (University of Glasgow)
• 1
Welcome to ESPM-16
Speaker: Eduard Kontar (University of Glasgow)
• 2
Magnetic flux emergence and the solar dynamo

In global dynamo models, the process of magnetic flux emergence through the stellar convection zone is thought to be crucial. In the Sun, it is the strong toroidal structures built at the base of the convection zone which are assumed to be unstable to a buoyancy instability and rise through the convection zone to produce sunspots. The details of how these buoyant structures are formed, evolve and interact inside the convection zone and emerge at the photosphere and beyond are still a matter of active research. Moreover, the ability of these emergence processes to take part into the whole dynamo process is also a matter of intense debate in the solar dynamo community.
We propose in this talk to review some aspects of magnetic flux emergence in the Sun and to connect those studies with the global picture of the solar dynamo mechanism.

Speaker: Laurene Jouve (IRAP Toulouse / France)
• 3

Mean-field dynamo theory is a rich field that tries to reproduce and often predict the large-scale behavior of a turbulent system. Numerical simulations play the same role as astronomical observations, except that the former ones can be done under controlled conditions while the latter ones have the advantage of being more in the asymptotic regime of large magnetic and fluid Reynolds numbers. Unfortunately, we do not yet understand much about the turbulence in the real Sun. Especially the extreme stratification in the uppermost layers is impossible to simulate properly.

Three-dimensional numerical simulations of the Sun are still far from the actual Sun, but they do produce features that are in agreement with appropriately tailored mean-field calculations. This has been achieved by measuring their mean-field transport coefficients using the test-field method, a reliable procedure whose accuracy enables us to pinpoint some previously unexpected phenomena.

Additional diagnostics concerning the measurement of magnetic helicity both at the solar surface and in the solar wind have widened the horizon of our understanding. In particular, it is now possible to solve mean-field models that encompass both dynamo and its wind emanating from it. In my talk, I will connect various aspects of solar wind physics with related aspects of solar magnetic activity. Looking around at other stars helps us further to put the Sun and our models into context and to appreciate to what extent our Sun might be special.

Speaker: Prof. Axel Brandenburg (NORDITA)
• 4
Babcock-Leighton solar dynamo models including the observed meridional circulation and surface flux loss

What sets the 11-year period of the solar activity cycle? Why do sunspots appear in the butterfly wings which propagate equatorward over the course of a cycle? The Babcock-Leighton flux-transport model attempts to answer these questions but has free paramters. Some of these free parameters have recently been observationally constrained. In particular, the meridional flow has recently been determined by Gizon et al. and the toroidal flux loss through the solar surface has been shown to remove most of the flux produced by the Omega effect. We show that the Babcock-Leigton FTD model can produce solar like solutions with these new constraints. We also show that the remaining parameters (mainly radial magnetic pumping and turbulent diffusivity) can produce butterfly diagrams similar to the Sun’s without artificial restrictions of the emergence latitude. The basic mechanism here is that the toroidal field is quickly transported to the bottom half of the convection zone where the meridional flow is equatorward. Thus the Babcock-Leighton FTD model remains an extremely promising candidate for how the Sun’s dynamo actually works.

Speaker: Simon Cloutier
• 5
Helioseismic evidence for large-scale flow cells in the solar convection zone

The results of helioseismological inferences of subsurface flow velocities by time–distance techniques are used to analyse the spatial structure of subphotospheric convection. The source data are obtained from the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory from May 2010 to September 2020. The helioseismic-inversion results produce the three-dimensional flow fields in a 19-Mm deep layer below the photosphere in a range of latitudes and Stonyhurst longitudes from − 60° to + 60° with an 8-hour cadence. We study the convective flow structures of various scales and shapes by applying a spherical harmonic transform to the horizontal-velocity-divergence field. The results reveal a signature of large-scale flow cells in the spatial power spectrum, which may represent the “giant cells” of solar convection. In particular, indications are found for meridionally elongated flow cells, which are considerably smaller in the longitudinal direction than in latitude. The physical properties of the multi-scale convection cells and their evolution in the course of the solar activity cycle are discussed.

Speaker: Alexander Getling (Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Russia)
• 10:15 AM
Coffee break
• Poster Session 1.1
Convener: Etienne Pariat (LESIA, Observatoire de Paris)
• 6
The rotation rate of active and ephemeral regions

The rotation rates of solar active and ephemeral regions depend on morphology and size of magnetic structures, although the reasons of the dependence are still being discussed. The aim of the work is to analyze the rotation rates of different types of magnetic tracers using magnetic field data rather than white-light images.

Magnetic field maps provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory were used to measure the rotation rates of 864 active and 322 ephemeral regions observed between 2010 and 2016. We found smaller magnetic tracers to show a tendency to rotate faster as compared to larger ones. Thus, ephemeral regions exhibit on average the fastest rotation rate. We further divided active regions into three classes. Class A comprised magnetic bipoles obeying Hale’s polarity law and Joy’s law. The second class B included active regions violating at least one of these laws. The third class U comprised unipolar active regions.

We found no significant difference between the rotation rates of active regions of classes A and B. In contrast, unipolar active regions exhibited lower rotation rate and narrower distribution of the rotation rate differences. Assuming the rotation rate to indicate the anchoring depth of the magnetic structure within the convection zone, we supposed that active regions of classes A and B might be anchored throughout the entire convective envelope while unipolar active regions are rooted within a thin layer located either near the base of the convection zone or at a shallow near-surface depth.

Speaker: Alexander Kutsenko (Crimean Astrophysical Observatory)
• 7
The N-S-asymmetry of the different magneto-morphological types active regions in the 23rd and 24th solar cycles

We use SOHO and SDO data to study 2046 active regions (ARs) of the 23rd and 1507 ARs of the 24th solar cycles. According to empiric regularities for sunspot groups (Hale’s polarity law, Joy’s law, etc.) and the magneto-morphological classification (MMC), all ARs were divided into three types: A-type – regular bipolar ARs; B-type – irregular ARs that violate at least one of empiric rules; U-type – unipolar spots. We found that regular/irregular ARs make near a half/quarter of all ones. The A-type ARs are distributed evenly between two maxima of each cycle, whereas the number of B-type ARs increases in the second maxima. Both regular and irregular ARs exhibit strong N-S-asymmetry. The number of A-type ARs in different hemispheres shows peaks in two maxima of each cycle alternatively (the sequence is different in studied cycles). The B-type ARs shows a similar pattern (although the A-peaks and B-peaks occurs in different cycle maxima), excluding immense B-peak in the S-hemisphere in the second maximum (cycle 24) that breaks the expected sequence. The cyclic variations of the normalized asymmetry index shows postponement for the B-type ARs (related to the S-hemisphere) with time difference ~1.5-2 years. The tendencies found are discussed in terms of the mean-field dynamo theory. The classification of ARs by AZh was supported by the RSF (Project 18-12-00131). VA thanks the MSHE of RF (Research 0831-2019-0006). The work of AKh was supported by the basic financial program of the FSR II.16. AKh thanks the support of the RFBRs grant 19-52-45002.

Speaker: Dr Anastasiya Zhukova (Crimean Astrophysical Observatory)
• 8
A catalog of bipolar active regions violating the Hale's polarity law

We present a catalog of bipolar active regions (ARs) violating Hale's polarity law over the period 1989–2018. The catalog is compiled based on visual inspection of solar data in web applications Debrecen Photoheliographic Data, Helioviewer and Solar Monitor. Also, data from catalogs of the Mount Wilson Observatory, Crimean Astrophysical Observatory (CrAO) and USAF/NOAA SRS were used. The compiled catalog includes bipolar ARs reverse to Hale's polarity law with sunspots/pores of both polarities and magnetic couplings between opposite polarities (in cases with available EUV data). Ambiguous and complex cases of ARs with violation of Hale’s polarity law included in the catalog are labeled with special marks. The catalog contains 275 ARs. This is about 3% of all observed ARs. For each AR, there are three information blocks in the catalog: USAF/NOAA SRS data, DPD data, and our special marks. The first and second blocks include data at the maximum of AR evolution (at the maximum of the area of sunspots). The catalog is available at the CrAO website https://sun.crao.ru/databases/catalog-anti-hale/. For details, see Zhukova et al. (2020, Solar Physics, Volume 295, Issue 12, article id 165). The AR classification by AZh was supported by RSF (Project 18-12-00131). VA is grateful to MSHE of RF (Research 0831-2019-0006). AKh is grateful to RFBR for awarding grant 19-52-45002.

Speaker: A.I. Khlystova (Institute of Solar Terrestrial Physics of SB of RAS, Irkutsk, Russia)
• 9
How good is the bipolar approximation of active regions for surface flux transport?

We investigate how representing active regions with bipolar magnetic regions (BMRs) affects the end-of-cycle polar field predicted by the surface flux transport model. Our study is based on a new database of BMRs derived from the SDO/HMI active region patch data between 2010 and 2020. An automated code is developed for fitting each active region patch with a BMR, matching both the magnetic flux and axial dipole moment of the region and removing repeat observations of the same region. By comparing the predicted evolution of each of the 1090 BMRs with the predicted evolution of their original active region patches, we show that the bipolar approximation leads to a 24% overestimate of the net axial dipole moment, given the same flow parameters. This is caused by neglecting the more complex multipolar and/or asymmetric magnetic structures of many of the real active regions, and may explain why previous flux transport models had to reduce BMR tilt angles to obtain realistic polar fields. Our BMR database and the Python code to extract it are freely available.

Speaker: Anthony Yeates (Durham University, UK)
• 10
Effect time-dependent reverse-flow in meridional circulation on the evolution of spot-producing toroidal fields and implications on observed short-term variability in polar faculae

In various classes of dynamo models, spot-producing magnetic fields are generated as axisymmetric toroidal flux-ropes. While mean-field models produce axisymmetric broad toroidal fields, full 3D convective models produce axisymmetric toroidal wreaths. All these models can reproduce various longitude-averaged features, such as solar-like butterfly diagrams. In recent simulations, we show that time-dependent non-axisymmetric m=1 type flows can originate due to nonlinear hydrodynamics of differential rotation. This non-axisymmetric flows will affect the axisymmetric meridional circulation, causing time-dependence in the reverse flow cell. We show that a dynamo, operating with such a time-dependent meridional circulation can explain the short-term variability (with periods of the order of a month) in the evolution of faculae around 75-degree latitudes, which have very recently been observed. We present results from various simulation experiments to determine what model conditions best-simulate the facular evolution. We also compare our results with the observations of surface active regions' evolutions as revealed from magnetograms.

Speaker: Dr Bernadett Belucz (1) Solar Physics and Space Plasma Research Centre, School of Mathematics and Statistics, University of Sheffield, Hicks Building, Hounsfield Road Sheffield, S3 7RH, UK, 2)Department of Astronomy, Eötvös Loránd University, 1/A Pázmány Péter sétány, H-1117, Budapest, Hungary, 3)Gyula Bay Zoltań Solar Observatory (GSO), Hungarian Solar Physics Foundation (HSPF), Petőfi tér 3., Gyula, H-5700, Hungary)
• 11
Reconstruction of solar magnetic field during Cycles 15--19 based on proxies from Kodaikanal Solar Observatory

The regular observation of the solar magnetic field is available only for about last five cycles. Thus, to understand the origin of the variation of the solar magnetic field, it is essential to reconstruct the magnetic field for the past cycles, utilizing other datasets. Long-term uniform observations for the past 100 years as recorded at the Kodaikanal Solar Observatory (KoSO) provide such an opportunity. We develop a method for the reconstruction of the solar magnetic field using the synoptic observations of the Sun's emission in the Ca IIK and H$\alpha$ lines from KoSO for the first time. The reconstruction method is based on the facts that the Ca II K intensity correlates well with the unsigned magnetic flux, while the sign of the flux is derived from the corresponding H$\alpha$ map which provides the information of the dominant polarities. Based on this reconstructed magnetic map, we study the evolution of the magnetic field in Cycles 15--19. We also study bipolar magnetic regions (BMRs) and their remnant flux surges in their causal relation. Time-latitude analysis of the reconstructed magnetic flux provides an overall view of magnetic field evolution. We identify the reversals of the polar field and critical surges of following and leading polarities. We found that the poleward transport of opposite polarities led to multiple changes of the dominant magnetic polarities in poles. Furthermore, the remnant flux surges that occur between adjacent 11-year cycles reveal physical connections between them.

Speaker: Bidya Binay Karak (Indian Institute of Technology (BHU) Varanasi)
• 12
Variation of the solar magnetic flux distribution over the solar cycle using HMI data.

We use statistical tools to analyse data from HMI to determine the magnetic flux distribution of photospheric magnetic features and its variation over a solar cycle. The nature of the observed flux distributions at different times during the solar cycle could be used to try and infer information about magnetic field generation mechanisms. In particular we compare whether the magnetic flux distributions are represented by a single power law or whether other distribution functions can provide a better representation of the data.

We find that the hypothesis that the distribution of observed photospheric fluxes follows a single power law over the full solar cycle cannot be discounted and investigate how the power law distribution changes over the solar cycle . We also consider whether a 'double' power law distribution could actually be more reasonable than that of a single power law. We also discuss potential implications of a double power law distribution for solar magnetic field generation.

Speaker: Callan Noble (University of St Andrews)
• Poster Session 1.2
Convener: Francesca Zuccarello (Istituto Nazionale di Astrofisica (INAF))
• 13
Direct evidence of a pre twisted magnetic flux rope emerging into the solar corona.

Flux ropes are known to be a critical structure in flaring, jet formation and coronal mass ejections. They can be observed in sigmoidal emission structures and are frequently found in magnetic extrapolations. Two leading theories for their formation are the emergence of pre-twisted magnetic structures from the convection zone and formation above the photosphere due to driving motions around the polarity inversion line. To the best of our knowledge there exists not direct (observational) evidence to conclusively demonstrate on or the other mechanism is responsible for flux rope formation; evidence is generally indirectly obtained by modelling. In this talk I will introduce the concept of magnetic winding, a measure of the entanglement of the magnetic field which can be applied to magnetogram data to directly infer the structure of the emerging field. We demonstrate conclusively with a number of examples that a pre-twisted magnetic field emerges from the convection zone. We anticipate the magnetic winding will become a staple quantity in the interpretation of magnetic field structure from magnetogram data.

Speaker: David MacTaggart
• 14
Characterising upflows in coronal holes and the quiet Sun

Recent observations from Parker Solar Probe and Solar Orbiter have emphasised the importance of small and short-lived phenomena in the solar atmosphere. The former has revealed a highly dynamic structure in the solar wind’s magnetic field, which are referred to as switchbacks. The latter has shown small extreme-ultraviolet brightenings in the solar corona, which were labelled campfires.
We have analysed 14 coronal upflows, which were derived as blue shifts in spectroscopic data and examined the potential sources for each upflow. This was done by using Hinode/EIS rasters and calculating the Doppler velocities for the Fe XII line. Then events, which are stronger blue-shifted than -6 km/s were extracted. Those blue-shift events in Hinode/EIS were then compared to SDO/AIA data in all extreme-ultraviolet wavelength bands. We could identify sources for 12 out of 14 events. Besides classical jets, four events are associated with bright points and seven events are connected to small scale sources. They are either short and faint brightenings or eruptions. Those small-scale events usually last for less than 25 minutes. In a last step those events were examined in Hinode/XRT and SDO/HMI data to get a better understanding.
Our work has shown that small-scale features, which are so faint and short-lived that they would usually be missed in imaging data can produce strong upflows. Their driving mechanisms and potential contribution to the solar wind however is not understood yet.

Speaker: Conrad Schwanitz (ETH Zürich | PMOD\WRC)
• 15
Energy generation, transport and dissipation in a coronal loop

Various processes have been proposed to explain the heating of loops to coronal temperatures, from braiding of magnetic flux tubes to waves. The relative contribution of different heating mechanisms is yet to be determined.

We study the coupling of a coronal loop to the solar surface and the transport and deposition of energy in different atmospheric layers.

Using 3D MHD simulations with the MURaM code, we model a coronal loop as a straightened-out magnetic flux tube. At each footpoint, the loop is rooted in a shallow convection zone layer, leading to self-consistent heating by magnetoconvection.

Energy transport may occur through slow, relative motions of magnetic flux tubes or by small-scale motions within magnetic flux concentrations. We compare the contribution of flows on various spatial and temporal scales to the energy transport.
We find that a large part of the energy injected into the loop is generated by internal coherent motions within strong flux tubes.
Small-scale vortices are ubiquitous in the intergranular lanes in the photosphere. These structures extend through the chromosphere into the corona and transport a significant fraction of the Poynting flux into the upper atmosphere and show enhanced heating rates.

The energy deposition in the corona gives rise to a clear substructure in the emission showing strands with widths of a few hundred km.

Our model allows us to follow the energy from its origin in the convection zone to dissipation in the corona and the resulting synthetic emission.

Speaker: Cosima Breu (Max-Planck-Institute for Solar System Research)
• 16
The SOLARNET Trans-national Access programme

Researchers in high-resolution solar physics should have access to research infrastructures that would otherwise be closed to them. That is the goal of the SOLARNET Trans-national Access programme which is funded by the European Union’s Horizon 2020 programme. The facilities include ground-based solar telescopes on the Canary Islands: GREGOR, SST, THEMIS, and VTT. In addition there is the SUNRISE 3 balloon mission, due to be launched in June 2022, and the Piz Daint supercomputer.

I will present the programme, introduce the participating facilities, explain the rules, and discuss the experience so far from the point of view of users and operators. Finally some words on the future.

Speaker: Dan Kiselman (Stockholm University)
• 18
Joint remote sensing-in situ measurements of solar wind plasma during the first Solar Orbiter-Parker Solar Probe quadrature

On January $18$, $2021$, Solar Orbiter (SolO) and Parker Solar Probe (PSP) were for the first time in a special orbital configuration, that is, in quadrature. At this time when traveling along its orbit very close to the Sun, PSP has been crossing the atmosphere of the Sun at a distance just above $20$ R$_{\odot}$. Because of the continuous expansion of the solar corona, the plasma crossed by PSP, which is moving outward at a speed above $100-200$ km s$^{-1}$ on the solar equatorial plane, is the same plasma observed with the Metis coronagraph just a few hours earlier at a distance of $3-7$ R$_{\odot}$ from the solar limb. It is, thus, the first time that the expanding coronal plasma - that is, the solar wind - fully characterized by observations remotely performed with Metis, encounters almost immediately in its way outward a suite of in-situ instruments that can directly measure its physical properties. This work deals with the joint SolO-PSP observations to study the transition of the solar wind plasma from the sub-Alfvénic solar corona to a region just above the Alfvén radius, thus aiming to investigate the evolution of the pristine solar wind not yet reprocessed by nonlinear interactions.

Speaker: Daniele Telloni (Istituto Nazionale di Astrofisica (INAF))
• 19
Extreme UV quiet Sun brightenings observed by Solar Orbiter/EUI

We present results of the first high cadence image sequence of the Extreme Ultraviolet Imager (EUI) taken on 2020 May 30, when Solar Orbiter was 31.5 degrees in solar longitude separated from Earth & SDO, and at 0.56AU from the Sun. At this distance, the two-pixel spatial resolution of EUI’s High Resolution EUV Telescope (HRIEUV) was 400 km. HRIEUV observed a quiet Sun region and detected small localised brightenings, nicknamed ’campfires’, with length scales between 400 km and 4000 km and durations between 10 sec and 200 sec. The smallest and weakest of these HRIEUV brightenings have not been previously observed. Simultaneous observations from the EUI High-resolution Lyman-α telescope (HRILYA) do not show localised brightening events, but the locations of the HRIEUV events clearly correspond to the chromospheric network. Comparisons with simultaneous AIA images shows that most events can also be identified in the 17.1 nm, 19.3 nm, 21.1 nm, and 30.4 nm pass-bands of AIA, although they appear weaker and blurred. Our differential emission measure (DEM) analysis indicated coronal temperatures. We determined the height for a few of these campfires to be between 1 and 5 Mm above the photosphere. We interpret these events as a new extension to the flare-microflare-nanoflare family. Given their low height, the EUI ‘campfires’ could stand as a new element of the fine structure of the transition region-low corona, that is, as apexes of small-scale loops that undergo internal heating all the way up to coronal temperatures.

Speaker: David Berghmans (Royal Observatory of Belgium)
• Poster Session 1.3
Convener: Stefaan Poedts (KU Leuven)
• 20
Two Classes of Eruptive Events during Solar Minimum

During Solar Minimum, the Sun is relatively inactive with few sunspots observed on the solar surface. Consequently, we observe a smaller number of highly energetic events such as solar flares or coronal mass ejections (CMEs), which are often associated with active regions on the photosphere. Nonetheless, our magnetofrictional simulations during the minimum period suggest that the solar corona is still dynamically evolving in response to the large-scale shearing velocities on the solar surface. The non-potential evolution of the corona leads to the accumulation of magnetic free energy and helicity, which is periodically shed in eruptive events. We find that these events fall into two distinct classes. One set of events are caused by eruption and ejection of low-lying coronal flux ropes and could explain the origin of occasional CMEs during Solar Minimum. The other set of events are not driven by destabilisation of low-lying structures but rather by eruption of overlying sheared arcades. These could be associated with streamer blowouts which are often considered as potential candidates for stealth CMEs. The two classes differ significantly in the amount of magnetic flux and helicity shed through the outer coronal boundary. We additionally explore how other measurables such as current, open magnetic flux, free energy, coronal holes, and the horizontal component of the magnetic field on the outer model boundary vary during the two classes of event. This study emphasises the importance and necessity of understanding the dynamics of the coronal magnetic field during Solar Minimum.

Speaker: Prantika Bhowmik (Department of Mathematical Sciences, Durham University, UK)
• 21
Simulating Atmospheric Heating with Bifrost

The stellar atmospheric simulation code $\textit{Bifrost}$ is useful for exploring the plasma dynamics of the solar atmosphere, but also for tracking magnetically energetic events that may be efficient in heating the chromosphere and corona. In this study, a cube of quiet Sun was modeled in order to track a) the evolution of quiet Sun photosphere and atmosphere, and b) the effect of a horizontal magnetic flux sheet inserted at the bottom boundary of the computational box, located 2.5 Mm beneath the Solar surface. Before the horizontal flux insertion, a significant magnetic heating event took place in the simulation. The characteristics of this heating event are presented including time series of Joule and viscous heating, magnetic field properties, and plasma velocities. Magnetic field lines are traced in order to understand the topology of the field responsible for the heating, and the field geometry appears to be a bipolar loop with kG-strength footpoints in the photosphere. This bipolar loop is associated with a plasma bubble that extends to roughly 7 Mm above the Solar surface. Analysis of the Joule and viscous heating parameters indicates that a significant amount of heating occurs in the atmosphere when the loop and its associated plasma bubble collapse. A newly-developed test particle module in $\textit{Bifrost}$ called $\texttt{corks}$ is employed in order to accurately trace the evolution of this magnetic feature from conception to collapse, and the associated heating may provide insight into how bipolar magnetic features can contribute to atmospheric heating, even in the quiet Sun.

Speaker: Rebecca Robinson (University of Oslo)
• 22
Are we resolving coronal strands? Analysis of substructure within coronal plasma loop cross-sectional profiles observed by NASA’s HiC

The High-Resolution Coronal Imager (Hi-C) was launched for a third time on 29th May 2018, resulting in 329 s of 17.2 nm data of target active region AR 12712 with a cadence of approx. 4 s, and a plate scale of 0.129 arcsec^2/pixel.

Co-aligned with SDO/AIA 17.1nm observations, this presentation outlines investigations of the widths of 49 coronal structures. Firstly, evidence of substructure within the loops that is not detected by AIA will be demonstrated. Hi-C 2.1 can resolve individual sub-loop strands as small as approx. 202 km, though the more typical strand widths seen are around 513 km. Strands from a region of low emission that can only be visualized against the contrast of the darker, underlying moss reveal the corona is filled with ubiquitous, low emission, low density magnetic threads.

Secondly, even at these spatial scales there may be evidence for further substructuring within the HiC strands themselves. Width profile intensity variations are reproduced by fitting multiple Gaussian profiles; 183 sub-elements are examined and most frequent structural widths are about 450–575 km with 47% of the strand widths beneath SDO/AIA resolution. These appear to be the result of multiple strands along the integrated line of sight rather than being the consequence of even finer sub-resolution elements.

Finally, the change of strand width along strand length is examined – open fan magnetic strand structures display a width increase from the base while closed structures show little variation. The implications of these results on coronal loop modelling will be discussed.

Speaker: Robert Walsh (University of Central Lancashire)
• 23
Stratification of canopy magnetic fields in a plage region

The role of magnetic fields in the chromospheric heating problem remains greatly unconstrained. Most theoretical predictions from numerical models rely on a magnetic configuration, field strength, and connectivity; the details of which have not been well established with observational studies for many chromospheric scenarios. High-resolution studies of chromospheric magnetic fields in plage are very scarce or non existent in general.
Our aim is to study the stratification of the magnetic field vector in plage regions. Previous studies predict the presence of a magnetic canopy in the chromosphere that has not yet been studied with full-Stokes observations. We use high-spatial resolution full-Stokes observations acquired with the CRisp Imaging Spectro-Polarimeter (CRISP) at the Swedish 1-m Solar Telescope in the Mg I 5173 Å, Na I 5896 Å and Ca II 8542 Å lines.
We have developed a spatially-regularized weak-field approximation (WFA) method, based on the idea of spatial regularization. This method allows for a fast computation of magnetic field maps for an extended field of view. The fidelity of this new technique has been assessed using a snapshot from a realistic 3D magnetohydrodynamics simulation.

Speaker: Roberta Morosin (Stockholm university)
• 24
Constraining the magnetic vector in the quiet solar photosphere and tracking the dynamics of small-scale magnetic features

With the advent of next generation high resolution telescopes, our understanding of how the magnetic field is organized in the internetwork (IN) photosphere is likely to advance significantly. We present high spatio-temporal resolution observations that reveal the dynamics of two disk-centre IN regions taken by the GREGOR Infrared Spectrograph Integral Field Unit (GRIS-IFU) with the highly magnetically sensitive photospheric Fe I line at 15648.52 Å. We apply inversions with the Stokes inversions based on response functions (SIR) code to retrieve the parameters characterizing the atmosphere, tracking the dynamics of small-scale magnetic features. We find linear polarization features (LPFs) with magnetic flux density 130−150 G and find LPFs appear preferentially at granule-intergranular lane boundaries. The weak magnetic field appears to be organized in terms of complex ‘loop-like’ structures, with transverse fields often flanked by opposite polarity longitudinal fields. We use snapshots produced from high resolution three-dimensional magnetohydrodynamic (MHD) simulations and employ SIR to produce synthetic observables in the same spectral window as observed by the GRIS-IFU. We then use a parallelized wrapper to SIR to perform nearly 14 million inversions of the synthetic spectra to test how well the true' MHD atmospheric parameters can be constrained statistically. Finally, we degrade the synthetic Stokes vector spectrally and spatially to GREGOR resolutions and consider the impact of stray light, spatial resolution and signal-to-noise. We studied a LPF exhibiting very similar magnetic flux density as those observed by the GRIS-IFU. Thus, we demonstrate that MHD simulations are capable of showing close agreement with observations.

Speaker: Ryan Campbell (Queen's University Belfast)
• 25
High-resolution UV observations of small-scale energy release events in the solar atmosphere

High-resolution UV observations of the solar atmosphere, complemented by photospheric measurements conveying information about the magnetic configuration of the region of interest, allow us to investigate the magnetic and plasma processes that drive coronal heating and energy release.
Here, we report on small-scale flux emergence and flux cancellation events and on the energy release phenomena simultaneously observed in coordinated campaigns involving ground- and space-based observatories (e.g., SST, IBIS, Hinode, SDO), focusing on recent results obtained by the IRIS satellite.
We discuss our findings illustrating how magnetic reconnection can explain the occurrence of such small-scale energetic events and how they are expected to be improved with upcoming observations from next-generation space missions, like Solar Orbiter and EUVST.

Speaker: Salvatore Luigi Guglielmino (Istituto Nazionale di Astrofisica (INAF))
• 26
NuSTAR Observations of the Quiet Sun

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a hard X-ray focusing telescope designed to observe astrophysical sources, but with the capability of observing the Sun. NuSTAR’s higher sensitivity in the HXR range compared to previous solar X-ray instruments, such as RHESSI, combined with the recent solar minimum has provided a unique opportunity to study the faint HXR emission from features on the quiet Sun. We present analysis of such features from several of the NuSTAR quiet Sun observations from the recent minimum. The X-ray spectra of these events have been fit, allowing their temperatures and emission measures to be determined. The contribution of these faint features to heating the solar atmosphere is investigated by finding the highest temperatures they can reach, as well as searching for the presence of any non-thermal emission. In order to study the multithermal emission from the features, the NuSTAR results are combined with corresponding observations from SDO/AIA and Hinode/XRT to produce Differential Emission Measures.

Speaker: Ms Sarah Paterson (University of Glasgow)
• Poster Session 1.4
Convener: Elena Khomenko (Instituto de Astrofisica de Canarias)
• 27
Different properties of microwave and metric QPPs in a solar radio burst

We analyse quasi-periodic pulsations (QPPs) which happened about 25 minutes after an M4-class solar flare on 5 September 2017. The QPP event was manifested as a bursty rapid enhancement of the flare microwave and decimeter radio emission, with almost no signatures pronounced in soft X-rays in comparison with the background flux caused by the decay phase of the preceding M4 flare. Simultaneous observations of this event with the two radioheliographs of a new generation — the Siberian Radioheliograph (4–8 GHz) and the MUSER radioheliograph (0.4–2 GHz) — reveal the presence of QPPs which were co-existing in the microwave and decimeter bands. Quasi-periodic narrow-band bursts at 0.4–0.8 GHz with a characteristic timescale of about 5 s and a fine structure inside each burst are seen in the coherent emission. In the 4–8 GHz band, we detect QPPs of the non-coherent emission, with a longer period that decreases gradually from 35 s to 25 s during the event. The work aims to identify the presence or absence of a connection between these two types of QPPs by analysing their temporal features in the two types of emission, as well as analysing the available spectral and spatial information. Based on this analysis, the most likely scenarios for the event are discussed.

Speaker: Elena Kupriyanova (Pulkovo Observatory of RAS)
• 28
Electron acceleration by Langmuir turbulence at an open magnetic trap

Fast electron streams in the plasma of the solar corona propagating along the magnetic field lines are able to drive Langmuir turbulence. The further resonant interaction of electrons with turbulent spectrum can lead to acceleration to high energies forming tails of a suprathermal distribution. Similar phenomena are observed in experiments at open magnetic traps with electron beams. An open magnetic trap for plasma confinement, to put it simply, is a magnetic configuration in which magnetic field lines are not closed and plasma can flow freely along them. Among other approaches, electron beams are used to create and heat plasma. Consequently, such a configuration may be interesting not only from the point of view of plasma confinement, but also for modeling some astrophysical phenomena occurring in the solar corona. In particular, the results of one such experiments at Gas Dynamic Trap (GDT) facility [1] are reported. In the experiment, electrons accelerated to energies an order of magnitude higher than the initial beam energy are observed. For the obtained results to be interpreted, the series of numerical particle-in-cell simulations with a continuously injected electron beam are carried out. It is demonstrated that with injection of a 30 keV electron beam in a plasma accelerated electrons achieve about 100 keV energies, power-law distribution of a suprathermal component are obtained. An interpretation of the presented results is given.
[1] Ivanov, A. A., Prikhodko, V. V. (2013). Plasma Physics and Controlled Fusion, 55(6), 063001.

Speaker: Mrs Evgeniia Volchok (Budker Institute of Nuclear Physics, SB RAS)
• 29
Diagnostics and fine structuring of coronal heating from 3D MHD modeling of twisted/braided coronal loops

Photospheric motions drive the twisting and braiding of coronal magnetic field confining loop plasma. Magnetic stress eventually leads to reconnection and conversion of magnetic energy into heat through current dissipation. We model this process by fully 3D MHD loop simulations and derive observables that directly track the dissipating currents, therefore providing evidence for the structuring and timing of the energy release and constraints for the observations.

Speaker: Fabio Reale (Istituto Nazionale di Astrofisica (INAF))
• 30
Proton energy spectra of energetic storm particle events and relation with shock parameters and turbulence

The proton energy spectra of 23 Energetic Storm Particle events, occurring either in association with (16 events) or in absence of (7 events) Solar Energetic Particles, are investigated by using data from particle instruments aboard STEREO A in the energy range from 84.1 keV to 100 MeV. For the SEP events at quasi-perpendicular shocks, the Weibull distribution provides good fits to the spectra, over the whole energy range for some events, and only at high energies for the others, being lower energies explained by the power law predicted by the DSA. Instead, the SEP spectra at quasi-parallel shocks are better reproduced by a double power law. In the cases non associated with SEPs, an Ellison-Ramaty form fits the observed spectra. Moreover, a significant correlation of the downstream turbulence level is found with the Weibull parameters for quasi-perpendicular shocks, and with the proton peak value in the intermediate energy range 4-6 MeV for all the 16 shocks. Our results suggest that the downstream turbulence is a relevant factor in particle acceleration and that stochastic acceleration can be a plausible mechanism for re-acceleration at interplanetary shocks.

Speaker: Federica Chiappetta (Università della Calabria)
• 31
Activities of the topical team on Coronal Shocks and Particle Acceleration for Solar Orbiter coronagraph METIS

The launch of the Solar Orbiter mission has motivated an intensification of the science activities in preparation for the exploitation of the data. For the METIS coronagraph, these activities are coordinated by topical teams, working on different physical problems. Here we present the activities carried out so far by the topical team on Coronal Shocks and Particle Acceleration. This team is trying to find the answer to some of the Solar Orbiter scientific objectives, like how and where do shocks form in the corona, and how and where are energetic particles accelerated at the Sun. In this poster, we will give an overview of the talks delivered up to now on coronal shock observations, shock 3D reconstruction, solar energetic particles, solar radio observations, shock acceleration, modelling and numerical simulations, and synergy with other instruments and other space missions.

Speaker: Prof. Gaetano Zimbardo (University of Calabria, Rende, Italy and INAF, Italy)
• Poster Session 1.5
Convener: Emilia Kilpua (University of Helsinki)
• 32
A CME followed by a prominence eruption as observed by the remote sensing instruments on-board Solar Orbiter

On February 12, 2021 two subsequent eruptions occurred above the West limb, as seen along the Sun-Earth line. The first event appeared in the SOHO/LASCO-C2 images as a typical Coronal Mass Ejection (CME), starting around 12:48 UT with a projected speed on the order of 120 km/s, as provided by CACTUS catalog. This slow CME was followed ~7 hours later by a smaller and collimated prominence eruption, originating Southward with respect to the CME, and propagating much faster at ~380 km/s. Interestingly, these two events were also observed not only by STEREO-A, but also by remote sensing instruments on-board Solar Orbiter, located at about 163 deg of separation angle from the Earth, hence observed the eruptions above the East limb. The two events were first observed by the SoloEUI imager, then crossed the field-of-view of the SoloMetis coronagraph, being finally observed by the SoloHI instrument. From the Solar Orbiter perspective the different source regions of the two eruptions were located just behind the limb, as suggested by SDO/AIA images, but the EUI imager followed very well the expansion of the prominence eruption. The Metis images show both in the VL and UV channels a faint but classical three-part structure CME, followed by a brighter plasma blob associated with the prominence eruption. The southward part of the CME was also observed higher up by SoloHI, while the prominence eruption (expanding more northward) was probably missed. This presentation will summarize the first results from the analysis of these two interesting events.

Speaker: Dr Alessandro Bemporad (Istituto Nazionale di Astrofisica (INAF))
• 33
Evolution of the critical instability height and CME likelihood in solar active regions

Working towards improved space weather predictions, we aim to quantify how the critical height at which the torus instability drives coronal mass ejections (CMEs) varies over time in a sample of solar active regions. We model the coronal magnetic fields of 37 bipolar active regions and quantify the critical height at their central polarity inversion lines throughout their lifetimes. We then compare these heights to the changing magnetic flux at the photospheric boundary and identify CMEs in these regions. When the magnetic flux of an active region increases due to the continuous emergence of a single bipole, the critical height tends to increase, whereas the emergence of a new bipole into an existing active region or quiet Sun environment often causes the critical height to decrease sharply. A similar invesitgation of periods of decreasing magnetic flux indicates that the critical height rises when magnetic concentrations disperse, and falls when polarities converge and cancel at their inversion line. These results support and extend previous studies by showing that the average critical height is generally proportional to the separation between active region sunspots through time. We find higher rates of CMEs at times when the critical height is falling rather than rising, and when magnetic flux is increasing rather than decreasing. With continued work to also study multipolar regions, we may be able to identify conditions that are more conducive to producing eruptions, and determine the relative likelihood of active regions producing CMEs using readily-available observational proxies.

Speaker: Dr Alexander James (European Space Agency, ESAC)
• 34
The energy partition in solar flares: current state, new observations, and new models

Solar eruptive events entail a complex interplay of energy release, transport, and conversion processes. A quantitative characterization of the different forms of energy therefore represents a crucial observational constraint for models of solar eruptions in general, as well as for magnetic reconnection, heating, and particle-acceleration processes in particular. These constraints are derived from X-ray, EUV and bolometric observations. We first review the results of recent statistical studies, focusing on possible explanations for apparent discrepancies between the different studies. We then present some of the first results on energy partition obtained with the novel STIX X-ray spectrometer/imager on Solar Orbiter. Finally, we present the first upper limits on energy input by nonthermal electrons given by the warm-target model.

Speaker: Dr Alexander Warmuth (Leibniz Institute for Astrophysics Potsdam (AIP))
• 35
Origin and Evolution of the 2021 May 7 Solar Eruption

Coronal Mass Ejections (CMEs) are large-scale eruptions expanding all the way from the low corona into the interplanetary space. Despite CMEs are spectacular events and attract wide interest among scientists and general public, these phenomena can seriously impact the Earth and potentially damage human facilities. CMEs have been studied quite extensively since their discovery, however different aspects governing this type of event still need to be understood.
From this perspective, the GOES M3.9 flare occurred on 2021 May 7 provides us with the unique set of observations to study the origin and the dynamics of its related fast CME and EUV wave. While the STIX instrument (Spectrometer/Telescope for Imaging X-rays), the Hard X-Ray (HXR) telescope onboard Solar Orbiter, allows us to investigate the flare and correlate the flare-accelerated electrons with the eruption, the combined near simultaneous observations from three different locations in the heliosphere, from Solar Orbiter, STEREO A and Earth, give us also the opportunity to disentangle the morphology of the CME, track the origin and evolution of the eruption, and deduce the kinematics of both the CME and the associated EUV wave.

Speaker: Andrea Francesco Battaglia (ETH Zürich and FHNW Windisch)
• 36
Validation of the PDFISS method for electric field inversions using magnetic flux emergence simulations

Information on electric fields in the photosphere is required to calculate the electromagnetic energy flux through the photosphere and set up boundary conditions for data-driven magnetohydrodynamic (MHD) simulations of solar eruptions. Recently, the PDFISS method for inversions of electric fields from a sequence of vector magnetograms and Doppler velocity measurements was improved to incorporate spherical geometry and a staggered-grid description of variables. The method was previously validated using synthetic data from anelastic MHD (ANMHD) simulations. We further validate the PDFISS method, using approximately one-hour long MHD simulation data of magnetic flux emergence from the upper convection zone into the solar atmosphere. We reconstruct photospheric electric fields and calculate the Poynting flux, and compare those to the actual values from the simulations. We find that the accuracy of the PDFISS reconstruction is quite good during the emergence phase of the simulated ephemeral active region evolution and decreases during the shearing phase. Analysing our results, we conclude that the more complex nature of the evolution (compared to the previously studied ANMHD case) that includes the shearing evolution phase is responsible for the obtained accuracy decrease.

Speaker: Dr Andrey Afanasyev (LASP, University of Colorado Boulder)
• 37

Speaker: Dr Arun Awasthi (University of Science and Technology of China)
• 38
Coronal dimmings as a diagnostics for solar and stellar CMEs

Coronal dimmings are sudden decreases of the solar EUV and X-ray emission caused by coronal mass ejection (CMEs). Dimming regions map to the bipolar ends of closed magnetic field lines that become stretched or temporarily opened during an eruption, and are a result of the depletion of coronal plasma caused by the expansion and mass loss due to the CME. Recently available multi-point imagery from satellites at different locations in the heliosphere provided us with unprecedented observations of the three-dimensional evolution of solar CMEs and their coronal dimmings. These studies showed distinct correlations between CME mass and speed with key parameters of the associated coronal dimmings such as their spatial extent and intensity drop (Dissauer+ 2019, Chikunova+ 2020). Here, we study Sun-as-a-star broad-band EUV light curves derived from SDO’s Extreme ultraviolet Variability Experiment (EVE) as a testbed to study whether coronal dimmings can be also observed on stars and used for stellar CME detection. We demonstrate that large eruptive flares are with a high probability associated with a post-flare coronal dimming, with intensity drops in the 15-25 nm full-Sun light curves up to 5%. Searching for similar patterns of post-flare dimmings in the X-ray and EUV light curves of solar-like and late-type stars, we identify 21 stellar CME candidates, which is more that all previous reports of stellar CMEs. The derived intensity drops are an order of magnitude larger than for the Sun, suggesting that a substantial part of the stellar corona gets ejected by the CME.

Speaker: Prof. Astrid Veronig (University of Graz)
• Poster Session 1.6
Convener: Manolis Georgoulis (RCAAM of the Academy of Athens)
• 39
Statistical study of CMEs, lateral overexpansion and SEP events

We present a statistical study on the early evolution of coronal mass ejections (CMEs), to better understand the effect of CME (over)-expansion and how it relates to the production of Solar Energetic Particle (SEP) events. We study the kinematic CME characteristics in terms of their radial and lateral expansion, from their early evolution in the Sun’s atmosphere as observed in EUV imagers and coronagraphs. The data covers 72 CMEs that occurred in the time range July 2010 to September 2012, where the twin STEREO spacecraft were in quasi-quadrature to the Sun-Earth line. From the STEREO point-of-view, the CMEs under study were observed close to the limb. We calculated the radial and lateral height (width) versus time profiles and derived the corresponding peak and mean velocities, accelerations, and angular expansion rates, with particular emphasis on the role of potential lateral overexpansion in the early CME evolution. We find high correlations between the radial and lateral CME velocities and accelerations. CMEs that are associated with SEPs tend to be located at the high-value end of the distributions of velocities, widths, and expansion rates compared to non-SEP-associated events.

• 40
Estimating uncertainties in the back-mapping of the fast solar wind

Although the sources of the fast solar wind are known (the coronal holes), the exact acceleration mechanism of the fast solar wind is still not fully understood. An important factor that can improve our understanding is the combination of remote sensing and in-situ measurements.
In order to combine them, it is necessary to accurately identify the source location of the in-situ solar wind with a process called back-mapping. Back-mapping consists mainly of two parts.
The first one is the ballistic mapping where the solar wind radially draws the magnetic field into the Parker Spiral, down to a point in the outer corona.
The second one is the magnetic mapping where the solar wind follows the non-trivial magnetic field line topology down to the solar surface. The magnetic field in this region is derived from a global model, like the potential field source surface extrapolations (PFSS).
In this study we focus on this back-mapping of the fast solar wind and try to determine all the uncertainties and sources of error that can affect the final location deduced on the solar surface. We compare different models for the ballistic mapping and also for the magnetic mapping and explore which free parameters have the greatest effect in the back-mapped locations.
Finally, we provide an uncertainty estimation for the back-mapped footpoints and compare our results with existing frameworks, like the Connectivity-Tool of IRAP.

Speaker: Alexandros Koukras (KU Leuven, Royal Observatory of Belgium)
• 41
Novel validation scheme for quality assessment of coronal models

The strive for improving space weather forecasts naturally comes with the need for a standardized validation scheme for the involved models. Especially the performance of coronal (magnetic field) models that form the lower boundary of forecasting simulations is crucial as their errors are further propagated by solar wind models. We therefore developed a benchmarking system, allowing the quality assessment for solar coronal magnetic field models in an easy-to-implement manner. Our system is constructed as a step-wise scheme, incorporating 1) visual inspection with multi-view point white-light data, 2) magnetic topology analysis with EUV on-disk data and 3) a feature matching implementation, also using white-light data from multiple perspectives. The strength of our assessment scheme lies in both the multi-view point aspect as well as the combination of both topological on-disk as well as off-limb structure analysis, providing a very detailed insight into a models magnetic configuration. A broad applicability is ensured by the possibility for customization of the sub-steps, while providing the framework for comparison. To showcase its application, also to derive ideal parameter sets for the model(s) under investigation, we used the validation system on the coronal model of EUHFORIA. We hereby run the model with 67 different parameter configurations and derive the best performing parameter sets.

Speaker: Andreas Wagner (University of Graz)
• 42
3D CME reconstruction model comparison: GCS vs. StereoCAT

For forecasting the arrival of coronal mass ejections (CMEs) at Earth, heliospheric 3D models such as EUHFORIA (EUropean Heliospheric FORcasting Information Asset) [Pomoell and Poedts,2018] rely on the correct observational input of the initial, near-Sun CME parameters (e.g. latitude, longitude, velocity, angular width, etc). The input parameters can be obtained by fitting the structure of the CME while it is still close to the Sun (up to 21.5 solar radii) by using different reconstruction methods such as the Stereo CME Analysis Tool (StereoCAT) and the Graduated Cylindrical Shell (GCS).

The StereoCAT online tool calculates the 3D kinematic properties of CMEs by triangulating their features based on up to three different coronagraph fields-of-view under the assumption that CMEs present a circular cross-section and maintain a constant angular width during their radial expansion, the so-called “cone model” [Millward,2013].

On the other hand, the GCS model [Thernisien,2011], consists of two cones that represent the "legs", attached to the ends of a tubular section, together forming the main body. Like StereoCAT, this tool relies on two different white-light coronagraph fields-of-view for the fitting of the CME shape with no description for the inside structure.

In this study, 19 Earth-directed CME events were selected and their kinematic parameters were calculated using both StereoCat and GCS tools. The aim of this work is to compare the CME properties obtained by both methods and understand how their potential differences lead to different modeling results at Earth when we propagate the CME structures with EUHFORIA.

Speaker: Mr Antonio Niemela (Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven)
• 43
Geo-effectiveness prediction of CMEs with EUHFORIA coupled to OpenGGCM

The EUropean Heliospheric FORecasting Information Asset (EUHFORIA, Pomoell and Poedts, 2018) is a physics-based heliospheric and CME propagation model designed for space weather forecasting and other scientific studies. Although EUHFORIA can predict the solar wind plasma and magnetic field parameters at Earth, it is not designed to evaluate geomagnetic indices like Disturbance-storm-time (Dst) or Auroral Electrojet (AE) that quantify the impact of the magnetized plasma encounters on Earth’s magnetosphere. Therefore, we coupled EUHFORIA with the Open Geospace General Circulation Model (OpenGGCM, Raeder et al, 1996), a magnetohydrodynamic model of Earth’s magnetosphere. In this coupling, OpenGGCM is driven with the synthetic solar wind and interplanetary magnetic field obtained from EUHFORIA simulations as input to produce the magnetospheric and ionospheric parameters of Earth. This coupling is validated with observed geo-effective CME events modelled with flux-rope CME models like Spheromak and FRi3D. We compare these simulation results with the indices obtained from OpenGGCM simulations driven by the measured solar wind data from spacecraft like WIND. We further discuss how the choice of CME model and observationally constrained parameters influence the input parameters, and hence the geomagnetic disturbance indices estimated by OpenGGCM. We also discuss some limitations of the coupling and suggest improvements for future work.

Speaker: Ms Anwesha Maharana (Centre for mathematical Plasma Astrophysics, KU Leuven, Belgium)
• 44
Including ephemeral regions in surface flux transport simulations and solar irradiance reconstructions

Time-series of historical solar irradiance variations is an important input to climate models. An extension of the record of direct irradiance measurements available since 1978 to earlier times is only possible with the help of models. For this, we need to know the evolution of the surface magnetic field. The longest record of direct observations of solar activity is the sunspot number covering the last four centuries. However, it has the shortcoming that it only allows tracing the evolution of the large-scale active region (ARs) harbouring sunspots, while the evolution of small-scale ephemeral regions (ERs) is heavily uncertain. At the same time, ERs play an important role in driving the long-term irradiance variations. We use a new model of the ERs emergence based on recent solar observations, where the emergence of all magnetic regions is described by a single power-law distribution with an exponent varying with solar activity, as represented by the sunspot number. The evolution of the magnetic field with time is simulated with a surface flux transport model, and the computed magnetic field is used to reconstruct solar irradiance.

Speaker: Bernhard Hofer (Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, Goettingen, Germany)
• 45
The Role of Magnetic Reconnection in the Acceleration and Expansion of A Coronal Mass Ejection on 2013 February 27

The role of magnetic reconnection in the acceleration of coronal mass ejections (CMEs) has been widely discussed. However, because CMEs are found to have expansion speed which is comparable to propagation speed in the corona, it is still not clear about which portion of the reconnection contributes to the acceleration and expansion separately. To address this question, we analyze a fast CME event on 2013 February 27, associated with clearly observed three-part structure. We find that the CME front undergoes a smooth but long-lasting acceleration in the high corona while the speed of its core remains constant. This acceleration is caused by the additional CME expansion along the radial direction (with acceleration speed of Ae). Magnetic reconnection is found to occur after the eruption of the CME and to continue during the CME propagation. We estimate the potential acceleration caused by the drag-based model (Ad), in which the CME deceleration due to the solar wind drag is thought to be compensated by the acceleration via reconnection. The comparison between Ae and Ad reveals that the contribution of the magnetic reconnection in the high corona to the expansion is comparable to or larger than its contribution to the acceleration. The result also brings a view that the expansion shall be considered separately when studying CME dynamics.

Speaker: Bin Zhuang (University of New Hampshire)
• 12:16 PM
Lunch break
• Plenary 2
Convener: Laurene Jouve (IRAP Toulouse / France)
• 46
Simulations of chromospheric heating: progress and challenges

The chromosphere is a very dynamic and complex layer where all the relevant physical processes happen on very small spatio-temporal scales. A few spectral lines that can be used as chromospheric diagnostics, give us convoluted information that is hard to interpret without realistic theoretical models. What are the key ingredients that these models need to contain? It is clear that what shapes the chromosphere and determines how and where the energy is deposited is the magnetic field configuration. In this review, we will discuss the formation and properties of chromospheric structures at different spatial scales. We will give an overview of what our models can reduce and what challenges we face. We will also touch on how chromospheric heating depends on emerging flux.

Speaker: Sanja Danilovic (Stockholm University)
• 47
Shocks and instabilities in the partially ionised plasma of the solar atmosphere

Observations of the Sun reveal a rich array of dynamics throughout all levels of the solar atmosphere. In many cases. the observed dynamic motions are driven by the magnetic field. However, the lower solar atmosphere, i.e. the photosphere and chromosphere, is a partially ionised plasma, with most of the species being neutral. This means that the driver of the fluid motions cannot directly influence the fluid itself, this only happens through the interaction of neutral and charged species. In this talk I will look at two areas of research into partially ionised plasma dynamics, relating to MHD shocks and instabilities, which are of particular relevance to our understanding of the dynamic solar atmosphere. For shocks, the neutral fluids decouples from the magnetic field in the shock front, creating a broad shock transition that contains substructure, and can influence the stability of the shock. For instabilities, partial ionisation is found to change the growth rate and the effects of non-linear transport. To date studies using two fluid models are often highly idealised, but progress is being made to look at more realistic settings. I will review some of the recent advances.

Speaker: Andrew Hillier (University of Exeter)
• 48
The formation and heating of chromospheric fibrils in a radiation-MHD simulation

How mass is loaded into the upper chromosphere and transition region is an important unclosed matter. The standard fibrilar mass loading scenario is of feeding material up relatively static fieldlines by the guiding magnetic field, resulting from initial impulses made by p-mode oscillations (Hansteen et al 2006, De Pontieu et al 2007). Instrumentation such as DKIST and EST will provide an excellent opportunity to address this issue.

We use passive tracer particles seeded into dense fibrils in 3D RMHD Bifrost simulations to investigate fibril creation and destruction. The most common “lift and drain” mass loading scenario found is markedly different to previous suggestions. Box oscillations, the simulation equivalent of the p-modes initiate the formation. Rather than loading material up the footpoints of static fieldlines, the fieldlines themselves rise, firstly near the footpoints where the plasma velocity is well aligned with vertical fieldlines. Material is then caught above the flattened apexes of rising fieldlines and lifted by the Lorentz force along the central lengths of the fibrils as the fieldline untwists and becomes more parabolic. Subsequently, material drains into one or both footpoints under gravity. Instances are also found of material with horizontal velocities that are simultaneously elevated in rising fieldlines, creating the illusion of parabolic motion up a static fieldline.

These mechanisms are not implausible additional solar scenarios for fibrilar mass loading. Criteria for discerning between this and standard mass loading scenarios are described. Experimental parameters required to achieve more standard fibrilar mass loading in the simulations are discussed.

Speaker: Malcolm Druett (Stockholm University)
• 49
Where is the base of the Transition Region? SDO, TRACE and ALMA observations

Atmospheric models place the Chromosphere-Corona Transition Region at $\sim2$Mm above the $\tau_{5000}=1$ level. Os course, the upper part of the chromosphere is highly inhomogeneous, with spicules intruding into the corona. There is, however, a more homogeneous lower region, as evidenced in the MgII triplet lines, extending to $\sim2$Mm (Alissandrakis etal.; https://doi.org/10.1007/s11207-018-1242-4). In SDO and TRACE images spicules appear in emission in the 1600, 1700 and 304A bands and in absorption in the EUV bands; the latter is due to photo-ionization of HI and HeI, which increases with wavelength. At the shortest available AIA wavelength and taking into account that the photospheric limb is $\sim0.34$Mm above the $\tau_{5000}=1$ level, we found that TR emission starts at $\sim3.7$Mm; extrapolating to $\lambda=0$, where there is no chromospheric absorption, we deduced a height of $3.0\pm0.5$Mm, above the value of 2.14Mm of Avrett and Loeser (2008,ApJS,175,229).
Another indicator of the extent of the chromosphere is the height of the network. This produces a limbward shift of features with respect to the position of their counterparts in magnetograms. Using this approach, we measured heights of $0.14\pm0.03$Mm(1700A), $0.39\pm0.06$Mm(1600A) and $3.29\pm0.23$Mm(304A), with a possible solar cycle variation.
A third indicator is the position of the limb in UV as well as in ALMA mm-λ images. This is not very reliable, as the limb position is affected by spicules, but it is indicative. We obtained values of $1.4\pm0.2$Mm(1600A), $2.4\pm0.7$Mm(ALMA 1.26mm), $4.2\pm2.5$Mm(ALMA 3mm) and $5.7\pm0.2$Mm(304A).
Putting everything together, we conclude that the average chromosphere extends higher than homogeneous models predict.

Speaker: Prof. Costas Alissandrakis (University of Ioannina, Greece)
• 50
SO/PHI on Solar Orbiter, the first magnetograph to leave the Sun-Earth line: The instrument, its first data and its scientific promise

The Solar Orbiter mission of ESA and NASA is currently on a trajectory that will take it into the inner heliosphere from where it will explore the Sun (and heliosphere) from close up and from out of the ecliptic plane. It aims to address the overarching questions of how the Sun creates and controls the heliosphere, and why solar activity changes with time. Among the instruments that Solar Orbiter carries is the Polarimetric and Helioseismic Imager (SO/PHI), which is the first magnetograph to observe the Sun from outside the Sun-Earth line. Already the trajectory provides SO/PHI with unique capabilities, although it also poses huge challenges, which could only be overcome by technology developments on a significant scale. Although Solar Orbiter is still in cruise phase first glimpses of SO/PHI’s capabilities have become apparent, including the excellent quality of the data. The promise for the science that can be done with SO/PHI data in the future is immense, both with standalone observations by SO/PHI and with SO/PHI data combined with observations made by other Solar Orbiter instruments, or with data gathered by instruments on other spacecraft or on the ground. The talk will give a brief description of the Solar Orbiter mission, introduce the SO/PHI instrument, show first data and describe the science goals for the different phases of the Solar Orbiter mission. The SO/PHI data policy will also be briefly introduced.

Speaker: Sami Solanki (Max Planck Institute for Solar System Research (MPS))
• 3:10 PM
Coffee/tea break
• Poster Session 2.1
Convener: Manolis Georgoulis (RCAAM of the Academy of Athens)
• 51
Towards measuring magnetic twist in active regions using SDO/HMI

Measurements of the magnetic field’s twist play an important role in constraining dynamo theory, models of flux emergence and the prediction of flares. We aim to characterize methods of measuring twist directly from SDO/HMI vector magnetograms by generating Monte-Carlo synthetic data sets. By studying several example sunspots we found that the temporal fluctuations in the HMI vector magnetograms are spatially correlated. We have developed an empirical model for noise that includes these spatial correlations.

Speaker: Christian Baumgartner (Max Planck Institute for Solar System Research)
• 52
Spotless days and geomagnetic index as the predictors of solar cycle 25

We study the sunspot activity in relation to spotless days (SLDs) during the descending phase of solar cycle 11–24 to predict the amplitude of sunspot cycle 25. For this purpose, in addition to SLD, we also use the geomagnetic activity (aa index) during the descending phase of a given cycle. A very strong correlation of the SLD (R=0.68) and aa index(R=0.86) during the descending phase of a given cycle with the maximum amplitude of next solar cycle has been estimated. The empirical relationship led us to deduce the amplitude of cycle 25 to be 99.13± 14.97 and 104.23± 17.35 using SLD and aa index, respectively as predictors. Both the predictors provide comparable amplitude for solar cycle 25 and reveal that the solar cycle 25 will be weaker than cycle 24. Further we derive that the maximum of cycle 25 is likely to occur between February and March 2024. While the aa index has been used extensively in the past, this work establishes SLDs as another potential candidate for predicting the characteristics of the next cycle.

Speaker: Dipali Burud (Ksv, gandhinagar india)
• 53
Global Solar Magnetic Variations Characterized by Excess Brightness Indices and Spectroscopic Proxies

On a global scale chromospheric magnetic activity is represented by plages and enhanced chromospheric network. These phenomena significantly contribute to the variation of the solar UV radiation and the enhancement in chromospheric emission in the two strong resonance Ca II H & K lines. We present a set of excess brightness and area indices based on disk-resolved UV 1600Å images of the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) and full-disk Ca II K line-core filtergrams of the Chromospheric Telescope (ChroTel) at Observatorio del Teide, Tenerife, Spain. In addition, we compute the spectroscopic S-index based on the quasi-synoptic observations in 2018 and 2019 with the Potsdam Echelle Polarimetric Spectroscopic Instrument (PEPSI) of the Large Binocular Telescope (LBT) at Mt. Graham International Observatory (MGIO), Arizona, U.S.A. All indices display signatures of rotational modulation, even during the very low magnetic activity in the minimum of Solar Cycle 24. The UV 1600Å and Ca II K image-based excess brightness indices reveal as strong peak of activity in 2012 and the double-peaked maximum in 2014. These features are driven by complex and large active regions. Furthermore, both spectral- and image-based indices demonstrate that the Solar Cycle 24 concludes with a deep minimum. Moreover, the SDO/AIA UV excess brightness indices reveal an intriguing aspect of activity asymmetry between the two hemispheres. In particular, starting in 2018, the rotational modulation in the southern hemisphere vanishes, indicating exceptionally low solar activity representing the basal activity level of the Sun.

Speaker: Ekaterina Dineva (Leibniz Institute for Astrophysics Potsdam)
• 54
Performance of solar far-side active regions neural detection

Far-side helioseismology is a technique used to detect activity signatures in the far hemisphere of the Sun, based on near-side wave field interpretation. We evaluated the performance of a new neural network approach, developed to improve the sensitivity of the seismic maps to the presence of far-side active regions, and thoroughly compared it with the standard method commonly applied to predict far-side active regions from seismic measurements, using STEREO extreme ultraviolet observations of the far hemisphere as a proxy of activity.
We have confirmed the improved sensitivity of the neural network to the presence of far-side active regions. Approximately 96% of the active regions identified by the standard method with strength above the threshold commonly employed by previous analyses are related to locations with enhanced extreme ultraviolet emission. For the same percentage of false positives, the neural network can provide a 47% increase in the number of far-side active region detections confirmed by their extreme ultraviolet brightness. Weaker active regions can be detected by relaxing the threshold in their seismic signature. For almost all the range of thresholds, the neural network delivers a higher number of confirmed detections and a lower rate of false positives.
The neural network is a promising approach to improve the interpretation of the seismic maps provided by local helioseismic techniques, which can lead to improvements in space weather forecasting.

Speaker: Elena García Broock (Instituto Astrofísico de Canarias)
• 55
Methodology for estimating the magnetic Prandtl number and application to solar surface small-scale dynamo simulations

It is still an open question if the quiet Sun small-scale magnetic field mainly originates of a turbulent cascade acting on fields produced by a global-scale dynamo, or if it is mainly generated locally by a small-scale turbulent dynamo. Consequently, a number of numerical studies focusing on turbulent dynamos was carried out in the recent past. Despite of these works providing extremely useful information on many aspects of small-scale dynamo action, it is still unclear how to extrapolate these results to the Sun, in particular because of the disparate regimes of magnetic Prandtl numbers, $Pr_m$, between numerical simulations and the real Sun. The present work addresses these issues in a twofold way. First, a general methodology for estimating the effective diffusivities stemming from radiative MHD simulations is proposed. It is based on the method of Projection on Proper elements, initially introduced in the plasma physics community to verify plasma turbulence simulation codes. It relies on a post processing step using different, higher order accurate numerical operators. Second, a study of how the magnetic field resulting from small-scale solar dynamo simulations depends on the Reynolds number, $Re$, and magnetic Reynolds number, $Re_m$, is presented. For this, several radiative MHD simulations with different effective viscosity and plasma resistivity are carried out with the CO5BOLD code and the resulting magnetic energy growth rate and saturated magnetic field are characterized in terms of $Re$ and $Re_m$. It is shown that it is possible to simulate small-scale dynamo action also in the regime $Pr_m<1$.

Speaker: Dr Fabio Riva (Istituto Ricerche Solari Locarno (IRSOL), Università della Svizzera italiana (USI), CH-6605 Locarno-Monti, Switzerland.)
• 56
Rotational velocity of the solar corona versus solar activity

This contribution describes the procedure by which we found relationship between the rotational velocity of the solar corona and the level of the solar activity (phase of the solar cycle) in the period of 2011 - 2020. The deviation of the speed from the mean value is ± 0.0239 °/day, which corresponds on the solar surface near the equator to ± 3.16 m/s, with a mean value of about 14.1 °/day (1864.3 m/s). The level of activity was determined using the coronal index (CI) and we used the data on coronal rotational velocity from our recent work (Dorotovič and Rybanský, 2019). The correlation coefficient between the monthly averages of the CI and the rotational velocity during the given period (120 months) is 0.752. We did not find theoretical explanation for this phenomenon.
Reference: Dorotovič, I., Rybanský, M. Rotation of Some Solar Coronal Bright Features as Derived from the Solar Dynamics Observatory/Atmospheric Imaging Array (SDO/AIA) 21.1 nm Images (for the Years 2011 – 2018). Sol. Phys. 294, 109 (2019). https://doi.org/10.1007/s11207-019-1501-z

Speaker: Ivan Dorotovič (Slovak Central Observatory)
• 57
Power law of coronal jets and their relationship with the solar cycle

Late-type stars often display multiple and variable levels of periodic magnetic activity. It is well established that our nearest stellar source, the Sun, manifests similar levels of magnetic activity, including the 11-year sunspot cycle discovered two centuries ago. This solar cycle dominates the properties of large-scale phenomena, such as flares and sunspots. However, little is known about how the solar cycle influences the dynamics of localized small-scale events. Here, we report our finding of the modulation of off-limb coronal jets by solar cycles on timescales of both 11 years and 1 year. The modulation is evidenced by the coronal jet butterfly diagram and the quasi-annual periodicities of their intensities, locations and structural characteristics. We also find a power-law distribution of the jet intensity, which is highly consistent with that found for solar and stellar flares. This suggests that the small-scale eruptive events studied here exhibit self-organized criticality similar to their large-scale counterparts.

Speaker: Jiajia Liu
• Poster Session 2.2
Convener: Ramon Oliver (University of the Balearic Islands)
• 58
Effect on heliospheric magnetic field and wind due to supergranular driving of streamers and pseudostreamers

Interchange reconnection has been proposed as a mechanism for the generation of the slow solar wind, and a key contributor to determining its characteristic qualities. We study the implications of interchange reconnection for the structure of the plasma and field in the heliosphere in the context of the "S-Web" model. We show that photospheric driving at supergranular scales leads to a corrugation (at low altitudes) of the separatrix surfaces that define the streamer and (to a lesser extent) the pseudostreamer. We demonstrate that newly-opened magnetic flux is distributed in a filamentary pattern in the heliosphere, suggesting that the pattern of granular and supergranular flows on the photosphere should leave an observable imprint in the heliosphere. As a result the connectivity of a heliospheric spacecraft trajectory to the photosphere exhibits high complexity.

Speaker: Prof. David Pontin (University of Newcastle, Australia)
• 59
Preparing for DKIST: Using SUNRISE/IMaX Data to Compute Energy Fluxes from the Quiet Sun

Understanding processes in the quiet Sun is crucial for understanding the Sun in general. The overarching goal of our study is to quantify the energy output of the quiet Sun, which can be expressed in terms of the Poynting flux. To compute this quantity, one can use Maxwell's equations, provided the full orientation of magnetic, electric, and velocity fields are available. All of those can be recovered from spectropolarimetric data using different inversion methods.

Quiet Sun magnetic fields have only recently become observable with the launch of missions such as Hinode and SUNRISE. The Daniel K. Inouye Solar Telescope (DKIST) is expected to improve the quality of these observations even further. While the SUNRISE/IMaX images have a resolution of 0.15 ''/pixel, the DKIST ViSP instrument available in the first cycle will provide magnetograms with a resolution of 0.05''/pixel. The cadence of images provided by the VBI is 7 seconds compared to 12 seconds by IMaX. The signal-to-noise ratio of Stokes vectors measurements is likewise expected to improve.

We present our preliminary results obtained from velocity and electric field inversions of photospheric images, magnetograms and Doppler velocities from SUNRISE/IMaX, the challenges associated with these inversions, and implications for DKIST observations. Specifically, we use Fourier Local Correlation Tracking (FLCT) and machine-learning-based algorithm, DeepVel, to obtain, respectively, optical flows and velocity fields, and compare these with quantities derived using the PDFI electric-field inversion method.

Speaker: Dennis Tilipman (NSO, CU Boulder)
• 60
Understanding the characteristics of solar WINQSEs

The confluence of the data from the Murchison Widefield Array and an imaging pipeline tailored for spectroscopic snapshot images of the Sun at low radio frequencies have led to enormous improvements in the imaging quality of the Sun. These developments have lowered the detection threshold for nonthermal emissions by up to two orders of magnitude as compared to earlier studies, and enabled our discovery of Weak Impulsive Narrowband Quiet Sun Emissions (WINQSEs). In our studies we find WINQSEs to be consistent with the radio signatures of coronal nanoflares, hypothesized by Parker (1988) to explain coronal heating in the quiet Sun emissions. As a first step towards exploring this tantalising possibility of making progress on the coronal heating problem, we have been pursuing multiple projects to better understand the observational characteristics of WINQSEs. These include attempts to look for WINQSEs in multiple independent datasets; attempting to characterise their morphologies in radio maps using Artificial Intelligence/Machine Learning based approaches; understanding their spectro-temporal structure by making solar images with a time resolution of a few milliseconds and frequency resolution of ~100 kHz. Here we present the current status of these projects.

Speaker: Prof. Divya Oberoi (National Centre for Radio Astrophysics, Tata Institute of Fundamental Research)
• 61
Inference of log(gf) values for atomic lines in NUV spectral range

The third flight of the Sunrise observatory carries the new Sunrise UV Spectropolarimeter and Imager (SUSI) which will cover the spectral range from 310 nm to 410 nm. In this region, the number density of spectral lines is at least four times larger than in the visible/infrared part of the spectrum, including a significant number of chromospheric lines. To extract accurate physical parameters of the solar atmosphere from observations with SUSI we need precise atomic line parameters, e.g., the log (gf) value. Here we review an extension of the spectropolarimetric inversion scheme to account for coupled inversion of atomic line parameters. In this scheme, atomic line parameters are inverted using information from the whole field of view. Additionally, we discuss the potential of this inversion scheme using synthetic spectra computed from 3D realistic magnetohydrodynamic simulations.

• 62
Acoustic/shock wave propagation in the gravitationally stratified partially ionised plasmas: the multi-dimensional effects

Numerical and observational evidences both support that wave propagation and dissipation may be an important mechanism that contributes to the heating of the solar chromosphere. For instance, acoustic waves and the resulting shock waves in the gravitationally stratified solar atmosphere, may provide at least part of the energy to compensate the radiative energy losses in the chromosphere. However, high-frequency waves are difficult to observe because of the limited instrumental resolution, and in fact, numerical modelling also needs sufficient spatial and temporal resolution for high-frequency waves to avoid artificial numerical dissipation. Therefore, numerous one-dimensional numerical simulations have been performed without taking into account multi-dimensional effects, thus reducing the computational costs. We have used a neutral-ion two-fluid model to investigate high-frequency acoustic wave propagation in one-dimensional gravitationally stratified partially ionised plasmas, showing that sufficient energy can be transported from the bottom of the photosphere up to the chromosphere to compensate the radiative energy losses (Zhang et al. 2021). The model assumes an initial hydrostatic and ionisation equilibrium, and takes into account the effects of neutral-ion collisions, ionisation and recombination. In this work, we further quantify the multi-dimensional effects in modelling acoustic/shock wave propagation. More specifically, as our previous simulations show that the shock wave damping yields an important heating process, in two-dimensional simulations the formation of shocks may change since the energy spreads also in the horizontal direction as the waves propagate in the vertical direction.

Speaker: Dr Fan Zhang (Centre for Mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven)
• 63
Will it rain today? The role of asymmetries in coronal rain formation during thermal non-equilibrium cycles

Thermal non-equilibrium (TNE) produces several observables that can be used to constrain the spatial and temporal distribution of coronal heating. Its manifestations include prominence formation, coronal rain, and long-period intensity pulsations in coronal loops. The recent observation of abundant periodic coronal rain associated with intensity pulsations by Auchère et al. allows to unify these two phenomena as the result of TNE condensation and evaporation cycles. On the other hand, many intensity pulsation events observed by Froment et al. show little to no coronal rain formation. Our goal is to understand why some TNE cycles produce abundant coronal rain, while others produce little to no rain.

We reconstruct the geometry of the event reported by Auchère et al., using images from STEREO/SECCHI/EUVI and magnetograms from SDO/HMI. We then perform 1D hydrodynamic simulations of this event, for different different heating parameters and variations of the loop geometry (9000 simulations in total). We compare the resulting behaviour to simulations of TNE cycles by Froment et al. that do not produce coronal rain. Our simulations show that both prominences and TNE cycles (with and without coronal rain) can form within the same magnetic structure. We show that the formation of coronal rain during TNE cycles depends on the asymmetry of the loop and of the heating. Asymmetric loops are overall less likely to produce coronal rain, regardless of the heating. In symmetric loops, coronal rain forms when the heating is also symmetric. In asymmetric loops, rain forms only when the heating compensates the asymmetry.

Speaker: Gabriel Pelouze (KU Leuven (Belgium))
• Poster Session 2.3
Convener: Georgia Tsiropoula (National Observatory of Athens)
• 64
Evidence of chromospheric molecular hydrogen emission in a solar flare observed by the IRIS satellite

We have carried out the first comprehensive investigation of enhanced line emission from molecular hydrogen, H$_{2}$ at 1333.79 Å, observed at flare ribbons in SOL2014-04-18T13:03. The cool H$_{2}$ emission is known to be fluorescently excited by Si IV 1402.77 Å UV radiation and provides a unique view of the temperature minimum region (TMR). Strong H$_{2}$ emission was observed when the Si IV 1402.77 Å emission was bright during the flare impulsive phase and gradual decay phase, but it dimmed during the GOES peak. H$_{2}$ line broadening showed non-thermal speeds in the range 7-18 km/s, possibly corresponding to turbulent plasma flows. Small red (blue) shifts, up to 1.8 (4.9) km/s were measured. The intensity ratio of Si IV 1393.76 Å and Si IV 1402.77 Å confirmed that plasma was optically thin to Si IV (where the ratio = 2) during the impulsive phase of the flare in locations where strong H$_{2}$ emission was observed. In contrast, the ratio differs from the optically thin value of 2 in parts of ribbons, indicating a role for opacity effects. A strong spatial and temporal correlation between H$_{2}$ and Si IV emission was evident supporting the notion that fluorescent excitation is responsible.

Speaker: Sargam Mulay (University of Glasgow, UK)
• 65
How common are counter Evershed flows?

The Evershed flow is one of the most outstanding features observed in sunspot penumbrae. It is a plasma flow running along penumbral filaments radially outwards and towards the moat of sunspots. This flow was first observed more than a century ago. Nevertheless, in the past decade, observations have shown unusual counter Evershed flows (CEFs) that run in the opposite direction, towards the sunspot umbra. Until now, very few CEFs have been reported. Thus, CEFs are still thought to be extremely rare events. In this work, we followed 97 active regions for an average of 9.6 days using SDO/HMI observations. Our sample includes various types of sunspots, from the most simple $\alpha$-spots to the more complex $\delta-$groups of sunspots. We report the detection of 384 CEFs, showing that CEFs are far more frequent than their rarity in the literature suggests. However, CEFs are still sparse as they are observed in just 5.9% of the average observed time. We explain that the small number of reports of CEFs was due to incomplete coverage of active regions, which was overcome by continuous observations taken from space.

Speaker: Sebastián Castellanos Durán (Max Planck Institute for Solar System Research (MPS))
• 66
Prevalence of thermal non-equilibrium over an active region

The solar corona is characterised by its puzzling multi-million degree component. On the other hand, observations in the last decade have shown that the corona also contains a large amount of coronal rain, 10-100 times cooler and denser than the surroundings. The properties of coronal rain are now known to be strongly linked to the coronal heating properties, but its origin, dynamics, and morphology are still not well understood. In particular, the spatial and temporal occurrence of coronal rain in an active region is unknown. In this study, we carry out an imaging and spectroscopic multi-wavelength statistical study of coronal rain observed in an active region off-limb with IRIS and AIA, spanning chromospheric to transition region temperatures. We use the Rolling Hough Transform (RHT) to automatically detect and measure the properties of coronal rain clumps. Over the 4.5 hr long observation, the entire coronal area in the plane-of-the-sky up to a height of 30-40 Mm is covered by coronal rain, suggesting a prevalence of thermal non-equilibrium (TNE). We estimate the fraction of coronal volume in TNE and the role of coronal rain in the mass and energy cycle. The rain is predominantly at chromospheric, suggesting complete catastrophic cooling, while not much difference is observed in the dynamics over the temperature range. A small subset of loops also exhibit long-period intensity pulsations. We discuss the spatio-temporal properties of the heating linked to this cooling behaviour.

Speaker: Seray Sahin (Northumbria University)
• 67
Microwave observations of coronal jets

We discuss the diagnostics of plasma jets in the solar corona from multiwavelength imaging observations in the microwave band. We present several events observed with RATAN-600, Siberian Radioheliograph, and Nobeyama Radioheliograph. Obtained data provide us with spatially resolved imaging information alongside microwave flux observations at several frequencies. To analyze the context information on the three-dimensional structure of the coronal magnetic field, we reconstruct the magnetic field in the lower corona from the SDO/HMI magnetograms and compare it with the magnetic field at the base of the corona derived from the RATAN-600 data. In this work we demonstrate that microwave observations of the events associated with coronal jets can (1) give insights into jet dynamics and excitation mechanisms and (2) provide important information on physical conditions in the corona of an active region where a jet is initiated and developed.
This work is supported by the Russian Foundation of Basic Research grant 18-29-21016.

Speaker: Sergey Anfinogentov (Institute of Solar-Terrestrial Physics SB RAS)
• 68
Turbulence driven by phase-mixed torsional Alfvén waves in nonuniform coronal loops

High resolution observations have shown the ubiquity of Alfvénic waves in the solar atmosphere. Recently, torsional Alfvén waves in coronal flux tubes have been first detected. Here, we perform numerical simulations of torsional Alfvén waves and study their nonlinear evolution. We consider a cylindrical, radially inhomogeneous, and straight coronal magnetic flux tube that is line-tied at two rigid walls representing the solar photosphere. Standing torsional Alfvén waves are excited by perturbing the azimuthal component of the velocity. The nonlinear evolution of these waves is obtained with the PLUTO code, which solves the ideal MHD equations using a finite-volume formulation with adaptive mesh refinement. Initially, torsional Alfvén waves undergo the process of phase mixing owing to the transverse variation of density, generating small scales across the magnetic field direction. After only few periods of torsional waves, azimuthal shear flows trigger the Kelvin Helmholtz instability (KHi), and the flux tube is subsequently driven to a turbulent state. Turbulence is anisotropic and develops transversely only to the background magnetic field. After the onset of turbulence, the effective Reynolds number decreases in the flux tube much faster than in the initial linear stage governed by phase mixing alone. We conclude that the nonlinear evolution of torsional Alfvén waves, and the associated KHi, is a viable mechanism for the onset of turbulence in coronal loops. Turbulence can significantly speed up the generation of small scales previously initiated by phase mixing.

Speaker: Sergio Díaz Suárez (University of the Balearic Islands)
• 69
Observational Detection of Drift Velocity between Ionized and Neutral Species in Solar Prominences

It has been stated for a long time that the solar atmospheric plasma is not in a neutral state nor in a fully ionized state. The solar plasma is composed of different species, and it can be considered that each of them behaves like a fluid interacting with the rest of the species via collisions. When the collisional coupling is strong, the plasma mostly behaves as a single fluid. If this coupling weakens in certain processes, there might be deviations between the dynamical and thermal properties of the different species. To detect ion-neutral effects, it is necessary to measure as accurately as possible the velocity of different species at the same spatial position and simultaneously. Our aim is the detection of non-ideal ion-neutral effects in the solar plasma. The best candidate targets for this goal are spicules, surges, and the lower part of prominences. For this study, we observed a quiet Sun solar prominence at the east limb in June 2017 with the German Vacuum Tower Telescope. We acquired simultaneous spectra of the Ca II 8542 Å, Hα 6562.8 Å, and He D3 5875.6 Å lines. The spectroscopic mode of the spectrograph was chosen to ensure the high cadence and signal to noise needed to detect these effects and shed light on the scientific questions proposed.

Speaker: Sergio Javier González Manrique (Instituto de Astrofísica de Canarias)
• 70
How wrong are the results of inverting Fe I lines when NLTE and 3D radiative transfer effects are ignored?

The Fe I 6301.5 Å and 6302.5 Å lines are widely used to probe the solar photosphere. They are known to be affected by the non-local thermodynamic equilibrium (NLTE) conditions due to the ultraviolet overionisation of iron atoms in the solar atmosphere. This leads to deviations in their level populations based on Saha-Boltzmann statistics. When inverting their Stokes profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE. In this work, we discuss how the routinely employed LTE inversion introduces errors in the derived atmospheric quantities. We show that when the NLTE effects are neglected, these errors can be as high as 13% in temperature, and in line-of-sight velocity and magnetic field strength the errors can even exceed 50%. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region with strong vertical gradients in the atmosphere. Similarly, strong horizontal gradients in temperature introduces 3D effects in these lines. We find that errors due to neglecting the 3D effects are more localised and are lower than 5% in temperature, and lower than 20% in both velocity and magnetic field strength. The NLTE and 3D effects are found to persist when the Stokes profiles are spatially and spectrally degraded to the resolution of the SST or DKIST. Our findings have wide-ranging consequences since many results derived in solar physics are based on inversions of these Fe I lines carried out in LTE.

Speaker: Dr H N Smitha (Max Planck Institute for Solar System Resesarch)
• Poster Session 2.4
Convener: Francesca Zuccarello (Istituto Nazionale di Astrofisica (INAF))
• 71
AIDA: Modelling and Machine Learning representation of kinetic processes in the solar wind

Many processes in the solar atmosphere and in the solar wind require a kinetic description. Additionally the latest missions, SolO and PSP, provide new unique view of kinetic processes. We report here the approach of the Horizon 2020 project AIDA (www.aida-space.eu): using a combination of massively parallel kinetic particle in cell simulations and machine learning data mining. The study of kinetic processes provide large data sets: for example a massively parallel PIC simulation of the solar wind produces several TB of data each time step. Space and solar missions provide increasingly massive dat sets as well. Human investigation or even traditional automatic methods struggle to deal with this size and complexity. Recently machine learning (ML) has become the focus of concentrated attention in our community. AIDA is an example of a community-building project to address practical problems in this area.
With the tools of AIDA we provide methods to study with compatible integrated methods, based on python, both results from observations and from simulation.
We will present the methods developed by AIDA, focusing on machine learning (ML) methods to analyse particle distributions, to analyse time series, and too analyse images (including higher dimensional data sets). We will show both supervised and unsupervised methods to identify extreme events such as reconnection. We will show the application to both simulation results and observational data.

Speaker: Giovanni Lapenta
• 72
Ionisation and recombination effects on current sheet dynamics in chromospheric partially ionised plasmas

Fast magnetic reconnection plays a fundamental role in driving explosive dynamics and heating in the solar chromosphere. The reconnection time scale of traditional models is shortened by the onset of the coalescence instability, which forms a turbulent reconnecting current sheet through plasmoid interaction. Non-equilibrium ionisation–recombination processes can significantly alter the time scale of magnetic reconnection by changing the plasma composition, therefore it is essential to evaluate their contribution in the development of plasmoid coalescence. In this talk I investigate the role of ionisation and recombination in the development of fast magnetic reconnection in a partially ionised plasma through the study of the coalescence instability of plasmoids. Unlike the processes occurring in fully ionised coronal plasmas, relatively little is known about how fast reconnection develops in partially ionised plasmas (PIPs) of the chromosphere. I will present 1D and 2.5D preliminary simulations of a two-fluid model of a partially ionised plasma (PIP) and show how the dynamics change in the presence and absence of ionisation and recombination processes. In our 1D calculations, as the current sheet collapses, it drives a burst of ionisation. This results in the current of the current sheet growing at a slower rate than calculations without ionisation and recombination, and in a thicker current sheet. I will discuss the consequences of ionisation and recombination on chromospheric plasmas, based on our 2.5D simulations.

Speaker: Giulia Murtas (University of Exeter)
• 73
Coronal forbidden lines in the DKIST era

Coronal forbidden lines in the visible and near infrared (NIR) provide a range of plasma diagnostics to probe the solar corona.
They have not been explored much, but this is changing, with several facilities coming into operation, primarily DKIST.
We briefly review the importance of accurate atomic rates and proper modelling for these forbidden lines.
We provide examples of new calculations for a few ions which are being made available via the CHIANTI database.
We then present the results of two eclipse observations, in 2017 and 2019, where we have combined NIR AIR-Spec observations with EUV observations by Hinode EIS. AIR-Spec is a pathway mission for DKIST: a NIR spectrometer designed to observe eclipses from an aircraft flying at an altitude of 14 km. We have obtained new measurements of temperatures, densities from line ratios, and elemental abundances. These plasma parameters are in overall agreement with our previous results. Most notably, that the quiet corona has a sulphur abundance close to the photospheric one.

Speaker: Giulio Del Zanna (University of Cambridge (UK))
• 74
Study of the striae in the tail of Comet Lovejoy as a diagnostic tool for coronal density

The passage of comets in the solar corona can be a powerful tool to probe the local plasma properties. One such case is offered by Comet Lovejoy, which gives us the possibility to infer the coronal plasma density along the magnetic field lines intersected by the comet trajectory, thanks to the emission by cometary ions injected along the magnetic field, forming so-called striae.

Here, we carry out a preliminary study of these striae, as observed by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) during the ingress phase of the comet orbit. We consider the images in the 171 Å line, where emission from oxygen ions O$^{4+}$ and O$^{5+}$ is found.

The striae are described as due to the beam of oxygen ions along the local magnetic field, with the injection beam velocity decaying because of collisions. Also, ion collisional diffusion contributes to ion propagation. Both the collision time for velocity decay and the diffusion coefficient depend on the ambient plasma density so that measurements of the striae length can be related to the local density. A probabilistic description of the ion beam density along the magnetic field is developed, where the beam position is given by the velocity decay and the spreading of diffusing ions is described by a Gaussian probability distribution. Profiles of emission intensity along the magnetic field are computed and compared with the profiles along the striae observed by AIA, in order to estimate the ambient plasma density.

Speaker: Dr Giuseppe Nisticò (Università della Calabria)
• 75
Source regions of type II radio emission on the surface of an interplanetary shock wave

Shock waves driven by Coronal mass ejections (CMEs) can accelerate particles. Radio signatures of electron beams accelerated at the shock front (type II bursts) are produced when the shock wave propagates through the solar atmosphere. Ground-based radio observations allow us to study shock waves in the low corona while space-based radio observations provide us the opportunity to track shock waves in the inner heliosphere.
The preferable location of the type II radio sources on the surface of the shock wave has been a long-discussed question. We address this question in the study of a shock wave associated with a flare/CME event and complex radio event on September 27, 2012. We employ a novel approach, combining the results from the radio triangulation analysis with shock wave modelling.
We first reconstruct the shock wave in 3D space using multi-viewpoint white light observations by SDO and STEREO spacecraft, and then model the evolution of the shock wave using a 3D MHD background coronal model produced by the MAS (Magnetohydrodynamics Around a Sphere Thermodynamic) model.
We first analyze the global evolution of the wave parameters and then localize the areas which could be the source regions of radio emission. We study the temporal evolution of the upstream plasma characteristics and the shock wave parameters. Our results indicate a complex relationship between the different shock wave parameters. However, the shock wave geometry and its relationship with the shock strength seem to play the most important role in the generation of type II radio emission.

Speaker: Immanuel Christopher Jebaraj (Royal Observatory of Belgium)
• 76
Precise Calcium Abundance Determinations for 207 Flare Decays observed by the BCS Channel 1 aboard SMM

The Bent Crystal Spectrometer (BCS) observed high resolution spectra of flares between 1980 and 1989. The instument's channel 1 recorded the triplet of lines arising from He-like Ca (Ca XIX) together with several satellite lines formed from doubly excited states of the Ca XVIII ion. Portions of the continuum emitted by flares were also observed, free from instrumental effects (crystal fluorescence). A collimator in front of the BCS allowed only emission from particular active regions, so preventing overlapping of spectra from multiple active regions. The SMM BCS spectra are still the best ever recorded for flaring hot plasmas. Early analysis of these Ca spectra revealed changing abundances of Ca between flares for ~146 events observed early in the Mission. In the present study, we extended this study for the spectra obtained over the entire SMM Mission duration using updated instrument response data. We determined/redetermined the absolute (relative to hydrogen) abundances for 207 flares during their decay phases with a precision of 1 to 3 % for some flares, the best abundance determinations for any coronal sources. We describe the new data reduction process and compare derived flare abundances with other selected flare characteristics as well as average calcium abundances determined from other measurements.

Speaker: Janusz Sylwester (Space Research Centre, Polish Academy of Sciences)
• 77
Type III radio bursts observed on April 03, 2019

During April, 2019, several groups of type III radio bursts were observed, starting from the metric wavelength range (ground based observations) and continuing in the hectometric to kilometric range (space based observations). A majority of the type III bursts was observed by LOFAR (Low Frequency Array), by radio instruments on Stereo A and Wind spacecraft, and also by the Parker Solar Probe (PSP).
We focus this study on a small group of type III bursts observed on April 03, in the time window from 16:40 UT until 17:00 UT. The aim of the study is to map the propagation of the type III bursts and compare the obtained plasma densities with the density profiles provided by EUHFORIA (EUropean Heliospheric FORecasting Information Asset) model, and with in-situ observations from PSP which was located in a close proximity. First results indicate that the type III bursts do not follow, as generally considered, Parker spiral but they propagate strongly southward from their source region. It is possible that this unusual propagation path is induced by the weak CME that preceded the radio bursts and disturbed the ambient solar conditions.

Speaker: Dr Jasmina Magdalenic (Royal Observatory of Belgium & KU Leuven)
• Poster Session 2.5
Convener: Alessandro Bemporad (Istituto Nazionale di Astrofisica (INAF))
• 78
Type III solar radio event observations on 22 August 2017 with LOFAR telescope

We hereby present the interferometric LOFAR observations of the solar radio event on 22 August 2017, during which the type III radio bursts have been detected. Solar radio image and dynamic spectra were recorded in the 10 – 90 MHz frequency band. Additionally to LOFAR observations, the data recorded by instruments onboard the Solar Dynamics Observatory (SDO) in the UV spectral range, GOES and RHESSI in X-ray spectral range complement observations in the radio field. Our study shows that the interferometric LOFAR observations, in combination with observations at other wavelengths can give better understanding of the environment in which the type III radio events occur.

Speaker: Bartosz Dabrowski
• 79
How connected is the solar atmosphere

The atmospheric conditions of the Sun are encoded within the line emission of different atomic transitions (spectra). To infer the physics of the solar atmosphere from spectra, we have to compare these observations with synthetic outputs from simulations. It is however often the case that the thermodynamic solutions satisfying an observation are degenerate, with different variations of vertically stratified temperatures, densities, and velocity fields leading to the same synthetic output. It is therefore desirable to restrain the solution space of our simulations by analyzing multiple spectral lines simultaneously. We use machine learning methods in combination with observations from the Interface Region Imaging Spectrograph (IRIS), to calculate the entire set of possible spectral responses over all spectral windows during a solar flare giving a single fixed Mg II spectrum. The results provide us with an automatic way of analyzing the Sun through a multithermal lens and provide us with a rich set of constraints for simulations and interpretations. This method helps shed light on the emission found at the edge of flare ribbons, showing central reversals in chromospheric and transition region lines, as well as deep atmospheric heating as indicated by enhanced Fe II emission.

Speaker: Dr Brandon Panos (University of Geneva)
• 80
Flare QPPs in AB Dor: I. Identical scaling laws to solar and stellar flares and clues of magnetoacoustic waves

AB Dor is an active rapidly rotating K0 dwarf which rotates $\sim$ 50 times faster than the Sun. We have studied flare light curves from AB Dor using the archival observations from the XMM-Newton satellite and detected quasi-periodic pulsations (QPPs). Periods and damping times of QPPs are derived and compared to those for solar and other stellar flares. These parameters are generally found to be larger than those from the solar and other stellar QPPs. Kolmogorov-Smirnov (K-S) test reveals that the QPPs found in the post-flare light curves of AB Dor are similar to the previously reported solar and stellar QPPs. The scaling law of damping period with oscillation time of flare generated QPPs interestingly exhibit the same nature as observed in the flares at the Sun and other magnetically active solar-type stars. This implies that QPPs at AB Dor may have a similar origin as in the Sun and other active stars. Thus the physical origin can be attributed due to the magnetoacoustic modes. To the best of our knowledge, the present findings are the first evidence of the wave-generated QPPs observed at AB Dor, and we discuss its physical implications in its stellar coronae.

Speaker: Chandrashekhar Kalugodu
• 81
LOFAR observations of fundamental and harmonic type III emission during an M class flare

We present observations of an M class flare with the LOw Frequency ARray (LOFAR) in the moring hours of 7 September 2017. The flare was accompanied by strong type III radio bursts. LOFAR interferometric images in the low band frequency range of 20 - 80 MHz show distinct sources that show variations in their positions, and intermittent dual source structures. We identify these as fundamental and harmonic emission, with the one or other being dominant at times. These distinct sources and their evolution allow for obtaining separate lightcurves for both fundamental and harmonic emission.
The data show that transport effects due to refraction and scattering play a significant role, both in source separation and drift of their apparent positions. Comparing the light curves of fundamental and harmonic pairs, e.g. 35 MHz fundamental and 70 MHz harmonic, enables studies of radio wave propagation in the solar corona. Observations of harmonic emission can provide information on source locations high in the corona, where fundamental emission would be near or below the ionospheric cutoff at 10 MHz. These are relevant for the transition into the solar wind, and for joint observing campaigns with Parker Solar Probe and Solar Orbiter that are currently investigating the inner heliosphere.

Speaker: Christian Vocks (Leibniz-Institute for Astrophysics Potsdam)
• 82
LOFAR observations of a jet-driven piston shock in the low solar corona

The Sun produces highly dynamic and eruptive events that can drive shocks through the corona. These shocks can accelerate electrons, which result in plasma emission in the form of a type II radio burst. Despite the large number of type II radio bursts observations, the precise origin of coronal shocks is still subject to investigation. Here we present a well observed solar eruptive event that occurred on 16 October 2015, focusing on a jet observed in the extreme ultraviolet (EUV) by the Atmospheric Imaging Assembly (SDO/AIA), a streamer observed in white-light by the Large Angle and Spectrometric Coronagraph (SOHO/LASCO), and a metric type II radio burst observed by the LOw Frequency Array (LOFAR). LOFAR interferometrically imaged the fundamental and harmonic sources of a type II radio burst and revealed that the sources did not appear to be co-spatial, as would be expected from the plasma emission mechanism. We correct for the separation between the fundamental and harmonic using a model which accounts for scattering of radio waves by electron density fluctuations in a turbulent plasma. This allows us to show the type II radio sources were located ∼0.5 R⊙ above the jet and propagated at a speed of ∼1000 kms−1, which was significantly faster than the jet speed of ∼200 kms−1. This suggests that the type II burst was generated by a piston shock driven by the jet in the low corona.

Speaker: Ciara Maguire (Trinity College Dublin)
• 83
First Time and Frequency Resolved Imaging Spectroscopy Observations of Solar Radio Spikes

Solar radio spikes are short duration and narrow bandwidth fine structures in dynamic spectra observed from tens of MHz to the GHz range. Their very short duration (10-1000 s) and narrowband emission is indicative of the possibly fastest small-scale energy release processes observed in the solar corona, yet the origin of the spikes is not established. We present, for the first time, spatially, frequency and time resolved observations of individual radio spikes associated with an eruptive solar flare-CME event using the LOw Frequency ARray (LOFAR). The spikes present low frequency drift rates, yet imaging spectroscopy of individual spikes between 30-45 MHz show apparent superluminal spike source motions. Comparison of spike characteristics with that of individual Type IIIb striae show similarities in duration, bandwidth, drift rate, and apparent areal increase, as well as the apparent motion in the image plane. The observed spatial, spectral, and temporal properties of the spike bursts are consistent with plasma radio emission escaping through anisotropic scattering density turbulence that induces the shift in the apparent source location over time.

Speaker: Daniel Clarkson (University of Glasgow)
• 84
Flux tubes and energetic particles in Parker Solar Probe orbit 5: magnetic helicity--PVI method and IS$\odot$IS observations

Plasma turbulence can be viewed as a magnetic landscape populated by large- and small-scale coherent structures, consisting notionally of magnetic flux tubes and their boundaries. Such structures exist over a wide range of scales and exhibit diverse morphology and plasma properties. Interactions of particles with turbulence may involve temporary trapping in, as well as exclusion from, certain regions of space; generally controlled by the topology and connectivity of the magnetic field. In some cases, such as SEP dropouts'', the influence of the magnetic structure is dramatic; in other cases, it is more subtle, as in edge effects in SEP confinement. With Parker Solar Probe now closer to the sun than any previous mission, novel opportunities are available for examination of the relationship between magnetic flux structures and energetic particle populations.

We present a synergistic use of the magnetic helicity-partial variance of increments (PVI) technique paired with IS$\odot$IS observations of energetic particles (EPs). The filtered magnetic helicity detects large-scale helical flux tubes while the PVI identifies their boundaries, thus giving a multi-scale finer description of the structures. The correlation with EP fluxes show direct observational evidence of the modulation provided by flux tubes.

We find that helical flux tubes, accompanied by bounding large-PVI patches, act as hard boundaries that inhibit EP transport either confining populations within the helical field, or excluding them from entering it.

Speaker: Francesco Pecora (Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA)
• Poster Session 2.6
Convener: Istvan Ballai (University of Sheffield)
• 85
How solar accelerated electron beams vary as a function of distance from the Sun

Solar electrons beams are accelerated in the corona, into the solar wind and beyond in the Solar system through eruptive mechanisms at the surface of the Sun such as flares. These beams of non-thermal electrons evolve as a function of distance from the Sun, interacting with the background plasma and growing Langmuir waves as they propagate. Subsequent radio and X-ray wavelength emission is also seen. Insight into electron beam transport effects allows us to disentangle them from the acceleration properties and provides a framework for using in situ measurements to diagnose coronal acceleration characteristics. Through beam-plasma structure simulations we study the interactions between these electron beams and the background plasma of the solar corona and beyond at different distances from the Sun. This allows us to determine what is the maximum electron velocity responsible for Langmuir wave production and growth, and consequently which electron energies these correspond to. Understanding the mechanisms driving the change in the maximum electron velocity will permit more accurate predictions in electron onset as well as arrival times, relevant for space weather applications and the understanding of the subsequent emissions at radio and X-ray wavelength. Moreover, our radial predictions can be tested against in-situ electron and plasma measurements from the instruments on-board the Solar Orbiter and Parker Solar Probe spacecrafts.

Speaker: Camille Lorfing (University College London)
• 86
Analysis of the solar wind distribution functions at 1AU.

This contribution focus on the analysis of the distribution function of different physical magnitudes of the solar wind. The data set comes from the ACE spacecraft located at the L1 point and covers almost two solar cycles, from 1998 to 2017.
We propose a bi-Gaussian distribution, formed by the addition of two Gaussian distribution functions, to characterize the solar wind. Each Gaussian function corresponds to the contribution of one of the different regimes of the bulk solar wind: fast and slow wind.
We apply this approach not only to the proton speed, but also for the interplanetary magnetic field magnitude, proton temperature and proton density, and we obtain a bimodal distribution which allow us to characterize each regime of the bulk solar wind.
Our results also show a clear correlation between some fitting parameters and the solar cycle.
We also apply the Bi-gaussian approach to the solar wind composition, more precisely to the average iron charge state. From these results we are able to identify interplanetary coronal mass ejections which were not included in previous catalogues.

Speaker: Mr Carlos Larrodera (University of Alcalá)
• 87
A dedicated high-energy particle environmental study for the Solar Orbiter coronagraph Metis

High-energy particles penetrate spacecraft and to a different extent affect the performance of instruments flown on board space missions. Consequently, spacecraft carry particle detectors for both diagnostics and scientific purposes that allow to monitor the galactic cosmic-ray and solar energetic particle fluxes during the whole mission elapsed time. These data allow us to disentangle long and short-term variations of the GCR flux associated with the solar activity, the passage of interplanetary counterparts of coronal mass ejections and high speed solar wind streams. Monte Carlo simulations of the GCR tracks observed in the Metis coronagraph visible light images have shown that the background of spurious pixels fired by cosmic rays is only a fraction of the order of 10-4 of the whole pixel sample and therefore they do not affect sensibly the quality of the Metis images. Moreover, the comparison of the simulations with the cosmic-ray matrices obtained from the Metis VL light images mainly consisting of primary and secondary particles generated by primary protons indicate that Metis may play the role of a detector to study the variations of GCRs and SEPs during the next decade.

Speaker: Catia Grimani (University of Urbino Carlo Bo and INFN Florence)
• 88
Propagation, in-situ signatures and geoeffectiveness of consecutive solar eruptions simulated in different solar wind conditions

Coronal Mass Ejections (CMEs) are some of the most energetic solar events that expel plasma and magnetic field into the interplanetary medium. Stealth CMEs represent a special type of solar eruptions that, in most cases, can be clearly seen in coronagraph observations, but lack distinct source signatures. We simulate consecutive CMEs ejected from the southernmost part of an initial configuration constituted by three magnetic arcades embedded in a globally bipolar magnetic field. The first eruption is driven through shearing motions at the solar surface. The following eruption is either a stealth blob-like CME, resulting from the reconnection of the coronal magnetic field, or another shearing driven flux rope. All CMEs are expelled into a bimodal solar wind.
We analyse the parameters that contribute to the occurrence of the second CME, as well as its influence onto the first eruption during the propagation to 1AU by simulating also a single erupting flux rope.
We track the two double-CME cases until Mercury and 1AU, and compare their simulated signatures with the in-situ data of a similar multiple CME event that occurred between 21-22 Sept. 2009, obtaining a good correlation.
Furthermore, we impose the same shearing speeds along the polarity inversion line of the southern arcade, but immersed into a faster solar wind, to analyze the effect of the overall magnetic structure and of the wind onto the resulting eruptions, propagation and geoeffectiveness. The latter is studied via the Dst index, computed using an empirical model from the simulated parameters of the ICMEs.

Speaker: Dana-Camelia Talpeanu (Departments of Mathematics, KU Leuven; SIDC, Royal Observatory of Belgium)
• 89
Metis first light Doppler dimming analysis

The observations of the resonantly scattered ultraviolet emission of the coronal plasma obtained with UVCS-SOHO, properly designed to measure the wind outflow speed by applying the Doppler dimming diagnostics, noticeably fostered the investigation of the wind in the inner solar corona. Metis on Solar Orbiter complements the UVCS spectroscopic observations, by simultaneously imaging the polarized visible light and the \ion{H}{i}~Lyman-$\alpha$ corona in order to obtain high spatial and temporal resolution maps of the outward velocity of the continuously expanding solar atmosphere. The polarized visible light (580-640~nm), and the ultraviolet \ion{H}{i}~Ly$\alpha$ (121.6~nm) coronal emissions, obtained in the two Metis channels, are combined in order to measure the Doppler dimming of the UV emission, relative to a static corona. The plasma outflow velocity is then derived as a function of the measured Doppler dimming. The Metis first light observations, obtained on May 15, 2020, near the end of the commissioning phase of the Solar Orbiter mission, provide the first instantaneous image of the speed of the plasma outflows in the corona during the minimum of solar activity, from 4~$R_\odot$ to 6~$R_\odot$, and allow us to identify the layers where the fast and slow wind flow is observed.

Speaker: Dr Daniele Spadaro (Istituto Nazionale di Astrofisica (INAF), OACt, Italy)
• 90
Active Region Contributions to the Solar Wind Over Multiple Solar Cycles

Both coronal holes and active regions are source regions of the solar wind. The distribution of these coronal structures across both space and time is well known, but it is unclear how much each source contributes to the solar wind.

We have used photospheric magnetic field maps from the past four solar cycles to estimate what fraction of magnetic open solar flux is rooted in active regions, a proxy for the fraction of all solar wind originating in active regions. The fractional contribution of active regions to the solar wind varies between 30% to 80% at any one time during solar maximum and is negligible at solar minimum, showing a strong correlation with sunspot number. While active regions are typically confined to latitudes $\pm$30$^{\circ}$ in the corona, the solar wind they produce can reach latitudes up to $\pm$60$^{\circ}$. Their fractional contribution to the solar wind is also highly variable, changing by $\pm$20% on monthly timescales within individual solar maxima.

These results quantify the importance of active regions as solar wind sources, providing motivation for understanding active region solar wind heating and acceleration using Parker Solar Probe and Solar Orbiter during solar cycle 25.

Speaker: Dr David Stansby (MSSL/UCL)
• 91
Three-Dimensional Tomographic Reconstruction and MHD Modeling of the low corona during the last solar minimum

Interest in predicting space weather conditions constantly pushes the advance of state-of-the-art three-dimensional(3D) magnetohydrodynamic (MHD) models of the solar corona and the solar wind, which need to be validated with observational data. Tomography of the solar corona is the only observational technique that can currently provide empirical data of the solar atmosphere in a 3D global fashion. In this work we carry out a validation study of the latest version of the Alfvén Wave Solar atmosphere Model (AWSoM), comparing its results with tomographic reconstructions of physical parameters of the solar corona. For this anlysis, we select rotations from the recent deep minimum of solar activity, between solar cycles 24 and 25, which renews the opportunity to study the Sun-Earth connection under the simplest solar and space environment conditions. Based on narrow band EUV coronal images provided by the SDO/AIA instrument, we carry out tomographic 3D reconstruction of the coronal electron density and temperature in the range of heliocentric heights $r \le 1.25~{\rm R}_\odot$. Based on polarized visible brightness (pB) coronagraph images provided by the SoHO/LASCO-C2 instrument, we carry out tomographic 3D reconstruction of the coronal electron density in the range of heliocentric heights $\approx 2.5 - 6.0~{\rm R}_\odot$. We compare the MHD model with the tomographic reconstructions in their respective range of heights and discuss the current capability of the AWSoM model to reproduce the reconstructions in specific coronal magnetic structures.

Speaker: Mr Diego Lloveras (Instituto de Astronomía y Física del Espacio (IAFE), Univ. of Buenos Aires - CONICET)
• Tuesday, September 7
• Poster Session 3.1
Convener: Etienne Pariat (LESIA, Observatoire de Paris)
• 92
Parameter study of 2D convection and supercomputer performance analysis

We perform hydrodynamic 2D simulations of solar convection. We aim to study the dependence of different physical parameters, the box size, and box aspect ratio on the actual convective motions we observe in the numerical model. Furthermore, we need to evaluate the performance of different Input/Output strategies on supercomputers, in order to plan and run future large-scale magneto-hydrodynamic models of the solar atmosphere. To this end, we use our convection model for a performance analysis of different numerical IO schemes. We use a large simulation domain together with a high grid resolution in order to provide high Reynolds numbers in the numerical model. This allows us to model the solar convection zone down to 20 Mm below the surface and include the solar atmosphere up to 10 Mm. We use 1024x256 grid points with initial conditions matching the gravity, temperature, and density stratification of the Sun. After the bottom of the box is heated sufficiently, the convective motions set in and transports energy towards the surface. At the top of the convection zone, material is ejected into the atmosphere, before falling down again.
We conclude that we require a minimum horizontal extent of the simulation domain and a minimum box aspect ratio, in order to obtain realistic convection cells. Also we find that HDF5 output is required for future large-scale computer models, because the traditional IO strategy of the Pencil Code puts too high demands on nowadays supercomputer file systems, both in storage space requirements and number of file operations.

Speaker: Johannes Tschernitz (University of Graz)
• 93
IRIS view of the solar cycle 24: variability of the Mg II k & h lines

The poster presents observations of variability of the emission cores of Mg II k & h lines in the solar cycle 24 and their center-to-limb variations acquired by the Interface Region Imaging Spectrograph (IRIS) in the years 2013-2020. We use 91 full-disk NUV IRIS mosaics covering the raising phase, maximum, decline, and the end of the solar cycle 24. For this analysis, the solar disk is divided into ten concentric equiareal zones. Data averaged over the zones represent the center-to-limb variation of the Mg II k & h line cores. The time series of IRIS spectral irradiance correlates well with the UV indices (Bremen Composite Magnesium II Index and the Composite Solar Lyman-alpha Index) with the correlation coefficients of 0.90 - 0.94, thus verifying the long-term stability of IRIS radiometric calibration. The Mg II k & h emission cores show significant center-to-limb variation. Their wavelength-integrated intensities decrease toward the limb suggesting overall limb darkening. However, broadening of the emission cores toward the limb invokes the limb brightening in narrow wavelength intervals where the intensities are higher than in the disk center. The time series of calibrated full-disk IRIS mosaics allows also to answer a question whether the quiet areas of the solar atmosphere change over the solar cycle 24 at chromospheric layers.

Speaker: Dr Julius Koza (Astronomical Institute, Slovak Academy of Sciences, Tatranska Lomnica, Slovakia)
• 94
Asymmetry of weak photospheric fields in solar cycles 21-24: Implications to magnetic field structure and diffusion

Weak magnetic field elements make a dominant contribution to the total magnetic field on the solar surface. Still, quite little is known of their properties and long-term occurrence.
We study the spatial-temporal evolution of the weak-field shift and skewness of the distribution of photospheric magnetic field values during solar cycles 21-24. We use WSO and SOLIS/VSM synoptic maps to construct butterfly diagrams for weak-field shifts and skewness.
Weak-field shift and full-field skewness depict a spatial-temporal solar cycle evolution, which is closely similar to that of the large-scale surface magnetic field. Magnetic field distribution has a systematic non-zero weak-field shift and a large skewness already at the emergence of active regions. We find evidence for coalescence of opposite-polarity fields during the surge evolution. This is clearly most effective at the supergranulation scale. However, a similar dependence of magnetic field coalescence on spatial resolution was not found in polar coronal holes.
Our results give evidence for the preference of even the weakest field elements toward prevailing magnetic polarity since the emergence of active regions, and for a systematic coalescence of stronger magnetic fields of opposite polarities to produce weak fields during surge evolution and at the poles. Our results suggest that supergranulation process is reduced or inhibited in polar coronal holes. This result is further supported, e.g., by the fact that the spectral index of diffusion of coronal bright points is larger in the polar coronal hole than at other latitudes, indicating that diffusion is less limited in the polar coronal hole.

Speaker: Prof. Kalevi Mursula (University of Oulu)
• 95
Modeling star spots on low mass stars observed by TESS

Since the release of the first TESS sector the possibility of examining stellar flares and stellar dynamo compared to the solar dynamo and solar flares has increased. Thanks to this observations we tried to estimate staining of low mass stars with visible variability of their luminosity. We managed to do this using newly created software called BASSMAN (Best rAndom StarSpots Model calculAtioN) that allows to estimate amount of spots on star and parameters of the estimated spots. Trying to recreate spots on star can help examine more deeply how inner structure of the star can be connected to staining of star, how the spots evolve or appear on the star or how spots are related to flares. Here I will present results of modelling of star spots for 2 stars with our new tool and compare the results with the previous reconstructions of the spatial distribution of spots.

Speaker: Mr Kamil Bicz (Astronomical Institute of the University of Wrocław)
• 96
Status of the Galileo Solar Space Telescope Mission

Here we present the concept feasibility study of the Galileo Solar Space Telescope Mission (GSST Mission) proposed by the Brazilian National Institute for Space Research (INPE). The study was conducted at the Space Missions Integrated Design Center (CPRIME - Centro de Projeto Integrado de Missões Espaciais). The GSST shall contribute to the understanding of the evolution of the magnetic structure of the Sun and its influence on the Earth's space environment. The requirements proposed for the mission include high spatial and temporal resolution observations. Those measurements involve observations of the photosphere and outer layers of the solar atmosphere, observations of the variability of TSI, and in situ observations. The study included: (a) the definition of the scientific objectives; (b) the identification of the system drivers; (c) the definition of the candidate solutions for the system; (d) the conceptual design of the mission’s architecture components, including the optical payloads; (e) the pointing accuracy analysis of the designed attitude control subsystem; (f) the simulation and verification of the mission operational concept; (g) the assessment of the ground segment required to fulfill the mission; (h) estimate of the schedule for the development of the mission; and (i) the risk analysis. The optical payload architecture, orbit, and ground segment were identified as the main system drivers. The concept of two full disk telescopes and one high-resolution telescope for visible and ultraviolet spectropolarimetric observations have been the basis for the solution of the optical payload architecture selected for scientific purposes. INPE's GSST Mission Working Group.

Speaker: Luis Eduardo Vieira (National Institute for Space Research (INPE/Brazil))
• 97
Modeling the evolution of LOS magnetograms of emerging solar active regions.

The presence of elongated magnetic polarities in active-region (AR) line-of-sight (LOS) magnetograms indicates the existence of twist in the flux tubes forming them. These elongations, a.k.a. magnetic tongues, which are mostly visible during the emergence phase of ARs, determine the magnetic field distribution observed in LOS magnetograms, and therefore, affect the measurement of AR characteristics such as their tilt angles. Obtaining a good estimation of tilt angle evolutions and spatial variations plays a key role in constraining flux-transport dynamo models, as Joy's law is fundamental for the formation and evolution of the polar field. In this work we aim to estimate the intrinsic geometrical properties of the twisted flux tubes, or flux ropes (FRs), that form ARs by comparing observed LOS magnetograms with synthetic magnetograms derived from a toroidal magnetic flux tube model. Analyzing a sequence of 66 magnetograms, corresponding to the emergence of NOAA AR 10268, we model the evolution of parameters such as the tilt angle, the length and cross-section size of the FR, and the emergence rate assuming a kinematic rise of the torus. Our method uses a probabilistic scheme based on the Bayes theorem to infer the most probable intrinsic parameters of the emerging flux tube, assuming a normal distribution for the differences between the model and the observations. We discuss the importance of the prior distribution for all the model parameters in order to avoid degeneracies of the optimal solution. We also propose a recursive method to constrain these priors directly from the observations.

Speaker: Mariano Poisson (Instituto de Astronomía y Física del Espaco (IAFE-UBA-CONICET))
• 98
The asymmetric solar rotation

Today’s picture of the internal solar rotation rate profile results essentially from helioseismic analyses of frequency splittings of resonant acoustic waves. This has the limitation that the rotation profile is an average over the northern and southern hemispheres.
Here we present another, complementary estimation of the internal solar rotation rate using the perturbation of the shape of the acoustic waves. For this purpose, we extend the global helioseismic approach developed previously for the investigation of the meridional flow to work on the
components of the differential rotation. We find that the
rotation rate profiles from the two different approaches are qualitatively in good agreement, while the new measurements provide the asymmetric rotation components in addition.

Speaker: Prof. Markus Roth (Leibniz-Institut für Sonnenphysik)
• Poster Session 3.2
Convener: D. Shaun Bloomfield (Northumbria University)
• 99
A Mechanism Driving Recurrent Eruptive Activity on the Sun

Active Regions (ARs) in their emergence phase are known to be more flare productive and eruptive than ARs in their decay phase. In this work, we focus on complex emerging ARs composed of multiple bipoles. Due to the compact clustering of the different emerging bipoles within such complex multipolar ARs, collision and shearing between opposite non-conjugated polarities produce “collisional polarity inversion lines” (cPILs) and drive rapid photospheric cancellation of magnetic flux. The strength and the duration of the collision, shearing, and cancellation are defined by the natural separation of the conjugated polarities during the emergence phase of each bipole in the AR. This mechanism is called “collisional shearing”. In Chintzoglou et al (2019), collisional shearing was demonstrated using two emerging flare- and CME-productive ARs (NOAA AR11158 and AR12017) by measuring significant amounts of magnetic flux canceling at the cPIL. This finding supported the formation and energization of magnetic flux ropes before their eruption as CMEs and the associated flare activity.

Here, we provide results from data-driven 3D modeling of the coronal magnetic field, capturing the recurrent formation and eruption of energized structures in support of the collisional shearing process. We discuss our results in relation to flare and eruptive activity.

Speaker: Dr Georgios Chintzoglou (Lockheed Martin Solar & Astrophysics Lab)
• 100
Disambiguation of vector magnetograms by stereoscopic observations from SDO/HMI and Solar Orbiter/PHI

Spectropolarimetric reconstructions of the photospheric vector magnetic field are intrinsically limited by the so-called 180$^\circ$ ambiguity in the orientation of the transverse component. The successful launch and operation of Solar Orbiter has made the removal of the 180$^\circ$-ambiguity possible using solely observations obtained from two different vantage points. While the exploitation of such a possibility is straightforward in principle, it is less so in practice and it is therefore important to assess the accuracy and limitations, as a function of both the satellites orbits and measurement principles. In this work we present a stereoscopic disambiguation method (SDM) and discuss a thorough testing of its accuracy in applications to modeled active regions and quiet Sun observations. The SDM is proven to to reach a 100% disambiguation accuracy when applied to moderately-to-well resolved fields. In such favourable conditions, the accuracy is almost independent of the satellites relative position, with the obvious exceptions of configurations where the satellites are within few degrees from co-alignment or quadrature. Even in the case of disambiguation of quiet Sun magnetograms with significant under-resolved scale, the SDM provides an accuracy between 82% and 98% depending on the field strength. Additionally, we provide an example of the expected accuracy as a function of time that can be used to optimally place remote-sensing windows during Solar Orbiter observation planning. Finally, a preliminary discussion of the effect of the viewing angle on the observed field as modeled by Solar Orbiter instrument simulations is presented.

Speaker: Gherardo Valori (MPS)
• 101
Analysis of Pseudo-Lyapunov Exponents of Solar Convection Using State-of-the-Art Observations

The solar photosphere and the outer layer of the Sun's interior are characterized by convective motions, which display a chaotic and turbulent character. In order to further investigate those motions, we estimated the pseudo-Lyapunov exponents of the overshooting convection described by current state-of-the-art observations of the Sun’s surface. In particular, we applied a method employed in the literature to estimate the pseudo-Lyapunov exponents, as well as another technique deduced from their definition, to the spectro-polarimetric data acquired with the ground-based Interferometric Bidimensional Spectrometer (IBIS) and Crisp Imaging SpectroPolarimeter (CRISP) instruments, and the space-borne Helioseismic and Magnetic Imager (HMI). Following previous studies in the literature, we computed maps of four quantities which were representative of the physical properties of solar plasma in each observation, and estimated the pseudo-Lyapunov exponents from the residuals between the values of the quantities computed at any point in the map and the mean of values over the whole map. We found that all the computed exponents hold negative values, which are typical of a dissipative regime, in contrast to previous results reported in the literature. We also found that the values of the estimated exponents increase with the spatial resolution of the data and are almost unaffected by small concentrations of magnetic field.

Speaker: Giorgio Viavattene (INAF - Osservatorio Astronomico di Roma)
• 102
Magnetically coupled atmosphere, the MHD waves transfer, and possible contribution to the outer atmosphere heating

We studied physical properties of the magnetic field above the sunspot which was observed on September 10, 2014 near the center of the solar disk during the time period 16:20-18:20 UT by the SDO/AIA/HMI and IRIS instruments. We detected a magnetic field flux tube above a sunspot umbra where vertical magnetic field lines formed a connection between the inner layers of the solar atmosphere and the solar corona. We studied a mechanism of MHD waves transfer from the photosphere without dissipation or reflection before reaching the corona. We also discuss a significance of such events in terms of the possible propagation and scattering of magnetic energy in the Sun's atmosphere, including solar corona. We can explain how the magnetic field flux tubes connecting the individual atmospheric layers can distribute the photospheric and chromospheric magnetic field energy across the active region. This mechanism can contribute to the coronal energy balance and improve our knowledge how the coronal heating is maintained.

Speaker: Dr Hana Meszarosova (Astronomical Institute AS, Czech Republic)
• 103
Observations and simulations of small-scale dynamic events with ALMA

Observations of the Sun with the Atacama Large Millimeter/sub-millimeter Array (ALMA) facilitate chromospheric studies at high spatial and temporal resolution.
We strive to evaluate observational data and determine the origin of the detected small-scale structures with the support of numerical simulations.
For this purpose, high-cadence Bifrost 3D simulations and radiative transfer calculations are used to construct brightness temperature maps at wavelengths corresponding to the spectral bands of ALMA.
A detailed study of shock waves in the simulation is made to characterise the corresponding signatures at mm-wavelengths.
Several hundred small-scale dynamic features are detected in the (~40 min) observation, which agrees well with shock wave signatures.
The ALMA bands effectively trace shock waves through the chromosphere. It is shown by degrading the resolution of the mm-maps, to what degree the dynamic signatures are expected to be observable, at different wavelengths and spatial resolutions. The specific spatial scales and contrasts, together with the size and shape of the effective resolution element, are very important for how the dynamic signatures are perceived.
In addition, the formation heights of the mm-wavelength radiation in connection to the small-scale dynamics is investigated and it is shown that the slope of brightness temperature within a receiver band can be used to give an estimation of the slope of the local gas temperature at the sampled layers and provides a better understanding of the underlying physical conditions of the dynamic features.

Speaker: Henrik Eklund (Rosseland Centre for Solar Physics, University of Oslo)
• 104
NuSTAR observations of a quiet Sun minifilament eruption

We present a unique set of observations of a confined minifilament eruption from the quiet-Sun during solar minimum. The Nuclear Spectroscopic Telescope Array (NuSTAR) spotted a tiny, compact hard X-ray (HXR) flare on 2019 April 26, peaking about 02:06UT lasting for a few minutes, finding brief emission >5MK. Observations with SDO/AIA and Hinode/XRT show this HXR emission was due to a tiny flare arcade underneath a confined minifilament eruption – behaviour similar to those seen in both major active-region filament eruptions and minifilament eruptions that lead to coronal jets. This eruption occurred near disk-centre, so the Earth orbiting observatories provide a top-down view of the event, but fortuitously a side-on view is obtained from STEREO-A/SECCHI, giving a clearer sense of eruption geometry. Line-of-sight magnetograms from SDO/HMI show that this eruption is due to opposite polarity flux moving together and cancelling and not due to flux emergence. We also explore the possibility of non-thermal emission due to accelerated electrons from the HXR observations of this tiny quiet Sun impulsive energy release.

Speaker: Iain Hannah (University of Glasgow)
• Poster Session 3.3
Convener: Stefaan Poedts (KU Leuven)
• 105
The Sun as a star: New insights from full-disk observations with ALMA

ALMA provides a new set of eyes to look at the stars including our Sun. In particular, the brightness temperatures provided by ALMA give insights about the thermal structure and activity of stellar atmospheres. The Sun, being the closest star, can be observed well resolved and thus be used as a reference case for solar-like stars. The overall aim of the presented study is to construct more robust solar/stellar activity indicators using ALMA observations in comparison with classical diagnostics.

Here, full disk solar maps from ALMA are compared with SDO-AIA and HMI maps and, with full disk H-alpha and Ca II maps to understand the correlation between them, which also provides constraints for the height range from where these mm emissions originate. The centre to limb variation in temperature observed for ALMA maps shows limb brightening which confirms the expectation that the radiation observed with ALMA originates from the chromosphere. In order to transfer the insights gained from solar ALMA observations to other stars, the full disk solar maps are converted into a corresponding stellar signal. Here we present the first results.

Speaker: Sneha Pandit (University of Oslo)
• 106
Study of Alfvénic vortex shedding past a cylindrical obstacle

In this paper, the phenomenon of vortex shedding around a circular cylindrical obstacle is studied numerically in magnetohydrodynamic (MHD) conditions in three spatial dimensions using the numerical code Lare3d. A parametric study was performed for different values of magnetic field perpendicular to the plasma flow plane. This model mimics coronal mass ejection flowing around a coronal loop, which is known as a probable mechanism for excitation of kink-mode oscillations in coronal loops. The phenomenon of vortex shedding has been widely studied in hydrodynamic conditions in both science and engineering, it has also been investigated by a number of numerical simulations in magnetic field environments, mainly in two dimensions. In MHD conditions, however, it is poorly understood.

Speaker: Sofya Belov (University of South Bohemia, Faculty of Science, Department of Physics)
• 107
Spectral inversion of simultaneous MSDP Hα and IRIS Mg II k & h lines in a quiescent prominence

Based on simultaneous spectroscopic measurements in the H$\alpha$ line and the Mg II k & h lines, Guiping et al. (2019) arrived at two solutions by comparing the observed and synthetic parameters of the H$\alpha$ line with 1D non-LTE modeling. They obtained relatively high temperatures at a microturbulent velocity of 8 km/s, while the temperatures at 16 km/s were standard values. Here we want to investigate the behavior of this prominence in more detail. We analyze both the H$\alpha$ line and the two Mg II lines detected by IRIS and compare the observed and synthetic profiles for all lines using five different spectral parameters. The analysis is based on spectral inversions using an extended grid of prominence 1D slab models. Preliminary results will be presented together with 2D maps of various physical parameters, and we will discuss our best-fit solutions with respect to the radiative-equilibrium models.

Speaker: Sonja Jejčič (University of Ljubljana, FMF)
• 108
On the multithermal nature of spicular downflows

Spectroscopic observations of the emission lines formed in the transition region (TR) commonly show persistent downflows of the order of 10--15 kms$^{-1}$. Their cause, however, is still not fully clear and has remained a matter of debate.

Using two sets of coordinated data from SST, IRIS, and SDO, we aim to understand the cause of such downflows by studying the coronal and TR responses to the recently reported chromospheric downflowing rapid red-shifted excursions (RREs), and their impact on solar atmospheric heating. To provide theoretical support, we use an already existing 2.5D magnetohydrodynamic simulation of spicules performed with the Bifrost code.

We show several examples of the spatio-temporal evolution of downflowing RREs across multiple channels, ranging from the cooler chromosphere to the hotter corona. Our analysis suggests that they are likely the returning components of the previously heated spicular plasma. Furthermore, the TR Doppler shifts associated with them are close to the average redshifts observed in this region, which implies that they could (partly) be responsible for the persistent downflows observed in the TR. We also propose two mechanisms (an upflow followed by a downflow and downflows along a loop), from the numerical simulation, that could explain the ubiquity of such downflows. A detailed comparison between the synthetic and observed spectra, reveals a distinctive match, and further suggests an impact on the heating of the solar atmosphere.

We present compelling evidence that suggests that many of the downflowing RREs are the chromospheric counterparts of the TR and lower coronal downflows.

Speaker: Souvik Bose (Rosseland Centre for Solar Physics)
• 109
Modelling Solar Ca II H & K Emission Variations

The emission in the near ultraviolet Ca II H & K lines, often quantified via the S-index, has been serving as a prime proxy of solar and stellar magnetic activity. Despite the broad usage of the S-index, the link between coverage of a stellar disk by magnetic features and Ca II H & K emission is not fully understood. In order to fill this gap we developed a physics-based model to calculate the solar S-index. To this end, we used the distributions of the solar magnetic features derived from simulations of magnetic flux emergence and surface transport, together with the Ca II H & K spectra synthesised using a non-LTE radiative transfer code.

We show that the solar S-index value is influenced by the inclination angle between the solar rotation axis and the observer’s line-of-sight, i.e. the solar S-index values obtained by an out-of-ecliptic observer are different from those obtained by an ecliptic-bound observer. This is important for comparing the magnetic activity of the Sun to other stars. We computed time series of the S-index as they would be observed at various inclinations dating back to 1700. We find that depending on the inclination and period of observations, the activity cycle in solar S-index can appear weaker or stronger than in stars with a solar-like level of magnetic activity. We show that there is nothing unusual about the solar chromospheric emission variations in the context of stars with near-solar magnetic activity.

Speaker: Sowmya Krishnamurthy (Max Planck Institute for Solar System Research)
• 110
Revised SDO/AIA Point Spread Functions to Correct for Long-Distance Scattered Light

The Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) is one of the most-used data sources in solar physics. Its point spread functions (PSF) were well calibrated by the instrument team, and allows high-quality observations of the solar corona at a resolution of ~1.5 arcsec for quiet Sun and active regions. However, it is suspected that low-emission features such as coronal holes still contain a significant contribution of diffusive, long-distance scattered light due to the microroughness of the mirrors, which is not accounted for by the PSFs provided by the instrument team.
We have developed a novel analytical technique that uses eclipse images and enables us to update the existing PSFs for diffusively scattered light. No preexisting knowledge on the shape of the missing part of the PSF is required. Applying this technique to the PSF provided by the instrument team, combined with analyzing 50 partial solar eclipses, shows that an additional 5% of the light is scattered farther away than 100 arcsec.
These missing 5% result in that the intensity of bright structures, such as active regions, is increased by an additional 5% as compared to AIA images deconvolved with the original PSFs. However, the intensity of dark structures, such as coronal holes, is decreased by about 40% as compared to AIA images deconvolved with the original PSFs. Our results demonstrate that for dark structures such as coronal holes, coronal dimmings, and filament channels, taking into account and correcting for the long-distance scattered light is essential.

Speaker: Stefan Hofmeister (stefan.hofmeister@columbia.edu)
• 111
Nanoflare distributions over solar cycle 23 based on AIA/SDO DEM analysis

We study the energy distributions of nanoflares in quiet Sun regions, using Differential Emission Measure (DEM) analysis of observations from the 6 EUV filters of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). In total, we analyzed 30 sets of AIA/SDO image series distributed evenly between the years 2011 and 2018 to characterize the nanoflare frequency distribution and their contribution to coronal heating throughout different levels of solar activity during solar cycle 23. Each series covers a $400''\times 400''$ field-of-view close to disc center over an observation time of two hours at the full AIA cadence of 12 seconds. DEM analysis was used to derive the emission measure and temperature evolution for each pixel. The nanoflare frequency distribution was then extracted from the DEM results for each data set by a threshold-based algorithm developed for the AIA data characteristics.
We find that the combined nanoflare frequencies follow a power-law distribution with a power-law index $\alpha = 2.3$ that covers five orders of magnitude in event energies ($10^{23}$ to $10^{28}$ erg). The power-law index obtained from individual data sets shows only minimal variation over the multi-year period and no significant correlation to solar activity. The steep slope ($\alpha>2$) suggests that the dominant energy contribution resides in the small flare events (nanoflares). However, we find that the combined energy flux from all the detected events is at least an order of magnitude too small to account for the heating requirement of the solar corona.

Speaker: Mr Stefan Purkhart
• Poster Session 3.4
Convener: Elena Khomenko (Instituto de Astrofisica de Canarias)
• 112
Energy distribution associated to 3D small scale magnetic reconnection in plasma turbulence

Understanding the energy dissipation in plasmas with low collisionality such as the solar wind is still a matter of ongoing research. While magnetic reconnection and turbulence are processes that can produce the appropriate conditions for heating and energy dissipation at subptroton scales, the energy distribution during 3D reconnection events that occur from a turbulent cascade is not entirely clear. To shed some light on this topic, we use an explicit fully kinetic particle-in-cell code to simulate 3D small scale magnetic reconnection events forming in anisotropic and Alfvénic decaying turbulence. We define a set of indicators to find reconnection sites in our simulation based on intensity thresholds. According to the application of these indicators, we identify the occurrence of reconnection events in the simulation domain and analyse one of these events in detail. The event involves two reconnecting flux ropes, and it is highly dynamic and asymmetric. We study the profiles of plasma and magnetic-field fluctuations recorded along artificial-spacecraft trajectories passing near and through the reconnection region as well as the energy exchange between particles and fields during this event. Our results suggest that the distribution of the internal energy is controlled by the region between the reconnecting flux ropes whereas the kinetic energy is associated to the inner part of the flux ropes.

Speaker: Jeffersson Andres Agudelo Rueda (University College London)
• 113
Deriving large coronal magnetic loop parameters using LOFAR J-burst observations

Fast-drift solar radio bursts' exciter velocities have been well studied by analysing type III and type J bursts. These bursts are produced by electron beams travel along open and closed coronal flux tubes. It is thought that electron beams travelling along different magnetic field structures have similar acceleration properties although it has not systematically been tested using radio bursts. By analysing a solar radio noise storm observed by the Low-frequency-array (LOFAR) on the 10th of April 2019, we identified 27 type-III and 27 type-J busts during a solar radio bursts and derived exciter velocities for all identified radio bursts from their frequency draft rates. The comparison shows their speeds are similar, inferring that electron beams experience a similar acceleration process in closed and open flux tubes during the same solar activity event.

We derived the ambient plasma density models that varied along the ascending leg of coronal loops and solar altitude by analysing selected 17 type J bursts. By estimating the density scale height, we inferred physical parameters of large coronal magnetic loops roughly 0.7 to 1.5 solar radii above the photosphere. Coronal loop tops had temperatures around 1.3 MK, pressures around 2.1 dynes/cm^2 and minimum magnetic field strength around 0.22 G. The physical parameters of such large loops are historically ill-defined.

Speaker: Jinge Zhang (University College London)
• 114
Torsional Alfvén pulses in zero-beta flux tubes

We investigate the generation of mass flux due to a torsional Alfvén pulse, as observed in e.g. photospheric magnetic bright points (MBPs). A flux tube model is developed, with the waves introduced at the lower, photospheric boundary of the tube as a magnetic shear perturbation. Due to the nature of MBPs we simplify the model by using the zero-beta approximation for the plasma inside the tube. We derive that the presence of torsional Alfvén waves can result in field-aligned plasma flux formed non-linearly due to the Lorentz force generated by the perturbations. Thus the model is consistent with jet formation observed in the lower solar atmosphere. The formation of the rising mass flux may even be a viable contribution to the generation of chromospheric mass transport, playing potential roles in the form of localised lower solar atmospheric jets. The analytical results are demonstrated by an example of the type of Alfvén wave perturbation that one might expect to observe, and comparison is made with properties of spicules known from observations.

Speaker: Mr Joe Scalisi (University of Sheffield)
• 115
Observing three-dimensional magnetic reconnection in solar flares

The latest expansions to the standard flare model in three dimensions (Aulanier & Dudík, 2019) led to identification of geometries in which field lines reconnect during eruptive flares. One of these geometries is the magnetic ‘ar-rf’ reconnection between the erupting flux rope and surrounding field lines. It occurs at ribbon hooks and leads to drift of flux rope footpoints across the solar surface. We report on observations of signatures of this reconnection geometry in multiple eruptive flares observed by the Atmospheric Imaging Assembly onboard SDO. Apart from drifting of CME footpoints, its presence is manifested e.g., in the conversion of filament strands to flare loops, spatial expansion of a core dimming region, and formation of newly-discovered saddle-shaped arcades of flare loops. We demonstrate how can the evolution and morphology of flare ribbons indicate the nature of processes powering flares and evolution of CMEs.

Speaker: Juraj Loricik (Astronomical Institute of the Czech Academy of Sciences, Ondrejov)
• 116
Impact of Hall MHD on the evolution of three-dimensional complex magnetic structures

Observations have revealed the diffusive nature of solar coronal plasma in the form of solar flares, coronal mass ejections, jets--manifestation of magnetic reconnections (MRs). The onset of reconnection in a perfectly conducting plasma like the solar corona requires the generation of small scales in consequence of large scale dynamics, eventually leading to a locally reduced characteristic length scale of the magnetic field variability. At such small scales, the standard magnetohydrodynamics is not valid and the Hall-magnetohydrodynamics (Hall MHD) takes over. Hall MHD gives the faster reconnection as well as resolves the details of dynamics at small scales. To explore the reconnection in Hall MHD, we have successfully extended the computational model EULAG-MHD to include the Hall forcing and reported the evolution of three-dimensional (3D) coherent magnetic structures recently. Simulations of evolution of a magnetic flux rope from solar corona like sheared magnetic arcades in the presence and absence of the Hall forcing have revealed that the rope evolves through intermediate complex structures, ultimately breaking locally because of reconnections in the presence of Hall forcing. Interestingly, the breakage occurs earlier in the presence of the Hall term, signifying faster dynamics leading to magnetic topologies favorable for reconnections.

Speaker: Kamlesh Bora (Physical Research Laboratory)
• 117
Oscillatory reconnection of a 2D magnetic X-point for systems with different base temperatures.

The propagation of magnetoacoustic waves about a 2D magnetic X-point has revealed the existence of oscillatory reconnection, which is a series of horizontal and vertical current sheets with associated changes in magnetic connectivity. Oscillatory reconnection has been proposed as a wave-generation mechanism to explain some of high-speed, quasi-periodic outflows/jets in the solar atmosphere, as well as one possible physical mechanism behind quasi-periodic pulsations (QPPs). In this study we expand the results of McLaughlin et al. (2009) by performing a parameter study over a wide range of base temperatures. We solve the full set of 2D MHD equations for a magnetic X-point with the use of the PLUTO code, with explicit resistivity included. Through a nonlinear wave, we initiate the collapse of the X-point into a current sheet, initiating oscillatory reconnection for systems of different base temperatures. We study the evolution of plasma beta and its effects on the oscillatory process. By increasing the base temperature of the system, we see that both the amplitude and the period of the oscillating current density profile change. Finally, we will discuss how thermal conduction affects the temperature evolution of our systems, and by extension the final non-potential state of our systems.

Speaker: Dr Konstantinos Karampelas (Northumbria University)
• 118
Relative field line helicity of active region 11158

Magnetic helicity is a physical quantity of great importance in the study of magnetized plasmas as it is conserved in ideal magneto-hydrodynamics and slowly deteriorating in non-ideal conditions such as magnetic reconnection. A meaningful way of defining a density for magnetic helicity is through field line helicity, which, in solar conditions, is expressed by relative field line helicity (RFLH). In this work we study in detail the behaviour of RFLH in a solar active region (AR) for the first time. The target AR is the large, well-studied, eruptive AR 11158. The computation of RFLH is based on a high-quality non-linear force-free reconstruction of the AR coronal magnetic field, and on the recent developments in its computational methodology. The derived photospheric morphology of RFLH is very different than that of the magnetic field or the electrical current. The large decrease in the value of helicity during an X-class flare of the AR is also depicted in the photospheric morphology of RFLH. Moreover, the area of the RFLH decrease coincides with the location of a flux rope, that is, of the magnetic structure that later erupted. The use of RFLH can thus provide important information about the value and location of the magnetic helicity expelled from the solar atmosphere during eruptive events.

Speaker: Kostas Moraitis (University of Ioannina, Greece)
• Poster Session 3.5
Convener: Jasmina Magdalenic (Solar-Terrestrial Centre of Excellence—SIDC, Royal Observatory of Belgium)
• 119
The Real Winding Number of Coronal Flux Ropes

Twist is routinely used to quantify the location of flux ropes and to understand their evolution. At the Sun, we rely on the quantity Tw that describes how much two infinitesimally close field lines wind about each other due to its simple derivation from parallel current. In this work we present a simple method for the identification of a flux rope axis and the calculation of the winding number for its field lines. The behaviour of this metric is explored in comparison to Tw.

Speaker: Dr Daniel Price (University of Helsinki)
• 120
The effect of a solar flare on chromospheric oscillations

Chromospheric oscillations with durations of approximately 3 minutes are prominent and thought to be related to the acoustic cut-off frequency of the medium. Here we used data from the CRISP instrument at the Swedish Solar Telescope and the Solar Dynamics Observatory to investigate intensity oscillations in an active region around the time of an M-class flare. Power spectrum fitting was used to characterise the locations and typical periods of the oscillations. Comparing results from before and after the impulsive phase of the flare, both the locations of the oscillatory signals and their typical periods were seen to change, with some periods seen to increase from ~100s to ~200s. The cut-off frequency can be linked to the magnetic field inclination, meaning these results could reveal how the flare has changed the magnetic environment in this active region.

Speaker: Mr David Millar (University of Glasgow)
• 121
Opening a new window of spectro-temporal polarimetric imaging study of low frequency radio Sun using SKA precursor

Polarization properties of solar radio emissions are known to be a rich source of information about the solar emission mechanisms and the magnetic field topology. Nonetheless, largely due to technical challenges, polarimetric imaging observations of the Sun at low radio frequencies have remained very rare. The degree of polarization of the solar radio emission varies dramatically over time, frequency and also in spatial structure depending on emission mechanism. The radio bursts show moderate to high degree of circular polarization and the quiet sun thermal emissions show very low degree of circular polarization (~<1%). When it becomes possible, detection of very low circular polarisation from quiet Sun thermal emission will be an important tool to measure quiet Sun coronal magnetic field. Simultaneous measurement of linear and circular polarisation from active emissions are important to understand the quasi-longitudinal and quasi-transverse propagation and will direct probe the magnetic field geometry. Due to large Faraday rotation, narrow bandwidth observations are allowing us to detect possibly the first ever linearly polarised emission from radio bursts at low-frequencies. Perhaps the most rewarding, and also challenging, will be the polarimetric observations of gyrosynchrotron emission from CME plasma, which will allow us to model the CME plasma parameters unambiguously. We are developing a radio interferometric imaging pipeline for snapshot spectroscopic polarimeteric solar images to enable such studies. Here we summarise its current status and showcase some early results. While this pipeline is optimised for the Murchison Widefield array, a SKA-low precursor, it can be adapted for the SKA-Low.

Speaker: Devojyoti Kansabanik (National Centre for Radio Astrophysics - Tata Institute of Fundamental Research, Pune, India)
• 122
Initial STIX Coarse Flare Locations

STIX on board Solar Orbiter uses an indirect imaging system to measure flare location, size and morphology. Pairs of tungsten grids create Moiré fringes on its coarsely pixelated CdTe detectors. Images are then reconstructed on the ground, using sophisticated imaging algorithms, after the full pixel has been download. STIX therefore uses a dedicated sub-collimator to estimate a rough (within a few arcminutes), but unambiguous, flare location on board in near real time. The Coarse Flare Locator (CFL) consists of a single grid with a specific pattern which selectively illuminates pixels of a dedicated detector based on the source location. The correlation between the counts in the pixels of this detector and a look up table of pre-caculated expectations allows the location to be estimated promptly, within the constraints of on board processing. Using the downloaded measured counts in each pixel the coarse flare location can also be reconstructed on the ground. This allows for more sophisticated algorithms which require greater computational power than is available on board; greater flexibility as to which time and energy intervals are combined; and more careful background subtraction. The first estimates of STIX flare locations calculated using the STIX Ground Processing Software (GSW) from data taken during the first year of STIX operations are presented here. Comparisons are made to the expected active region and source locations, using data from several other instruments. Pending a successful flight software update the on board location estimates will also be analysed and compared with expectations.

Speaker: Dr Ewan Dickson (FHNW, Windisch, Switzerland; University of Graz, Graz, Austria )
• 123
Polarization signatures during the X1.6 flare observed in NOAA 12192

The X1.6 flare observed on 22 October 2014 (SOL2014-10-22T14:28) was among the strongest flares occurred in the great and magnetically complex active region NOAA 12192. It was a confined flare, without an accompanying CME, despite the large amount of released energy.
We attempt to deepen our understanding of the magnetic field configuration of NOAA 12192. We analyzed the polarization signatures during the flare using spectro-polarimetric data acquired by the IBIS/DST instrument along the photospheric Fe I 617.3 nm and the chromospheric Ca II 854.2 nm lines in a time interval immediately following the peak of the X1.6 flare. The results obtained provided evidence of significant changes in the magnetic field configuration during the analyzed time interval.

Speaker: Fabiana Ferrente (Università Degli Studi di Catania)
• 124
Radio and EUV analysis of a shock wave reflected by an equatorial coronal hole

We report the first unambiguous observational evidence in the radio range of
the reflection of a coronal shock wave at the boundary of an equatorial coronal hole.
The eruption occurred on 2011 August 11 above active region NOAA 11263 and
was accompanied by an EUV wave, as evinced by AIA/SDO and EUVI/STEREO
observations, and an anomalous reverse-drifting metric type II radio burst,
interpreted as a signature of the reflected shock. By combining EUV images
from AIA and EUVI with radio observations obtained by the Nancay Radioheliograph (NRH),
we show that the reverse-drifting type II radio emission was produced at the
intersection of the shock front, reflected at a coronal hole boundary, with an
intervening low-Alfvén speed region characterised by an open field configuration.
Moreover, we provide a data-driven reconstruction of the
spatio-temporal evolution of the shock-accelerated electron beams produced
by the reflected shock.

Speaker: Federica Frassati (Istituto Nazionale di Astrofisica (INAF))
• Poster Session 3.6
Convener: Emilia Kilpua (University of Helsinki)
• 125
Multispacecraft CME event modelled with EUHFORIA: translating observations to model input parameters

Solar eruptions, such as flares and coronal mass ejections (CME), are key drivers of space weather phenomena. CMEs are large plasma eruptions, with magnetic flux ropes frozen-in the plasma. They can travel within the heliosphere with speeds from a few hundred up to 1000 km/s and can have a profound impact on the near-Earth environment, as well as human activity in space and on ground. Modelling and successfully reconstructing CMEs is essential for space weather forecasting purposes. In this study we investigate the implementation of the spheromak CME-type flux rope model used in EUHFORIA (EUropean Heliospheric FORecasting Information Asset). To this purpose we select a CME event from the 6th of January 2013 as a test case. We translate the observable properties of the CME into model inputs. Particular attention is given in translating the observed angular widths of the CME, as captured in white-light images, to a spheromak radius. In addition, we investigate the insertion angle of the spheromak necessary to capture the observed magnetic field topology at the source region. To assess the model output, we compare it with in-situ observations at two spacecraft locations, Venus Express (~0.7 AU) and STEREO-A (1 AU), which were radially aligned during the eruption. To further understand the modelling results we conducted detailed investigations on the dynamics of the magnetic structure as it propagates.

Speaker: Eleanna Asvestari (University of Helsinki)
• 126
Probing multiscale properties of plasma turbulence in space and time at sub-ion scales with Iterative Filtering.

Turbulence in space and astrophysical plasmas is an intrinsic multiscale phenomenon, which involves nonlinear coupling across different temporal and spatial scales. As a result, the physical mechanisms responsible for the turbulent dissipation and the heating of the plasma below the proton characteristic scales remain largely unknown. In this talk, we will present recent results from a statistical multiscale study of Hall-MHD and hybrid-kinetic numerical simulations of plasma turbulence, performed employing Iterative Filtering, a technique designed for the analysis of nonstationary nonlinear multidimensional signals. A spatial decomposition reveals that turbulence at sub-proton scales is highly intermittent, formed by localized magnetic structures and/or perturbations organized in a filamented network where dissipation is enhanced. Further spatiotemporal analysis shows that such perturbations (with temporal frequencies smaller or comparable to the ion-cyclotron frequency) cannot be described in terms of either kinetic Alfvén or ion-cyclotron waves. Implications of these results and comparison with solar wind turbulence observations will be discussed.

Speaker: Emanuele Papini (Università degli studi di Firenze)
• 127
The multi-spacecraft observations of the sheath region of April 2020 magnetic cloud and the associated energetic ions

The sheath regions driven by coronal mass ejections (CMEs) are large-scale heliospheric structures. Turbulent and compressed sheaths could contribute to the acceleration of charged particles in interplanetary space, but their internal structure and its relation to energization process is still poorly understood. We report here the analysis based on observations by Solar Orbiter, BepiColombo and the L1 spacecraft to explore the structure of a coronal mass ejection (CME)-driven sheath and enhancements of energetic ions that occurred on April 19-20, 2020. Our detailed analysis of the magnetic field, plasma and particle observations show that the enhancements were related to the Heliospheric Current Sheet crossings related to the reconnecting currents sheets in the vicinity of the shock and a mini flux rope that was compressed at the leading edge of the CME ejecta. This study highlights the importance of smaller-scale sheath structures for the energization process. These structures likely formed already closer to the Sun and were swept and compressed from the upstream wind past the shock into the sheath. The upcoming observations by the recent missions (Solar Orbiter, Parker Solar Probe and BepiColombo) provide an excellent opportunity to explore further their role.

Speaker: Emilia Kilpua (University of Helsinki)
• 128
How the shape of coronal holes influences the high-speed stream peak velocity at Earth

Coronal holes (CHs) are regions with open magnetic field topology in the solar corona. They are characterized by lower densities and temperatures compared to the ambient solar environment. Further, they are the source regions of high speed solar wind streams (HSSs), which propagate through the interplanetary space. Throughout the years, many authors have performed statistical studies comparing the CH properties and the characteristics of the HSSs at Earth in order to understand their relationship. This topic is of utmost importance for modelling the solar wind environment in interplanetary space as well as for space weather predictions.

In this study, we perform a statistical analysis of a set of 45 CHs during a period of solar minimum activity. We focus on the geometrical shape of the CHs and we divide our sample in different groups based on various geometrical criteria. We then examine the relationship of each group with the HSS peak velocity at Earth. Our results show that the HSS peak velocity in situ is better constrained by CHs of specific properties. We further quantify the geometrical complexity of the CHs by employing two different ways: (a) by defining a ratio of geometrical properties and (b) by calculating their fractal dimension.

Speaker: Evangelia Samara (Royal Observatory of Belgium and KU Leuven)
• 129
On the uncertainties of the sunspot area measurements

Sunspot areas are important quantities that can be obtained from the analysis of the images of the solar photosphere. In particular, sunspot areas extracted from historical solar drawings are very useful due to its strong correlation with other solar activity indices as the Group Number or the Sunspot Number (related to the solar irradiance too). Usually, sunspot areas corrected from foreshortening (in millionths of solar hemisphere, or msh) are determined using the well-known equation

$A_{M}= \frac{10^{6} A_{S}}{2\pi R^{2} \cos\rho}$

where $A_{S}$ is the sunspot area measured directly on the image, $R$, the radius of the solar disk and $\rho$ the angle between the direction of the centre of the solar disk and the direction of the sunspot.

We have analysed the uncertainties in the measurement of $A_{M}$ due to the different factors included in above equation, with special interest in the influence of the binarization threshold in the measure of $A_{S}$. Moreover, as the equation is an approximation valid for relatively small sunspots where the angle $\rho$ is the same for all the sunspot surface, we have analysed its validity for unrestricted size sunspots of circular and elliptical shapes.

• 130
First results on production of magnetosheath jets during to SIRs and CMEs

Coronal mass ejections (CMEs) and stream interaction regions (SIRs) are different large-scale structures in the solar wind. When interacting with Earth, they may cause the most severe Space Weather effects. Between solar wind and Earth’s magnetosphere also other possible geoeffective phenomena are generated, namely “magnetosheath jets”. They are defined mainly as dynamic pressure enhancements and constitute a significant coupling effect between the solar wind and the magnetosphere of the Earth. How they are related to CMEs and SIRs is so far not explored. To shed light on this relation and on the generation of these jets in general, we present the first-ever statistical results of the jet production during CMEs and SIR times by using THEMIS data. We find that the number and total time of observed jets decrease while CMEs hit the Earth's magnetic field. On the other hand, the number of jets increases during SIR phases. We discuss possible physical processes to explain these statistical differences.

Speaker: Florian Koller (University of Graz, Austria)
• 10:31 AM
Coffee break
• Poster Session 4.1
Convener: Etienne Pariat (LESIA, Observatoire de Paris)
• 131
Features of the evolution of polar and non-polar coronal holes over the past 11 years

This paper presents the results of the analysis of the dynamics of coronal holes on the Sun during the period May 13, 2010- May 13, 2021. In our study used images in the extreme ultraviolet in the Fe XII, XXIV line (193 Å) obtained with the Atmospheric Imager Assembly of the Solar Dynamics Observatory (AIA/SDO). To localize coronal holes and determine their areas, we used the Heliophysics Event Knowledgebase (HEK), which is available at http://www.lmsal.com/hek/hek_isolsearch.html . Information on coronal holes was extracted with the Spatial Possibilistic Clustering Algorithm (SPoCA). The separation of all coronal holes of the considered period into polar and non-polar ones showed that the daily total area of polar coronal holes increases at the minima of solar activity and decreases at the maximum of the cycle. This is consistent with the general concept of polar coronal holes as the main source of the solar dipole magnetic field. There is an asymmetry in the areas of polar coronal holes in the northern and southern hemispheres. It is shown that the areas of nonpolar coronal holes vary quasi-synchronous with the sunspot activity of the Sun, which suggests the existence of a physical connection between these two phenomena. Apparently, the nature of the magnetic fields of polar and non-polar coronal holes is different. Non-polar coronal holes are possibly very high loops that close through the corona in other regions of the Sun, while polar coronal holes extend far into the heliosphere.

Speaker: Mrs Olga Andreeva (Crimean Astropysical Observatory)
• 132
The Polar Precursor Method for Solar Cycle Prediction: Comparison of Predictors and Their Temporal Range

The polar precursor method is widely considered to be the most robust physically motivated method to predict the strengths of an upcoming solar cycle. It uses in form of indicators, the magnetic field concentrated near the poles around sunspot minimum. Here, we present an extensive performance analysis of various such predictors, based on observational data like (WSO magnetograms, MWO polar faculae counts and Pulkovo A(t) index) and outputs (global dipole moment) of various existing flux transport dynamo models. We have calculated Pearson correlation coefficients (r) of the predictors with the next cycle amplitude as a function of time measured from solar cycle maximum and polar field reversal. Setting r = 0.8 as a lower limit for acceptable predictions, we find that observations and models alike indicate that the earliest time when the polar predictor can be safely used is 4 years after polar field reversal. This is typically 2 to 3 years before the solar minimum and about 7 years before the predicted maximum. Re-evaluating the predictors another 3 years later, at the time of solar minimum, further increases the correlation level to r > 0.9. As an illustration of the result, we determine the predicted amplitude of Cycle 25 based on the value of the WSO polar field at the official minimum date of December 2019 as 126 ± 3. A forecast based on the value in early 2017, 4 years after polar reversal would have only differed from this final prediction by ~ 3.1 %.

Speaker: Mr Pawan Kumar (Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi, India)
• 133
Peak magnetic field in sunspots: a comparison between the CrAO and SDO/HMI data

We performed a digitization of maximum magnetic field measurements in sunspots. The original data were acquired as drawings at the Crimean Astrophysical Observatory of the Russian Academy of Sciences (CrAO RAS). About 1000 sunspots observed in 2014 were analyzed. The data were compared to the corresponding measurements from the SDO/HMI instrument. For the same sunspot, the maximum modulus of the magnetic field derived at CrAO was compared to the corresponding value from HMI. The Crimean data and the space-based data were found to be in direct proportion to each other. A linear approximation over the entire range of measurements (1–4) kilogauss (kG) shows a Pearson correlation coefficient of 0.71 (with the 95 % confidence boundaries of 0.68–0.74) and a slope of linear regression of 0.65±0.02. A linear approximation over the range of strong fields B(CrAO) > 1.8 kG gives a similar correlation, however the slope of linear regression is far closer to unity and constitutes 0.90. In the range of weak fields B(CrAO) < 1.8 kG, a non-linear deviation (exceeding) of the space-based data is observed. Non-linearity can be explained, in part, by a specific routine of the magnetic field measurements at CrAO, however further investigations are needed to explore sources of possible non-linearity in the HMI data. The Crimean measurements of the maximum magnetic field in sunspots are concluded to be in good agreement with the corresponding SDO/HMI measurements, and therefore they can be used for scientific purposes.

Speaker: Ms Regina Biktimirova (Crimean Astrophysical Observatory)
• 134
On the minimum - maximum method for the solar cycle prediction

One of the methods to predict the future solar activity is the minimum - maximum method, which is based on a linear relationship between relative sunspot number in the minimum and maximum epochs of solar cycles. It belongs to the precursor class of the solar activity forecasting methods. In present work we apply a modified version of this method using data not only from the minimum year, but also from a couple of years before and after the minimum. The new version of the 13-month smoothed monthly total sunspot number data set from SILSO-SIDC is used. We investigate the value of the correlation coefficient of the mentioned relationship as a function of the time lag around the solar minimum. Further, a statistical significance of the results and inclusion/exclusion of the curious solar cycle no. 19 are discussed. For the next solar maximum of the cycle no. 25 we predict a similar amplitude as the previous one, or even something lower. This is in accordance with the overall middle-term lowering of the solar activity after the secular maximum in the 20th century and consistent with the Gleissberg period of the solar activity. The reliability of the method is experimentally checked by applying it to reconstruct a couple of previous solar cycle maxima, using the earlier data. Finally, the minimum-maximum method and its results are discussed in the context of the well-known empirical findings: the extended solar cycle and the Waldmeier effect, as well as various solar dynamo models.

Speaker: Roman Brajša (Hvar Observatory, Faculty of Geodesy, University of Zagreb, Croatia)
• 135
Prediction of 11-year solar cycle strength with Hemispheric Sunspot Number

The 11-Year solar cycle is driven by the sun’s magnetic field. The sunspot number is the most-common long term index of solar activity and prediction of its amplitude can help to understand the effects of space weather & solar activity. Previous studies have shown that analyzing the solar activity of the two hemispheres separately instead of the full sun can provide more detailed information on the activity evolution. However, the existing Hemispheric Sunspot Number(HSN) series (1945 onwards) is too short for the purpose of solar cycle predictions. Based on a newly created HSN catalogue for the time range 1874-2020 (Veronig et al. 2021) that is compatible with the International Sunspot Number from WDC-SILSO, we investigate the evolution of the solar cycle for the two hemispheres, and demonstrate that empirical solar cycle prediction methods can be enhanced by investigating the solar cycle dynamics in terms of HSN. We develop a method for predicting the solar cycle amplitude based on the peak growth rate in the ascending phase of the cycle using HSN for cycles 12-24. We show that using this technique, the sum of the predictions (North + South) of the two hemisphere give better estimates of the cycle amplitude than the Total Sunspot Numbers. In addition, we estimate the cycle peaks with 1st order and 3rd order regressions and find that the HSN provide better estimates of cycle peak than total sunspot numbers with the obtained correlations lying in the range r = 0.8-0.9 depending on the applied smoothing window

Speaker: Mr Shantanu Jain (Skolkovo Institute of Science and Technology)
• 136
Cycle dependency of the Quasi-Biennial Oscillation (QBO)

The quasi-biennial oscillation (QBO) is a low amplitude oscillatory signal commonly seen in solar activity proxies and may hold the key to a greater understanding of the solar dynamo. This will help us mitigate the risks that come with space weather which is driven by the solar magnetic field. In addition, mid-cycle oscillations have been observed on other stars, making the impact of this work extend beyond our own solar system. We use helioseismology to probe the solar interior by examining frequency shifts of p-modes to isolate the QBO and its associated periodicity. We also investigate the p-modes’ spatial and frequency distribution, and examine how these distributions change over Cycles 23 and 24. We use data from the Global Oscillations Network Group, Michelson Doppler Imager and Helioseismic & Magnetic Imager in the intermediate-degree range, as well as solar activity proxies to investigate Cycles 21-24. We use Empirical Mode Decomposition which is adept at picking out quasi-oscillatory signals where the signal is allowed to vary in period, shape, phase, and amplitude. We find evidence of the QBO in both Cycles 23 and 24, although it is less significant in Cycle 24, which raises the possibility of a multiplicative relationship between the activity of a solar cycle and the presence of the QBO. We also see the evidence of the QBO across depths from 0.2-0.95 solar radii, suggesting that the magnetic field driving the QBO is likely to be located in the near surface region.

Speaker: Tishtrya Mehta (University of Warwick)
• 137
Diagnostics of turbulent component of solar dynamo in the convective zone from the morphology of strong-flaring active regions

To find out observational evidences for the turbulent component of the solar dynamo in the convective zone is a very challenging problem because the dynamo action is hidden below the photosphere. Here we present results of a statistical study of active regions (ARs) with strong flares (>X1.0) occurred during the 23rd and 24th solar cycles. A suggested magneto-morphological classification of ARs allowed us to diagnose the degree of intervention of the turbulent (mid-scale and small-scale) component of the dynamo. We found that in 72% of cases, the strong-flaring ARs do not comply empirical laws of the global dynamo (the Hale polarity law, the Joy's law, the leading spot prevalence rule; ARs-"violators") and therefore they may be attributed to the influence of the turbulent dynamo action inside the convective zone on scales of typical ARs. We found that strongest flares occur in the ARs-"violators", so the flaring capability of the Sun is controlled by the turbulent component of dynamo. ARs-"violators" with strongest flares tend to happen during the second maximum and the descending phase, when the toroidal field ceases and the turbulent component of the dynamo should be more pronounced. These observational results are in consensus with a concept of essential role of non-linearity and turbulent intermittency in the magnetic fields generation inside the convective zone, which follows from simulations of dynamo.

Speaker: Valentina Abramenko (Crimean Astrophysical Observatory)
• Poster Session 4.2
Convener: Jasmina Magdalenic (Solar-Terrestrial Centre of Excellence—SIDC, Royal Observatory of Belgium)
• 138
(When) Can wave heating balance optically thin radiative losses in the corona?

Why the atmosphere of the Sun is orders of magnitudes hotter than its surface is a long standing question in Solar Physics. Over the years, many studies have looked at the potential role of MHD waves in sustaining these high temperatures. In this study, we use 3D MHD simulations to investigate (driven) Alfvenic waves in a coronal loop. Due to the radial density profile, resonant absorption (or mode coupling) and phase mixing take part in the boundaries of the flux tube and the large velocity shears are subject to the Kelvin Helmholtz instability (KHI). The combination of these effects leads to enhanced energy dissipation and wave heating. By considering a variety of wave drivers and coronal loop profiles, we determine when this energy release is sufficient to balance radiative losses.

Speaker: Prof. Ineke De Moortel (University of St Andrews; Rosseland Centre for Solar Physics, University of Oslo)
• 139
Bayesian evidence for the nonlinear damping of coronal loop oscillations

Recent observational and theoretical studies indicate that the damping of solar coronal loop oscillations depends on the oscillation amplitude. We consider the mechanisms of linear resonant absorption and of nonlinear damping due to the development of the Kelvin-Helmholtz instability. We confront theoretical predictions from these models with observed data in the plane of observables defined by the damping ratio and the oscillation amplitude. The structure of the Bayesian evidence in this plane displays a clear separation between the regions where each model is more plausible relative to the other. There is qualitative agreement between the regions of highest marginal likelihood and Bayes factor for the nonlinear damping model and the arrangement of observed data. A quantitative application to 101 loop oscillation cases observed with SDO/AIA results in the marginal likelihood for nonlinear damping being larger in the majority of them. The cases with conclusive evidence for nonlinear damping outnumber considerably those in favor of linear resonant absorption.

Speaker: Iñigo Arregui (Instituto de Astrofisica de Canarias)
• 140
Detection and multi-wavelength analysis of swirling structures in the solar atmosphere

High-resolution solar observations from both space-borne and ground-based telescopes have revealed ubiquitous photospheric vortical motions in quiet, as well as in active regions. In observations of the chromosphere, obtained in spectral lines, such as the Hα and the Ca II IR, they appear as spiral-shaped or circular dark patches. These so called “chromospheric swirls” are considered to be of great significance due to their ubiquity, and to their suggested contribution to the energy transfer from the sub-photospheric layers to the transition region and corona. Therefore, statistical information concerning their population and a number of significant physical properties, such as radii, lifetimes and line-of-sight velocities are imperative in order to understand their nature and estimate the amount of the upwards energy transfer. We have developed a novel, automated chromospheric swirl detection method based on their morphological characteristics, in an attempt to complement visual inspection methods and velocity field derivation techniques used to date, with a more reliable to chromospheric observations method. We will be presenting the designed algorithm, and its evaluation on different observational datasets in several chromospheric spectral lines, obtained with the CRisp Imaging SpectroPolarimeter (CRISP) and the CHROMosheric Imaging Spectrometer (CHROMIS) of the Swedish 1-m Solar Telescope (SST). In addition to the swirl detection results, various parameters derived from a multi-line and multi-wavelength analysis conducted on detected swirls, within selected areas, will be presented and discussed.

Speaker: Mr Ioannis Dakanalis (IAASARS, National Observatory of Athens)
• 141
On the correlation between the millimeter brightness temperature and the solar magnetic field

ALMA millimeter wavelength images of the Sun show significant correspondence with the solar magnetograms. We analyze the observed correspondence by comparing ALMA solar images taken at 1.2 and 3.0 mm with SDO/HMI line-of-sight magnetograms. We find that the active regions and the chromospheric network show a positive correlation where the brightness temperature increases with the line-of-sight magnetic field strength, while sunspots display the opposite behavior with a negative correlation. On the other hand, quiet Sun regions do not show any dependence of brightness temperature with the magnetic field. Several radiation mechanisms are explored to explain the observed (anti)correlations. Thermal free-free emission is given as the most probable explanation and the main contributor to the observed correlations with enhanced heating in the active regions and a decrease of temperature in the sunspots due to the suppression of convection by the magnetic field.

Speaker: Ivica Skokić (Hvar Observatory, Faculty of Geodesy, University of Zagreb, Croatia)
• 142
Prominence formation by levitation-condensation at extreme resolutions

We revisit the so-called levitation-condensation mechanism for the \textit{ab-inito} formation of solar prominences: cool and dense clouds in the million-degree solar atmosphere. A flux rope is formed in response to the deformation of a force-free coronal arcade by controlled magnetic footpoint motions and subsequent reconnection. Existing coronal plasma gets lifted within the forming rope, therein isolating a collection of matter now more dense than its immediate surroundings. This denser region ultimately suffers a thermal instability driven by radiative losses and a prominence forms. Our modern open-source grid-adaptive simulation code [amrvac.org] enables a resolution of 5.6 km within a 24 Mm x 25 Mm domain size; the full global flux rope dynamics and local plasma dynamics are captured in unprecedented detail. Our 2.5D simulation demonstrates that the thermal runaway condensation can happen at any location, not solely in the bottom part of the flux rope where the majority of prominence material is assumed to reside. Intricate thermodynamic evolution and shearing flows develop spontaneously, themselves inducing further fine-scale (magneto)hydrodynamic instabilities. Our analysis touches base with advanced linear magnetohydrodynamic stability theory, e.g. with the Convective Continuum Instability or CCI process as well as with in-situ thermal instability studies. We find that condensing prominence plasma evolves according to the internal pressure and density gradients as found previously for coronal rain condensations, but also misalignments therein suggesting the relevance of the Rayleigh-Taylor instability or RTI process in 3D. We also find evidence for resistively-driven dynamics in the prominence body, in close analogy with analytical predictions.

Speaker: Jack Jenkins (Centre for mathematical Plasma Astrophysics, KU Leuven, Belgium)
• 143
Setting observational constraints to the chromospheric heating problem

The solar atmosphere is hotter than predicted by assuming radiative equilibrium. This is most obviously evidenced by the high temperature of the solar corona, but the bulk of the energy deposition happens already down in the much cooler chromosphere. While in recent years we have gain detailed understanding of many important processes that must be at work in the chromosphere, also from numerical simulations, their exact contribution to the total energy budget remains unclear.

Chromospheric heating or cooling can be estimated by calculating the radiative losses whenever a model atmosphere is available. Most comparisons between simulations and observations have used canonical values of radiative losses that have been derived from 1D models of spato-temporal averages of solar spectra (e.g., FAL / VAL models). Such approach cannot capture the high complexity and fine structures that is observed in high resolution observations. Recent studies have evidenced that spatially resolved radiative cooling can be up to five times higher in active regions than those canonical values that are usually assumed.

In this talk, we present spatially resolved radiative cooling rates computed from the inversion of high spatial resolution chromospheric datasets observed with the Swedish 1-m Solar Telescope in Ca II K, 8542 and Fe I 6301/6302. We study the distribution of radiative cooling across the FOV in different targets in active regions and in quiet-Sun. Our results will help modellers to set better constraints on theoretical predictions and models.

Speaker: Dr Jaime de la Cruz Rodriguez (Institute for Solar Physics, Stockholm University)
• 144
Spectroscopic Observation of a Transition Region Network Jet

Network jets at transition region temperatures of ca. 0.1 MK have been observed to be widespread enough to provide substantial mass and energy to the upper solar atmosphere. Previous studies of this phenomena have mostly focused on near-limb and broadband imagery data and ascribed propagating intensity disturbances as mass motions. Thus, the nature of plasma flows in these jets and their driving mechanism remain unclear. Using co-aligned IRIS and SDO/HMI data, we present observations of a small-scale jet located within a coronal hole at disc-center. The analysis of the event spectra of the Si IV line at 1394 Å showed clear blue-shifts at transition region temperatures along the upper portion of the jet. The jet can be traced to its base where heightened chromospheric activity in Mg II overlaps areas of strong line-of-sight magnetic field concentrations. While single-gaussian fits only show obvious blue-shifts near the top of the jet, closer inspection of the individual spectra reveals a double-peaked profile along the spire of the jet. Near the base of the jet, a weak, blue-shifted secondary component exists. This secondary component, exhibiting flow speeds of 30-60 km/s, becomes stronger farther along the jet from its base, eventually influencing the single-gaussian fits to show blue-shifts of the Si IV spectral line profiles. This observation allows for a unique investigation of the true mass motions of a jet clearly identified in the transition region and down into the chromosphere, where it can be connected to changes in the underlying magnetic field.

Speaker: Jamie Gorman (Max Planck Institute for Solar System Research)
• Poster Session 4.3
Convener: Stefaan Poedts (KU Leuven)
• 145
Propagating localized brightenings in small loop-like structures: Observations from Solar Orbiter/ EUI

Localized brightenings observed in extreme-ultraviolet (EUV) images, are generally interpreted as signatures of micro-or nanoflares occurring in the transition region or lower corona. These brightenings are omnipresent and hence, could well play a major role in heating up the solar corona. Recent observations with the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter have revealed such localized brightenings (‘campfires’) down to an unprecedented size of only of 0.08 Mm^2 (2 EUI pixels). These are the smallest such events yet observed in the quiet-Sun corona. In our study, we find a number of these loop-like brightenings to host propagating features that are manifestations of the internal dynamics. Typically, the propagation speeds range from 30 km/s to 60 km/s. Assuming the loop plasma to be at million Kelvin, these apparent motions would be below the local sound speed, but still quite substantial. Most interestingly, we find non-trivial propagation characteristics, in particular bifurcation, merging, and reflection. These signatures could provide important insights about the dynamic response of the (loop) plasma to the heating events and the location of the heating events themselves. For example, initiation of the brightening on one side of the loop and reflection at the other suggests energization at the footpoint, while an origin of the propagating feature near the apex would be consistent with heating near the top of the bundle of magnetic fieldlines defining the loop. We will discuss these observational results and put them into the context of recent 3D modeling of the small transient brightenings.

Speaker: Sudip Mandal (Max Planck Institute for Solar System Research)
• 146
Kinetic and magnetic vortices in the solar atmosphere

Small scale solar vortices are ubiquitous in the photosphere, and they are believed to organize the magnetic fluxes, creating efficient conduits for energy transport and wave propagation. Another process linked to vortical flows is the generation of twisted magnetic flux tubes. In this work, the twisted magnetic flux is defined as a new typology of a solar vortex, and we applied forefront methodologies to detect both the kinetic and magnetic vortices tubes in realistic magnetoconvection simulations. Our results show that those types of vortices are found in distinct parts of the intergranular downflow. The magnetic vortices appear mostly in shear flow areas where plasma-$\beta>1$, whereas the kinetic vortices are found in low plasma-$\beta$ regions. The solar vortices present similar dynamics at different solar atmosphere levels, concentrating and perturbing the magnetic field lines. The kinetic vortices show upward jets and tend to encompass high magnetic fluxes; thereby, their dynamics is mainly dominated by magnetic forces. Based on the magnetic and kinetic energy ratio obtained for magnetic vortices, we determined that they can be classified into two distinct types, with considerable differences in the overall geometry of the magnetic field line. Our results indicate that the presence of rotational motion in the flow is not necessary for the appearance of magnetic vortices.

Speaker: Suzana S. A. Silva (The University of Sheffield)
• 147
Five years of observing the solar chromosphere with ALMA - First results and future opportunities

Five years ago, regular observations of the Sun with the Atacama Large Millimeter/sub-millimeter Array (ALMA) started. Since then, an increasing number of data sets have been acquired. At the same time, the Solar ALMA Pipeline (SoAP) for processing these data sets has been improved substantially. As a result, the first version of the Solar ALMA Science Archive (SALSA) was constructed. The aim of SALSA, being the final product of the now concluding ERC-funded Solar ALMA project, is to provide science-ready ALMA data to solar physicists. The contained data is mostly comprised of high-cadence (1-2s) time series of millimetre continuum images for different receiver bands and solar targets. A particular advantage is that these ALMA maps provide direct measurements of (brightness) temperatures in different chromospheric layers that are complementary to other chromospheric diagnostics.
After a brief introduction to the opportunities and challenges connected to observing the Sun with ALMA, examples of SALSA data and first scientific results regarding, e.g., the imprint of magnetic fields, propagating shock waves, and oscillations are presented. Finally, the diagnostic potential for the Sun and other stars and the future development of ALMA’s solar capabilities are discussed.

Speaker: Dr Sven Wedemeyer (Rosseland Centre for Solar Physics, University of Oslo, Norway)
• 148
The Missing Cool Corona in the Flat Magnetic Field Around Solar Active Regions

SDO/AIA images the full solar disk in several EUV bands that are each sensitive to coronal plasma emissions of one or more specific temperatures. We observe that when isolated active regions (ARs) are on the disk, full-disk images in some of the coronal EUV channels show the outskirts of the AR as a dark moat surrounding the AR. We will present several specific examples, selected from time periods when there was only a single AR present on the disk. Visually, moats are observed to be most prominent in the AIA 171 Angstrom band, which has the most sensitivity to emission from plasma at log10 T = 5.8. By using the emission measure distribution with temperature, we find the intensity of the moat to be most depressed over the temperature range log10 T ~ 5.7 - 6.2 for all the cases. We argue that the dark moat exists because the pressure from the strong magnetic field that splays out from the AR presses down on underlying magnetic loops, flattening those loops -- along with the lowest of the AR's own loops over the moat -- to a low altitude. Those loops, which would normally emit the bulk of the 171 Angstrom emission, are restricted to heights above the surface that are too low to have 171 Angstrom emitting plasmas sustained in them, while hotter EUV-emitting plasmas are sustained in the overlying higher-altitude long AR-rooted coronal loops. This potentially explains the low-coronal-temperature dark moats surrounding the ARs.

Speaker: Talwinder Singh (University of Alabama in Huntsville)
• 149
A statistical study of plasma composition in solar active regions

The plasma in the solar corona originates from the photosphere and would, therefore, be expected to have similar elemental composition. However, elements with a low first ionisation potential (FIP) have been observed to have an increased abundance in certain regions of the corona. This phenomenon is known as the FIP effect and the degree of enhancement is measured using the FIP bias parameter. The increased elemental abundance is typically observed in active regions.

In this statistical study, we analyse how the degree of enhancement varies in active regions of different sizes, ages and level of complexity. We explore whether the average FIP bias is linked to the evolution of active regions and the photospheric magnetic field at the scale of an active region. First, by exploring whether there is a correlation between average FIP bias and the total magnetic flux and age of the active region. Second, if there is an average FIP bias dependence on magnetic flux density. Third, if the average FIP bias varies depending on whether the plasma is above the leading or following polarity of an active region.

Speaker: Ms Teodora Mihailescu (Mullard Space Science Laboratory, University College London, UK)
• 150
How do driving time scales effect energy release in the solar corona?

Since the discovery of the hot solar corona, a wide variety of mechanisms have been proposed for maintaining the high temperatures. The majority of these models fall into one of two broad categories, either AC (alternating current) or DC (direct current) heating. The distinction between these two groups arises from the characteristic time scales of the photospheric motions which are the source of the required energy. AC models are associated with short time scale driving and DC models with long time scales.

Despite decades of investigation, debate continues about the relative importance of AC and DC heating in different regions of the corona. In either case, the rate of energy injection is sensitive to both the imposed velocity profile (driver) and the form of the atmospheric magnetic field. The interaction of the driver with the coronal field has important consequences for energy budgets. With this in mind, I will present the results of a series of numerical simulations of coronal heating in general settings. By modifying the characteristics of an imposed, random driver, I will compare the expected energy release rates and the atmospheric response for AC and DC driving.

Speaker: Thomas Howson (University of St Andrews)
• 151
Effects of spatial resolution on Stokes profiles

Both numerical simulations and observations of the solar atmosphere are limited in how small spatial variations they can resolve. The smallest processes in simulations are limited by the numerical grid employed, but the sun is under no such constraints. Therefore, observations and simulations at the same nominal resolution are not directly comparable, since sub-resolution effects not present in the simulations may still influence the observed Stokes profiles. A natural question, then, is when inferring quantities such as velocities or magnetic fields, how much is lost by not including the sub-resolution effects in the modelling? This will affect both the selection of numerical grids for simulations (to compare with observations), and the use of inversions to infer properties from observations, which often assume 1D atmospheres at the scale of each pixel. In the present work we study the effects of spatial resolution on Stokes profiles. We make use of 3D rMHD Bifrost simulations run with spatial resolutions of 6, 12, and 23 km, all starting from the same initial state. From these we compute synthetic spectra using the RH1.5D code for photospheric and chromospheric lines, which we degrade and downsample to the pixel scale of the lowest-resolution simulation (23 km). We then compare the inferred physical quantities (in particular magnetic field and line-of-sight velocities) for varying amounts of sub-pixel resolution. We find that important differences persist despite the downsampling and degradation to a lower resolution, and quantify the variation in the inferred physical quantities caused by the imprint of sub-resolution effects.

Speaker: Thore Espedal Moe (University of Oslo)
• Poster Session 4.4
Convener: Elena Khomenko (Instituto de Astrofisica de Canarias)
• 152
The properties of compressive oscillations in hot coronal loops

Flare-associated hot coronal loops often display compressive plasma oscillations involving a density perturbation moving back and forth between the foot points of the loop. These oscillations, sometimes referred to as, sloshing oscillations", exhibit properties very similar to the standing slow waves that were discovered much earlier with the SoHO/SUMER and other spectroscopic instruments. Utilising the multi-wavelength high-resolution imaging observations from SDO/AIA, we have recently shown that the sloshing oscillations eventually transform into a standing slow wave. The oscillation properties and the associated change between the two phases with respect to the evolving physical conditions within the loop, will be discussed. By analysing multiple examples, we also present the possible initial conditions under which such an oscillation may be setup.

Speaker: Dr S. Krishna Prasad (KU Leuven)
• 153
NuSTAR observations of a repeatedly microflaring active region

The Nuclear Spectroscopic Telescope Array (NuSTAR) is an astrophysical X-ray telescope capable of observing the Sun with direct imaging spectroscopy providing a unique sensitivity above 2.5 keV. We use NuSTAR to investigate highly frequent and weak flares thought to contribute to heating the Sun's atmosphere particularly in active regions. I will present several X-ray microflares from a recently emerged active region, AR12721, that were observed on 2018 September 9-10 with NuSTAR. In combination with SDO/AIA, I describe the temporal, spatial, and spectral evolution of these GOES sub-A class microflares that reach temperatures above those of the surrounding active region (>5 MK). One of the microflares presented is the faintest non-thermal microflare so far observed with NuSTAR with an equivalent GOES class of A0.1. Using SDO/HMI, I also present evidence of photospheric magnetic flux cancellation/emergence at the footpoints in 8 of the NuSTAR microflares.

Speaker: Kristopher Cooper (University of Glasgow)
• 154
Interaction of particles beams with chromosphere during impulsive phase of solar flares

Energy transport by fast, non-thermal particles from primary energy release place - located in the top of flaring loop - to the loop foot-points, plays very important role in solar flares. A fully understanding of the processes of transportation, energy deposition in the chromosphere and radiative response of chrompospheric plasma requires both: high (sub-second) cadence observations and numerical models consistent with them.

The analysis of two compact solar flares with similar GOES-class (C1.1 and C1.6), but with significance differences during impulsive phase is presented. Variations of the position and the vertical extent of the energy deposition layer (EDL) as well as variations of the flaring spectra and emission intensities recorded in the H$\alpha$ hydrogen line are studied.

The variations of the HXR fluxes and H$\alpha$ intensities were well-correlated in time during the impulsive phases of the flares, and they agreed with the variations of the calculated position and vertical extent of the EDL. Impulsive variations of the H$\alpha$ emission were caused by individual, short-time episodes of energy deposition by the electron beams on various depths in the chromospheric plasma.

The compared flares were observed in H$\alpha$ line with very high time resolution (20 spectra-images per second) using the MSDP spectrograph at Bialkow Observatory (University of Wroclaw, Poland), as well as by GOES and RHESSI satellites in X-ray domain. The numerical model was calculated assuming that the external energy is delivered to the flaring loop by non-thermal electrons.

Speaker: Dr Krzysztof Radziszewski (Astronomical Institute, University of Wrocław, Poland)
• 155
Can we estimate coronal wave energies using synthetic non-thermal line widths from MHD simulations?

Estimates of coronal wave energies remain uncertain as a large fraction is hidden in the non-thermal line widths of emission lines. To estimate these wave energies, previous studies have taken the root mean squared wave amplitudes to be a factor of $\sqrt{2}$ greater than the non-thermal line widths. However, other studies have used different factors. To investigate this discrepancy, we consider the relation between wave amplitudes and the non-thermal line widths within a variety of 3D magnetohydrodynamic (MHD) simulations. To generate the synthetic emission required to analyse the non-thermal line widths, the forward modelling code FoMo is used. In order to estimate wave energies, an appropriate relation between the non-thermal line widths and root mean squared wave amplitudes must be found. However, evaluating this ratio to be a singular value, or even providing a lower or upper bound on it, is not realistically possible given the sensitivity it has to various MHD models. It depends on a variety of factors, including line-of-sight angles, velocity magnitudes, wave interference and exposure time. Indeed, some of our models achieved the ratios claimed in recent articles while other more complex models deviated from these values. As the ratio between wave amplitudes and non-thermal line widths is not constant across our models, this widely used method for estimating wave energy is not robust.

Speaker: Ms Lianne Fyfe (St. Andrews University)
• 156
Simulating Rayleigh-Taylor induced magnetohydrodynamic turbulence in Solar Prominences

The purpose of our study is to deepen our understanding of the turbulence that arises from the Rayleigh Taylor Instability and leads to the formation and evolution of prominence. Quiescent prominence formed in the solar corona are cool and dense condensates comprising of varied dynamics of physical processes and scales. It is dominated by the vertical motion of flows in the upper atmosphere where the mean magnetic field is predominantly in the horizontal direction, and the magnetic pressure holds the dense suspended prominence material. Previous studies with Hinode Solar Optical Telescope's observations revealed turbulent behavior in these prominence structures having evolving rising plumes and descending pillars. We perform numerical simulations from MPI-AMRVAC to study the 2.5D Rayleigh Taylor Instability at the prominence-corona transition region, using the ideal-magnetohydrodynamic approach. High-resolution simulations achieved spatial grid resolution of $\sim 23$ km having a cadence of $\sim 0.85$ sec for $\sim 25$ min transitioning from a multi-mode perturbation instability to the non-linear regime and finally a fully turbulent region. We use statistical methods to relate that the dynamics due to the instability in quiescent prominence indicate turbulence behavior that occurs distinctly on different prominence scales for the turbulent magnetic and velocity field fluctuations.

Speaker: Mr Madhurjya Changmai (Centre for mathematical Plasma-Astrophysics , KU Leuven)
• 157
First Detection of Torsional Oscillations within a Magnetic Pore in the Solar Photosphere

Alfvén waves have proven to be important in a range of physical systems due to their ability to transport non-thermal energy over long distances in a magnetised plasma. This property is of specific interest in solar physics where the extreme heating of the atmosphere of the Sun remains unexplained. In an inhomogeneous plasma like a flux tube in the solar atmosphere, they manifest as incompressible torsional perturbations. However, despite evidences in the upper atmosphere, they have not been directly observed in the photosphere. Here, we report the detection of anti-phase incompressible torsional oscillations observed in a magnetic pore in the photosphere by the Interferometric BIdimensional Spectropolarimeter (IBIS). State-of-the-art numerical simulations suggest that a kink mode is a possible excitation mechanism of these waves.
The excitation of torsional waves in photospheric magnetic structures can significantly contribute to the energy transport in the solar atmosphere and the acceleration of the solar wind, especially if such signatures will be ubiquitously detected in even smaller structures with the forthcoming next generation of solar telescopes.

Speaker: Marco Stangalini (ASI Agenzia Spaziale Italiana)
• Poster Session 4.5
Convener: Emilia Kilpua (University of Helsinki)
• 158
Signal processing for the automatic detection of flare events in the Soft-X GOES signal in the period 1986-2020

An automatic algorithm has been used for detection of flares in the GOES data in the period 1986-2020. The detection process starts from soft-X solar signal, provided by the NASA-GOES Satellite Network which is devoted to the surveillance of the Earth. Since flares represent one of the main events associated with Space Weather their statistics are of particular importance in this context.
The algorithm produced a database, tested with official catalogs for validation, that increased the number of detected flares by a factor of about four compared to the official GOES catalog.
In particular, the problems connected to the soft-X background flux in pre-flare conditions, multiple flares and lower intensity flares were dealt with.
In addition to briefly presenting the algorithm, we show the results of the analysis of the distributions in the last four solar cycles.

Speaker: Nicola Plutino (Dipartimento di Fisica - Università di Roma Tor Vergata)
• 159
Coronal Dimmings associated with Coronal Mass Ejections

We conducted a statistical analysis of the dimming events associated with Earth-directed coronal mass ejections (CMEs) that were observed in quasi-quadrature by the SDO and STEREO satellites. We derived the properties of the dimmings as observed above the limb by STEREO EUVI and compared them with the mass and speed of the associated CMEs. The unique satellite constellation allowed us to compare our ﬁndings with the results from Dissauer et al. (2018, 2019), who studied these events observed against the solar disk by SDO AIA. Such statistics is done for the ﬁrst time and conﬁrms the close relation between characteristic dimming and CME parameters for the off-limb viewpoint. We find that the dimming areas are typically larger for off-limb observations while the total extreme ultraviolet intensity decrease is similar (c ∼ 0.6). Parameters describing the total dimming extent, i.e., off-limb area and total brightness, strongly correlate with the CME mass (c ∼ 0.7–0.8). The derivatives of these parameters show a high correlation with the CME speed (c ∼ 0.6). In addition, we studied whether the coronal dimmings can be used to identify a predominant direction of the CMEs. Using the same set of events, we estimated the dimming growth direction and compared it with modeling results of the CMEs propagation, derived from existing GCS reconstructions and provided in catalogs. Our ﬁndings suggest that coronal dimmings have the potential to provide early estimates of the Earth-directed CMEs parameters, relevant for space weather forecasts, for satellite locations at both L1 and L5.

Speaker: Galina Chikunova (Skolkovo Institute of Science and Technology)
• 160
Simulation of thermal sub-THz emission from solar flares

Since the beginning of this century it became possible to make observations of solar flares in subterahertz (sub-THz) frequency range at a few frequencies in the range of 100-400 GHz. Within these observations some of M- and X-class solar events had a spectral (sub-THz) component that grew with frequency. To understand the origin of this phenomenon, we simulated the plasma density and temperature evolution in a flare loop, caused by interaction between the injected accelerated electron beams, which travel from the reconnection site in the region of the coronal loop-top down to lower layers of the solar atmosphere. The numerical code FLARIX used for simulations allowed us to define the dynamics of the flare plasma parameters at different heights, based on the equations of radiation hydrodynamics. Using the simulation results, we inferred the thermal bremsstrahlung emission in the sub-THz and X-ray ranges. The comparison of the obtained simulation results with observational data is under discussion. This work was partly supported by the RFBR (N20-52-26006), the Ministry of Education and Science (Research Work No. 0831-2019-0006), RVO: 67985815, project LM2015067: EU-ARC.CZ, 21-16508J of the Grant Agency of the Czech Republic.

Speaker: Galina Motorina (Astronomical Institute of the Czech Academy of Sciences)
• 161
Solar type III radio burst fine structure from Langmuir wave motion through turbulent plasma

The Sun frequently accelerates near-relativistic electron beams that travel out through the solar corona and interplanetary space. Undergoing wave-particle interactions with Langmuir waves, these beams are the driver for type III radio bursts, the brightest radio bursts produced by the Sun. The formation and motion of type III fine frequency structures is a puzzle but is commonly believed to be related to plasma turbulence in the solar corona and solar wind. Combining a theoretical framework with kinetic, wave-particle simulations and high-resolution radio type III observations, we quantitatively show that the fine structures are caused by the moving intense clumps of Langmuir waves in a turbulent medium. Our results show how type III fine structure can be used to remotely analyse the intensity and spectrum of compressive density fluctuations, and can infer ambient temperatures. This new plasma diagnostic for the solar corona and solar wind is at distances from the Sun where these properties normally cannot be measured, and significantly expands the current potential of solar radio emission.

Speaker: Hamish Reid (University College London)
• 162
Blowout expansion of a coronal mass ejection and subsequent filament eruption

We present a comprehensive multi-wavelength analysis of a coronal mass ejection (CME) associated with a M3.2 flare and filaments eruption on 8 January 2014 from the active region NOAA 11947. Observations from the AIA 171 A images reveal the origin of pre-CME arcade $\approx$ 1 hr prior to the eruptive events. After formation, the pre-CME arcade undergo through slow rise evolution with a speed $\approx$ 3 km s$^{-1}$. Subsequently, a typical arcade-to-bubble" transformation is observed, providing the evidence of CME in the form of bubble at lower coronal heights. Differential emission measure analysis suggests the presence of multi-thermal plasma in the bubble structure and strong plasma heating at the core of the active region. Further, the bubble undergo through a blowout expansion with a speed of $\approx$ 420 km s$^{-1}$. The blowout expansion of CME is accompanied with gradually varying EUV and X-ray emissions from the hot core and multiple type III radio bursts, indicating the magnetic reconnection as a possible triggering mechanism for CME. During the impulsive phase, we observed compact hard X-ray sources at energies up to $\approx$ 50 keV from its source region. A temporal correlation between the blowout expansion of the CME and enhanced X-ray fluxes suggests a feedback association between kinematical evolution of CME and impulsive phase of the flare. With the impulsive phase, the activation and subsequent eruption of two successive filaments take place. From these observations, we propose that CME in association with filaments eruption is a result of successive magnetic reconnections.

Speaker: Hema Kharayat (USO/PRL, Udaipur)
• 163
The Most Energetic GOES/XRS Flares

The soft X-ray photometric records from the GOES/XRS instruments
now approach a half century of coverage, and constitute the most
complete space-based proxy for solar flare X-ray luminosities and
energies. Observations continue with the GOES-16 and -17 and
forthcoming satellites, which use a slightly different detector
technology. Some of the most powerful earlier events saturated the
telemetry range of their ionization-chamber detectors, making it
difficult to study the event distribution function at its most
interesting top end. Over the interval 1976--2015 a total of 11
events saturated the key 1--8 Achannel, many of which were from
NOAA region 6659 in 1991. We have devised an algorithm for best
estimates of the peak fluxes for these saturated events. Based on
this new input, and our understanding of the calibration issues for
GOES-15 and earlier, we discuss the event distribution function
(e.g. Aschwanden & Freeland, 2012) looking for evidence of a
downward break in the event distribution in the vicinity of magnitude
X10. Note that the absolute calibration of this level is under
investigation at present, but this does not concern the relative
magnitude distribution. Any departure from the power law will
crucially affect our understanding of superflare'' occurrence on
the Sun.

Speaker: Hugh Hudson (UC Berkeley / U of Glasgow)
• 164
Spatial and temporal features in microwave and ultraviolet emission for eruptive and confined events

Solar flares and coronal mass ejections (CME) are the most powerful manifestations of solar activity. Both phenomena associated with the evolution of the spatial structure of the magnetic field of active regions (AR). It is known that not all powerful flares accompanied by CME. In some cases, CME are observed, associated with very low intensity bursts. At the same time the observational signs that determine the ability of AR to cause the eruption of matter from AR into the high layers of the solar corona are still not clear. This makes it difficult for us to understand the eruption initiation physical mechanism (CME trigger). The purpose of this work is to search for observational signs of the onset of eruptive process. For this, we conducted a comparative analysis of pre-flare and flare conditions for flare events, accompanied by CME, and events not accompanied by CME. We studied the features of the spatial dynamics of the microwave and ultraviolet emission (data from the Nobeyama Radioheliograph and SDO/AIA) of AR for selected events.

Speaker: Dr Irina Bakunina (National Research University Higher School of Economics)
• Poster Session 4.6
Convener: Istvan Ballai (University of Sheffield)
• 165
On the Drag Parameter Probability Distribution Function for Coronal Mass Ejection propagation models

Interplanetary Coronal Mass Ejections (ICMEs) are among the main drivers of Space Weather and are responsible for the strongest variations in the near-Earth solar-wind conditions. Forecasting the Time of Arrival and Speed of Arrival of an ICME more than an hour ahead is a rather complicated task, since it means propagating a poorly determined plasma and magnetic field structure into an essentially undetermined interplanetary environment.
The Drag-Based Model, despite its simplicity, is still among the most used models to simulate the propagation of an ICME in the heliosphere.
To model the interaction of the ICME with the background solar wind it uses the parameter γ, which is the parameter that modulates the coupling between the solar wind fluid and the ICME body.
Since this parameter incorporates much of the physics of the ICME-wind interaction and its precise value is poorly understood, we think it deserves further investigation.
We built a database with a large number of ICMEs to create a new empirical Probability Distribution Function for γ and to find a suitable functional form to model it.

Speaker: Gianluca Napoletano (University of Rome "Tor Vergata")
• 166
Effects of the chromospheric Lyα line profile shape on the determination of the solar wind outflow velocity using the Doppler dimming technique

The determination of solar wind outflow velocity is fundamental to probe the mechanisms of wind acceleration in the corona. Using the Doppler dimming technique, we studied the effects that the chromospheric Lyα line profile shape causes on the determination of the outflow speed of coronal HI atoms. Starting from UV observations (SOHO/UVCS) of the coronal Lyα line and simultaneous measurements of pB (LASCO/SOHO and Mk3/MLSO), we analysed the impact of the pumping chromospheric Lyα line profile through measurements from SOHO/SUMER, UVSP/SMM and LPSP/OSO-8, taken from representative on-disk regions and as a function of time during the solar activity cycle. In particular, we considered the effect of four chromospheric line parameters: line width, depth of the central reversal, asymmetry and distance of the peaks. We find that the range of variability of these parameters is of about 50% for the width, 69% for the depth of the central reversal, 15% for the asymmetry, and 50% for the distance of the peaks. We derive that the variability of the pumping Lyα profile affects the estimates of the coronal HI velocity by about 5-10%. Therefore, this uncertainty is smaller than other physical quantities uncertainties, and a constant in time and unique shape of the Lyα profile over the solar disk can be adopted to estimate the solar wind outflow velocity.

Speaker: Giuseppe Emanuele Capuano (Istituto Nazionale di Astrofisica (INAF))
• 167
No evidence of a solar diameter change during the Maunder Minimum from measurements made in the Basilica of San Petronio

We analyze the measurements of the solar diameter made at the Basilica of San Petronio (Bologna, Italy) from 1655 to 1736 using a meridian line. This series includes the Maunder Minimum, a period of abnormally low solar activity between 1645 and 1715. Some authors have suggested an increase of the solar diameter during the Maunder Minimum. We have applied statistical analyses to compare the solar diameter measured at San Petronio during the Maunder Minimum (1655–1715) and that in a subsequent period (1716–1736). No statistically significant differences are found in the medians and averages of the solar diameter in both periods. In fact, we have found differences around 0.6ʺ, which are below the mean accuracy of the instrument. Therefore, we conclude that there is no difference between the solar diameter value measured during the Maunder Minimum (1655–1715) and that for the subsequent period (1716–1736).

• 168
DOUBLE PROMINENCE ERUPTION OBSERVED BY SOLAR ORBITER AND PSP

A pair of off-limb eruptions was observed by the Extreme Ultraviolet Imager (EUI) onboard Solar Orbiter (SolO) on 2021 April 23/24. At the time, the spacecraft was at 0.87 AU from the Sun. As seen from Earth, SolO was roughly near the Sun’s east limb (as seen from the Earth) and in quadrature with Parker Solar Probe (PSP), which was at the Sun’s far side. The eruptions are remarkably well observed in the Full Sun Imager (FSI) in both its EUI/FSI174 and EUI/FSI304 channels. The first eruption starts as a slowly growing prominence eruption culminating in an apparent untwisting and ejection from April 23 22:25 UTC onwards. This eruption is immediately followed by a yet more spectacular and faster eruption from the same source region less than 4 hours later. The absence of any corresponding signature in Earth-bound datasets confirms this eruption pair is back-sided from the Earth. STEREO-A COR2 observations confirm that the prominence eruptions observed by EUI/FSI correspond to a complicated interaction of 2-3 coronal mass ejections. In this paper we will use the favourable quadrature orientation of SolO and PSP to link SolO's extreme ultraviolet and coronagraphic observations with PSP's off-limb and in-situ observations. We will highlight the lessons learned for upcoming PSP and Solar Orbiter quadratures such as in February 2022.

Speaker: David Berghmans (Royal Observatory of Belgium)
• 169
Mining valuable data in observations of solar active regions

We implemented multiple data analysis and machine learning algorithms to extract information of solar active regions that can be used in the future to perform CME and flare forecasting.

We use the data set produced by Angryk et al. (Sci.Data,2020) containing 51 magnetic field parameters and the associated X-ray activity of solar active regions from 2010 to 2018. We performed a data reduction by eliminating redundant parameters in each entry. The reduction was performed using a combination of Common Factor Analysis (CFA) and Principal Component Analysis (PCA). This reduced the mount of data to 5 parameters. To increase the separability of different types of active regions we projected all the points to a 7-dimensional manifold using Sparse Autoencoders (SAE). We then performed supervised and unsupervised classifications of the reduced parameters of each active region. We demonstrate that it is possible to differentiate flaring from non-flaring active regions. We also show that there are tenuous differences between active regions with different flaring activity. However, the current data is not sparse enough to allow a clear differentiation between the different levels of flare activity.

We propose an alternative parametrization of solar active regions. Using Disentangled Variational Autoencoders (beta-VAE) we produce a different representation of the active regions which can be used for classification. We show that this method can be used also to create new unseen artificial active regions to study the evolution of flaring activity, or to solve the problem of data imbalance.

Speaker: Jorge Amaya (KU Leuven, Mathematics Department, CmPA)
• 170
Do conditions of the chromosphere and corona foretell active region flaring?

Solar flares are powered by the evolution of the magnetic field, but it is still impossible to deterministically predict whether an active region will flare or not solely based on photospheric information. Observational case studies of the upper solar atmosphere reveal increased levels of magnetic reorganization, dynamics, and temperature variation prior to solar flares. Whether such signatures play a physical role in event initiation, and therefore could improve flare prediction, is still unclear. To this aim, we statistically analyze the coronal and chromospheric conditions prior to solar flares and during flare-quiet periods using data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory. AIA Active Region Patches (AARPs), region-targeted extractions of AIA time-series data in (extreme-) ultraviolet are matched to the HMI Active Region Patches (HARPs) for 2010-2018. Pre-event dynamics and heating is parametrized with high-order moment summaries of brightness and running-difference images, plus emission measure, temperature, and density images; temporal behavior is captured by sampling each parameter over a 7hr time-series. NorthWest Research Associates’ Classification Infrastructure (NCI), a well-established statistical classifier system based on Non-Parametric Discriminant Analysis is used to statistically evaluate whether parameters describing the upper atmosphere differ significantly between flaring-imminent vs. flare-quiet populations. Preliminary results and their physical implications will be presented. AARPs are constructed daily, from 15:48-21:48 UT (13 min intervals each hour, time cadence 72s) and will be freely available with this study's publication at www.nwra.com/AARP. This work is supported by NASA Grant HGI/80NSSC19K0285.

Speaker: Dr KD Leka (NWRA)
• 171
Solar flares and their impacts on the Earth’s lower ionosphere.

Solar flares have direct consequences on the Earth’s ionosphere. The enhancement of extreme ultraviolet (EUV) and X-ray emission during a flare results in the rapid increase of ionization on the entire dayside ionosphere causing a sudden ionospheric disturbance (SID). While SIDs occur in all regions of the ionosphere, the effect on the lowest-lying D-region (60-100km in altitude) is the most apparent. In particular, X-rays with wavelengths < 10 angstroms penetrate down to D-region altitudes and dominate photoionization of all neutral constituents there. This can cause substantial electron density increases in the lower ionosphere to extents large enough to affect the propagation of radio waves used in high-frequency radio telecommunications. Here, by using a combination of solar flare multi-wavelength observations (X-ray and EUV) together with remote sensing of the ionospheric conditions using very low frequency (VLF: 3-30kHz) radio wave propagation, we probe the geophysical effects of flare emission. This work presents a statistical study of flares from the past solar cycle and their impacts on the ionospheric D-region electron density. We look at the relationship between the incident X-ray flux and the D-region response including amplitude variations, heliographic position of flare, relative time-delay analysis and select several events to further investigate the spectral components of a flare and their geophysical impact. We will also discuss the opportunities looking towards solar cycle 25 with both ground-based remote sensing observations of the ionospheric conditions that can be combined with newly available space-based solar which can be utilized in future studies.

Speaker: Laura Hayes (Dublin Institute of Advanced Studies (DIAS))
• 12:31 PM
Lunch break
• Plenary 3
Convener: Ioannis Kontogiannis (Leibniz-Institute for Astrophysics Potsdam (AIP))
• 172
What drives the hot solar corona?

The outer atmosphere of the Sun, the corona is comprised of tenuous, highly ionized plasma, that is governed by magnetic fields and is heated to more than a million Kelvin. Such hot coronal plasma is thought to be powered by numerous impulsive heating events called nanoflares. What drives these impulsive nanoflares? What role does magnetic field play in coronal heating? We address these long-standing questions through multi-wavelength observations of the Sun that span from the photosphere through the corona. In this talk, we will present new results that reveal an intricate link between the impulsive coronal heating and the evolution of magnetic fields at the solar surface. In particular, we will discuss the role of magnetic reconnection, a process through which magnetic energy is liberated, in the heating of the solar corona.

Speaker: Lakshmi Pradeep Chitta (Max Planck Institute for Solar System Research)
• 173
Solar Orbiter/EUI very wide field observations of the EUV corona.

At 3.8°, the field of view (FOV) of the Full Sun Imager (FSI) on Solar Orbiter is by far wider than that of any previous solar EUV imager. Depending on the distance of the probe to the Sun along its orbit, this corresponds to 14 to 4 solar radii, to be compared to the 3.5 Rs of STEREO/EUVI or Proba2/SWAP. This very large field of view opens up a new discovery space into a region largely unexplored in the EUV. Since it was expected that stray-light would dominate beyond 2 Rs, a moveable occulting disk can be inserted in the optical path to block light rays up to 0.78° off the optical axis. On March 21 2021, at 0.51 AU, FSI acquired deep exposures at 17.4 and 30.4 nm with the occulting disk in place. The data reveals solar structures extending up to 5 Rs which, to our knowledge, is the furthest ever recorded at these wavelengths. We compare the morphology of the observed structures with close in time observations in white light by the Metis coronagraph. We present a comparison of the measured signal fall-off as a function of distance to Sun-center with a model of coronal emission taking into account collisional excitation and resonant scattering.

Speaker: Frédéric Auchère (Institut d'Astrophysique Spatiale)
• 174
On the dynamics of vortices in the solar atmosphere

Small scale vortices in the solar atmosphere have received increasing attention in recent years. They are ubiquitous at photospheric and chromospheric levels and are a viable candidate mechanism for the exchange of waves, energy, and mass with the upper solar atmosphere.
In order to identify and study the dynamics of these vortices, we seek a suitable mathematical criterion for which an evolution equation exists. The vorticity and vorticity equation would be the classical options for this task. However, it is now well known that they can be biased by shear flows. Therefore, we adopt the swirling strength criterion, a generalisation of the vorticity, for which we derived an evolution equation from the basic equations of (magneto)hydrodynamics. We suggest the swirling strength and its dynamical equation as a novel tool for the analysis of vortex dynamics in numerical simulations.
We apply this novel tool to realistic numerical simulations of a small portion of the solar atmosphere realized with the radiative magnetohydrodynamic code CO5BOLD. We find a tight relation between unidirectional, vertical swirling motions and torsional magnetic field perturbations, which propagate upward at local Alfvén speed and are driven by magnetic tension forces. All together, these are clear characteristics of torsional Alfvén waves in the form of compact wave-packets or unidirectional pulses. These pulses naturally arise from a self-consistent numerical simulation of the solar atmosphere and we estimated the mean Poynting flux at the bottom of the chromosphere.

Speaker: José Roberto Canivete Cuissa (IRSOL)
• 175
Curved coronal arcades: proliferation and heating of an MHD avalanche

MHD avalanches involve small, intensely localized instabilities that spread across neighbouring regions in a magnetic field.
Cumulatively, many small events release vast amounts of stored magnetic energy.
Straight cylindrical flux tubes, in Parker (1972)'s model of coronal loops, are liable to such avalanches: one unstable flux tube can cause instability to proliferate through reconnection, resulting in an ongoing chain of like events.
True coronal loops are curved, arching between different footpoints on one photospheric plane.
Using three-dimensional MHD simulations, we here verify the viability of MHD avalanches within the curved magnetic geometry of a multi-threaded coronal arcade.
In contrast to the behaviour of straight cylindrical models, a kink mode occurs more readily and preferentially upwards in this new geometry.
Such instability spreads over a region wider than the original flux tubes, and arguably wider than is seen in a model of straight flux tubes.
Consequently, the release of substantial amounts of energy is sustained, in a series of nanoflare-type events, contributing significantly to coronal heating.
Overwhelmingly, viscous heating dominates over Ohmic, attributable to the shocks and jets produced around these small events.
Reconnection is not the greatest contributor to heating, but rather the facilitator of those processes that are.
Localized and intermittent, the heating shows no strong spatial preference, except for a small bias away from footpoints.
Effects of realistic plasma parameters, and the implications for one-dimensional, loop-aligned models of energetic transport, are discussed.

Speaker: Dr Jack Reid (University of St Andrews)
• 176
Photospheric and coronal sources of the solar wind observed by PSP and Solar Orbiter: topological links, spatial and temporal periodicities

Parker Solar Probe (PSP) and Solar orbiter have made a number of important discoveries in its exploration of the inner heliosphere/outer corona. Their observation of ubiquitous large amplitude Alfvénic fluctuations, regardless of solar wind speed, in all wind streams except for narrow areas surrounding the heliospheric current sheet, together with large s-shaped inversions of the magnetic field, called switchbacks, begin to call into questions standard ideas of solar wind acceleration. Initial Parker Solar Probe results have shown that slow Alfvénic solar wind intervals appear to be a frequent, if not standard, component of the nascent solar wind inside 0.5 AU. In addition to the strong presence of Alfvénic fluctuations propagating away from the Sun, such intervals also display the huge oscillations - switchbacks, where the Alfvénic fluctuation is accompanied by a fold in the radial magnetic field and a corresponding forward propagating radial jet. Switchbacks often come in patches, separated by short intervals depleted with fluctuations, and periods without switchbacks may also show a striking quiescence, with the magnetic field remaining mostly radial and very small amplitude velocity and magnetic field fluctuations. These observations pose a series of questions on the origins of the solar wind and the role of coronal structure, as well as of the evolution of fluctuations within the solar wind. Here we discuss how the sources of the solar wind measured in situ are related to photospheric and solar magnetic network structures.

Speaker: Olga Panasenco (Advanced Heliophysics Inc.)
• 3:20 PM
Coffee break
• Plenary 4
Convener: Andrew Hillier (University of Exeter)
• 177
Coronal spectroscopy of pre-flare and active regions

Spectroscopy of active regions reveals details of physical processes through measurement of a variety of plasma parameters. The Hinode EUV Imaging spectrometer (EIS) and the NASA IRIS instruments continue to provide spectroscopy from the chromosphere through to the corona. In 2020 the Solar Orbiter mission launched with the SPICE spectrometer onboard. Plans are well underway for a new spectrometer on the next Japanese solar mission, Solar-C. In this review, we will look at results on the behaviour of active regions in non-flaring states and in the build-up to flares. Plasma diagnostics have been used to probe how the active regions loops stay hot and how the flows seen at the edges of active regions can contribute to the slow solar wind - and indeed create energetic particles. In the pre-flare phase, measurements have been made that indicate an increase in non-thermal velocity before the flare begins. We discuss these results and look towards the future of coronal spectroscopy.

Speaker: Louise Harra (PMOD/WRC and ETH-Zurich)
• 178
Observational and numerical characterization of a wave-like front propagating along a coronal fan

On 2011 July 6 EUV channels of AIA instrument onboard SDO detected a recurrent, arc-shaped intensity disturbance over an active region. The intensity disturbance fronts were observed to propagate along a coronal loop bundle rooted in a small area of the dark umbra of the sunspot. Neither signatures of flare activity nor of a coronal mass ejection event were observed in association with the phenomenon. Analysis of EUV wavelengths reveals that the fronts are accelerated in 171 Å and propagate with a projected, averaged plane-of-sky phase velocity of about 60 km/s while for the other coronal channels the values are about 40 km/s. All the channels exhibit a periodic recurrence with a period of about 3 minutes in the umbra and longer periods towards the penumbra. To shed light on the physical nature of the event, we perform 2D numerical simulations based on a simple symmetrical potential magnetic field configuration embedded in a gravitationally stratified atmosphere in hydrostatic equilibrium. We perturb the atmosphere using a driver located below the photosphere with a period of 3 min. We compare the kinematical properties of the event as observed at the different temperature regimes covered by the SDO/AIA images with the results from the numerical simulations. The speed values obtained from numerical simulations are similar to those estimated from observations for 171 Å. The analysis suggests that the accelerated profile seen on 171 Å is due to a projection effect and responds to the slow magnetoacoustic propagating wave scenario.

Speaker: Valeria Sieyra (KU Leuven)
• 179
Kink instability of jets in the solar atmosphere

We studied the kink instability of triangular jets sandwiched between magnetic tubes/slabs and its possible connection to observed properties of the jets in the solar atmosphere. A dispersion equation governing the kink perturbations is obtained through matching of analytical solutions at the jet boundaries. The equation is solved analytically and numerically for different parameters of jets and surrounding plasma. The analytical solution is accompanied by a numerical simulation of fully nonlinear MHD equations for a particular situation of solar type II spicules. MHD triangular jets are unstable to the dynamic kink instability depending on the Alfven Mach number (the ratio of flow to Alfven speeds) and the ratio of internal and external densities. Jets with an angle to the ambient magnetic field have much lower thresholds of instability than field-aligned flows. Growth times of the kink instability are estimated as 6-15 min for type I spicules and 5-60 s for type II spicules matching with their observed life times. Numerical simulation of full nonlinear equations shows that the transverse kink pulse locally destroys the jet in less than a minute in the conditions of type II spicules. Dynamic kink instability may lead to full breakdown of MHD flows and consequently to observed disappearance of spicules in the solar atmosphere.

Speaker: Teimuraz Zaqarashvili
• 180
Direct observational indications of wavelike disturbances in MBP tracks

Magnetic Bright Points (MBPs) are small-scale solar atmospheric structures of highly concentrated magnetic fields that can be studied in ever more details thanks to current high precision measurements and a strong theoretical framework greatly supported by Magnetohydrodynamic (MHD) simulations and its analysis. In this study, we consider a large dataset taken from the HINODE mission from two consecutive solar atmospheric layers and analyze how MBPs evolve over time. We developed new computational techniques to track these small-scale structures and reconstruct their behavior during their lifetime. We found direct evidence for MHD wavelike disturbances within the evolution of their movement paths. We interprete these detected signatures within the MBP tracks as Kink and Sausage wave contributions which can effectively transport energy into the higher atmosphere and thus shedding light on how energy can be transferred throughout the solar atmosphere via the help of flux tubes as wave guides.

• 181
Whistler instability driven by the sunward electron deficit in the solar wind: High-cadence Solar Orbiter observations

Solar wind electrons exhibit complex electron velocity distribution functions (VDFs) shaped by expansion, Coulomb collisions, and field-particle interactions. We investigate how the suprathermal electron deficit in the anti-strahl direction, which was recently discovered in the near-Sun solar wind, drives a kinetic instability and creates whistler waves with wave vectors that are quasi-parallel to the direction of the background magnetic field.

Our study is based on high-cadence measurements of electron pitch-angle distribution functions combined with measurements of electromagnetic waves provided by Solar Orbiter during its first orbit.

The suprathermal electron deficit creates kinetic conditions under which the quasi-parallel whistler wave is driven unstable. We directly test our predictions for the existence of these waves through solar wind observations. We find whistler waves that are quasi-parallel and almost circularly polarised, propagating away from the Sun, coinciding with a pronounced suprathermal electron deficit in the electron VDF. The cyclotron-resonance condition is fulfilled for electrons moving in the direction opposite to the direction of wave propagation, with energies corresponding to those associated with the suprathermal electron deficit.

We conclude that the suprathermal electron deficit acts as a source of quasi-parallel whistler waves in the solar wind. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.

Speaker: Laura Bercic (Mullard Space Science Laboratory, UCL)
• 5:00 PM
Coffee/Tea break
• ESPD Business meeting - all welcome

ESPD Board report
ESPD Board election results

Convener: Tiago Pereira (Institute of Theoretical Astrophysics, University of Oslo)
• Wednesday, September 8
• Poster Session 5.1
Convener: Etienne Pariat (LESIA, Observatoire de Paris)
• 182
Historical Total Solar Irradiance Reconstruction: a new approach

It is well established that the total solar irradiance (TSI) varies on
timescales of minute to centuries. On timescales of minutes to hours
the TSI varies due to the globally-averaged superposition of solar
turbulent convection and oscillations, while on solar-cycle and
solar-rotation timescales the majority of the TSI-amplitude
fluctuations are the result of opposing brightenings caused by
faculae/plages and shorter-duration dimmings caused by sunspots. While
TSI variations from minutes to a few decades have been continuously
monitored from space since the late 1970s, TSI variations over much
longer periods of time can only be estimated using either historical
observations of solar surface magnetic features, namely sunspots and
plages, possibly supported by surface flux transport models, or from
the measurements of the cosmogenic isotope (i.e., 14C and 10Be)
concentrations in tree rings and ice cores.

In this work we present a new approach to reconstruct the TSI
variability from the pre-industrial era to the present using component
analysis of time series of historical observations of plages and
sunspots, and the secular trend of the solar potential modulation.

Speaker: Matteo Cantoresi (Dipartimento di Fisica - Università di Roma Tor Vergata)
• 183
Observational study of the solar convective pattern in the vicinity of sunspots during their decaying phase.

The solar photosphere is characterized by the presence of several structures being sunspots the most visible manifestation of the magnetic field immersed in the convective plasma. The study of such active regions in the photosphere includes the analysis of formation, growth and decay of sunspots. These evolutionary processes are directly related to phenomena of solar activity occurring at upper atmospheric layers. High-resolution solar observations have made it possible to study in detail the dynamics of the solar plasma at different spatial and temporal scales, in particular, through time series of images. Sheeley (1969) first reported radial high-speed flows in the convective pattern around sunspots and called it "moat flow". The study of the moat flow origin is crucial to understand its connections with the structure of the sunspot, for example, the penumbra and their different evolutionary stages. Previous studies have reported links between the moat flow, the Evershed flow and small-scale magnetic elements called Moving Magnetic Features (MMFs). In this work, local correlation tracking techniques are employed over time series of solar images acquired with the Solar Dynamics Observatory in the continuum, in order to study proper motions in the plasma granular pattern in the vicinity of sunspots during the decaying phase and characterize the endmost evolution of plasma flows in the active regions.

Speaker: Valeria Quintero Ortega (Universidad Nacional de Colombia)
• 184
Electron thermal escape in the Sun

Magnetic field vector observations in the solar photosphere have generally revealed a non-zero value of the divergence: the vertical field component gradient is found on the order of 3 G/km when the horizontal field component gradient is of 0.3 G/km only. This has first to be assigned to the fact that the measured quantity is the magnetic field H, which is related to the divergence-free magnetic induction B by the law B=µ0(H+M), where M is the magnetization. In plasmas like the solar photosphere, magnetization results from plasma diamagnetism and spiral movement of charged particles about the magnetic field. It can be observed that in the solar interior the electron thermal velocity is much larger than the escape velocity from both gravity and proton attraction. A model of this is presented in the related paper (Bommier, V., "Solar photosphere magnetization", 2020, A&A, 634, A40). The electrons escape from lower layers in a quasi-static spreading, and accumulate in the photosphere. Therefore, the electron density at surface is increased but decreases with height at surface, which enables the observed values because divH = -divM. Such a structure is probably at play in the solar-type stars. Besides, solar magnetohydrodynamics must include the fact that the quantity measured by Zeeman effect is H (4 demonstrations in the paper), which is related to the electric current j by the Maxwell relation rotH = j, where j is itself submitted to the Lorentz force j x B, where B is the divergence-free magnetic induction.

Speaker: Véronique Bommier (Observatoire de Paris)
• 185
Subcritical dynamo and hysteresis in a Babcock-Leighton type kinematic dynamo model

A large-scale magnetic cycle is possible in the Sun and other stars as long as the large-scale shear and helicity of the plasma flow in their convection zones are sufficiently strong. Hence, there is a critical dynamo number for each star for the operation of a large-scale dynamo. As a star spins down, it is expected that the large-scale dynamo ceases to operate above a critical rotation period. Our study explores the possibility of the operation of the dynamo in the subcritical region using the Babcock-Leighton type kinematic dynamo model. In some parameter regimes, we find that the dynamo shows hysteresis behavior, i.e., two dynamo solutions are possible depending on the initial parameters---decaying solution if started with weak field and strong oscillatory solution (subcritical dynamo) when started with a strong field. However, under large fluctuations in the dynamo parameter, the subcritical dynamo mode is unstable in some parameter regimes. Therefore, our study supports the possible existence of subcritical dynamo in some stars which was previously shown in a mean-field dynamo model with distributed $\alpha$ and MHD turbulent dynamo simulations.

Speaker: Vindya Vashishth (Indian Institute of Technology, Varanasi)
• 186
A new generation of Babcock-Leighton solar dynamo model

The Babcock-Leighton (BL) mechanism, in which the poloidal field is produced by the emergence and subsequent dispersal of sunspot groups, has been received more and more observational evidence during the past decade. Most BL-type dynamo models assume that the toroidal field is generated in the tachocline. However, recently the importance of the tachocline has been questioned by magnetic activity of fully convective stars. We aim to develop a new generation of BL-type dynamo model, in which the dynamo operates mainly within the convection zone. We introduce the near-surface turbulent pumping into the BL-type dynamo model with a vertical upper boundary condition. Other ingredients include solar-like internal differential rotation, a single-cell meridional flow with penetration depth of 0.7R, and the magnetic diffusivity of $5.0\times10^{11}cm^{2}s^{-1}$. The pumping causes the poloidal field within the convection zone to be almost in the purely latitudinal direction. Due to the latitude shear in the bulk of the convection zone, most of the toroidal field is generated there. Our model reproduces the basic properties of the solar cycle, including a) 22 years magnetic cycle; b) equatorward propagation of the activity belts; and c) phase difference between the activity belts and polar field. The results do not depend on the existence of the tachocline or not. The near-surface pumping enhances the coupling of the poloidal field between the northern and southern hemispheres to reproduce solar-like dipolar parity.

Speaker: Zebin Zhang (Beihang university, Beijing, China)
• 187
Algebraic quantification of an active region contribution to the solar cycle

The solar dipole moment at cycle minimum is considered to be the most reliable precursor to determine the amplitude of the subsequent cycle. Numerical simulations of the surface flux transport (SFT) model are widely used to effectively predict the dipole moment. An algebraic method was recently proposed to quickly predict the contribution of an active region (AR) to the axial dipole moment at cycle minimum instead of SFT simulations. The method assumes a bipolar magnetic region (BMR) configuration of ARs, however most ARs are asymmetric in configuration of opposite polarities, or have more complex configurations. Such ARs evolve significantly differently from those of BMR approximations. We propose a generalized algebraic method to describe the axial dipole contribution of an AR with an arbitrary configuration, and evaluate its effectiveness compared to the BMR-based method by comparing its results with SFT simulations of observed ARs and artificial ARs. We also compare the results with those from the BMR-based method. The generalized method is equivalent to the SFT model, and precisely predicts the contributions of ARs to the dipole moment, but has a much higher computational efficiency. Although the BMR-based method has similar computational efficiency to the generalized method, it is only accurate for symmetric bipolar ARs. The BMR-based method systematically overestimates the dipole contributions of asymmetric bipolar ARs, and randomly miscalculates the contributions of more complex ARs. The generalized method quick and precisely quantifies an AR's contribution to solar cycle evolution, which paves the way for application in physics-based solar cycle predictions.

Speaker: Zi-Fan Wang (National Astronomical Observatories, CAS)
• 188
Solar prominence diagnostics from non-LTE modeling of Mgii h&k line profiles

We investigate a new method to obtain the plasma parameters of solar prominences observed in the Mg II h&k spectral lines by comparing line profiles from the IRIS satellite to a bank of profiles computed with a one-dimensional non-LTE radiative transfer code. The prominence observations were carried out by the IRIS satellite on 19th April 2018. Using a grid of 1007 one-dimensional non-LTE radiative transfer models, some including a prominence-corona transition region (PCTR), we are able to recover satisfactory matches in areas of the prominence where single-line profiles are observed. Large values of ionization degree are found by the procedure in areas where the line of sight crosses mostly plasma from the PCTR, correlating with high mean temperatures and correspondingly no Hα emission. The models were unable to recover satisfactory fits in the regions where we see Hα emission. This is due to the complex line shapes manifesting from many unresolved independently moving threads. This issue might be solved in future by increasing the microturbulent velocities in the models to simulate these unresolved movements.

This new method naturally returns information on how closely the observed and computed profiles match, allowing the user to identify areas where no satisfactory match between models and observations can be obtained. The inclusion of the PCTR was found to be important, as regions where satisfactory fits were found were more likely to contain a model encompassing a PCTR.

Speaker: Mr Aaron Peat (The University of Glasgow)
• Poster Session 5.2
Convener: D. Shaun Bloomfield (Northumbria University)
• 189
Diagnostics of the off-limb solar corona using EUV and forbidden lines

We report on diagnostics of the off-limb solar corona using coordinated observations of EUV allowed lines together with the coronal forbidden lines. We show that the electron densities obtained from the Fe XIII line ratios observed by both Hinode/EIS and COMP are in good agreement once the photoexcitation and background subtraction are both accounted for. In addition, the Hinode/EIS observations of Fe XII indicate that the 195.1 A line has an anomalous width and is likely optically thick. The Fe XII 1349 A forbidden line observed by IRIS shows enhanced intensities in the active region spectra, but not in the quiet Sun. These observations can be explained by presence of high-energy electrons in active region corona.

Speaker: Jaroslav Dudík (Astronomical Institute of the Czech Academy of Sciences)
• 190
Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere

Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun, also known as quiet Sun Ellerman bombs (QSEBs), are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the lower solar atmosphere. We analyze high spatial resolution observations (0.″1) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the Hβ line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the field of view for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier Hα observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightenings found in the Hβ line wings also persist in the line core with a temporal delay and spatial offset toward the nearest solar limb. The observed QSEBs vary significantly in their properties, such as lifetime, brightness, and size. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the lower solar atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.

Speaker: Jayant Joshi (Rosseland Centre for Solar Physics, University of Oslo, Norway)
• 191
The Solar Line Emission Dopplerometer (SLED), a state-of-the-art imaging spectrograph for the dynamics of solar coronal structures

Analysis of the dynamics of the hot coronal plasma is the most promising method
to examine the contribution of wave-like phenomena in the global heating of the solar corona.
We present here a new, state-of-the art instrument for imaging spectroscopy:
the Solar Line Emission Dopplerometer (SLED). It is based on the Multi-channel
Subtractive Double Pass (MSDP) principle, which combines the advantages of
filters and slit spectrographs. The SLED (presently under construction) will observe
coronal structures in the forbidden lines of FeX 637.4 nm and FeXIV 530.3 nm. It will
measure Doppler shifts up to 150 km/s with high precision (50 m/s) and fast cadence (1 Hz),
over a 1000" x 150" rectangular FOV (for six meter telescope’s focal length).
The SLED is optimized to detect high-frequency wave-like plasma motions
which could be the signatures of the coronal heating processes and allows
studies of the dynamics of fast evolving events. A numerical simulation of
observations is shown to demonstrate the capabilities of the instrument.
The regular observations will be performed with the high-altitude coronagraph
at Lomnicky Stit Observatory (LSO), and during total solar eclipses.

Speaker: Dr Jean-Marie Malherbe (Observatoire de Paris)
• 192
A study of plasma β in Quiet Sun: multi-instrument view

A joint campaign of various space-borne and ground-based observatories, comprising the Hinode-SOT, IRIS, EIS (HOP 381, 10 – 22 October 2019), and the GREGOR Solar Telescope, investigated Quiet Sun regions for inferring the plasma β at photosphere, transition region, and corona. This campaign provided co-spatial and co-temporal observations, which can provide values of the magnetic field, temperature, and density in the solar atmosphere. They can help us to complete a more detailed depiction of the plasma β with height. We present the preliminary results of coordinated multiwavelength observations. Temperature estimates in the photosphere were obtained using High-resolution Fast Imager (HIFI at GREGOR) in blue continuum and G-band wavelengths. In the transition region, density diagnostics were obtained through the emissivity ratio method on OIV and SIV lines of the Interface Region Imaging Spectrograph (IRIS). Finally, coronal density estimates were derived from the line pair Fe XII 186/195 of the Extreme-ultraviolet Imaging Spectrometer (Hinode-EIS) and coronal temperatures from differential emission measure (DEM) using Atmospheric Imaging Assembly (AIA-SDO) datasets.

Speaker: Jenny Marcela Rodriguez Gomez (Research scientist)
• 193
Multi-instrument STIX microflare study

Solar flares are generally thought to be the impulsive release of magnetic energy giving rise to a wide range of solar phenomena that influence the heliosphere and in some cases even conditions on Earth. Part of this liberated energy is used for particle acceleration and to heat up the solar plasma. The Spectrometer/Telescope for Imaging X-rays (STIX) instrument onboard the Solar Orbiter mission promises considerable advances in the understanding of electron acceleration and plasma heating. It observes X-rays in the energy range from 4 to 150 keV, enabling it to diagnose thermal plasma with temperatures of ≳ 10 MK, as well as nonthermal bremsstrahlung emission of flare accelerated electrons. During the spacecraft commissioning phase in 2020, STIX observed 68 microflares, all of which originated in an active region that was also visible from Earth. These events provided a great opportunity to demonstrate the great potential in combining the STIX observations with multi-band EUV imagery from the Atmospheric Imaging Assembly (AIA) instrument on board the Earth orbiting Solar Dynamics Observatory.
We show first results from two GOES B-class events for which we combined STIX spectroscopic analysis with plasma diagnostics through differential emission measure analysis performed on AIA observations to analyze thermal flare plasma outside STIX’s sensitivity. We find that the thermal parameters inferred from STIX and AIA differ due to the different temperature ranges covered by each instrument. The values deduced from STIX are consistent with similar sized flares in the literature. We conclude that STIX spectroscopy is science ready.

Speaker: Jonas Saqri (Institute of Physics, University of Graz)
• 194
Optically thin cooling curves and thermal instability

Condensations are observed in many astrophysical environments. In solar physics, common phenomena are coronal rain and prominences. Coronal rain consists of transient dense blobs that form in magnetic loops and rain down along the magnetic field lines. The prominences are cold, dense structures suspended in the hot, tenuous corona by the magnetic field.

Those structures are formed due to energy loss via optically thin radiative emission. Instead of solving the full radiative transfer equations, precomputed cooling curves are typically used in multidimensional, magnetohydrodynamic (MHD) simulations assuming an optically thin and fully ionised plasma. Precomputed cooling curves used in literature differ greatly, depending on the incorporated plasma emission processes, atomic physics data, and solar abundances.

We study the effect of the optically thin cooling curves on the formation and evolution of condensations. The condensations are formed due to thermal instability. We use the open-source software MPI-AMRVAC to setup idealised simulations, i.e. a thermal equilibrium in a 2D local coronal volume perturbed by interacting slow MHD waves. For all cooling curves, condensations are formed by thermal instability. However, the differences are twofold. First, the growth rates of the thermal instability are different, leading to condensations being formed at different times. Second, the morphology of the condensations is widely varying. This is influenced by the low-temperature treatment of the cooling curves. Furthermore, we discuss a bootstrap procedure that allows us to continue high-resolution simulations of thermal instability far into the nonlinear regime.

Speaker: Mr Joris Hermans (Centre for mathematical Plasma-Astrophysics, KU Leuven)
• Poster Session 5.3
Convener: Mats Carlsson (Rosseland Centre for Solar Physics, University of Oslo, Norway)
• 195
A neural network approach to 3D NLTE radiative transfer

Computing synthetic spectra from model atmospheres is a fundamental tool to understand the formation of complex spectra and help guide observations. Forward modelling using 3D MHD simulations is becoming widespread, and with it the need to efficiently carry out radiative transfer computations. This is particularly acute for the solar chromosphere, where out of equilibrium conditions dictate the need for non-LTE calculations, which in 3D are computationally extremely expensive and require a supercomputer. We have developed a neural network approach to greatly speed up 3D non-LTE spectral synthesis. We used a convolutional neural network to learn the full 3D mapping between LTE and non-LTE populations for a given model atom. Once we have the non-LTE populations, synthetic spectra can be quickly computed for any viewing angle and spectral line, assuming complete redistribution (CRD). The `true'' values of the non-LTE populations are taken from runs with the Multi3D code. The proposed network architecture successfully learns the population mappings using a combination of simulations with different conditions and magnetic topology. Testing with different snapshots of the simulations used in the training gives a very good agreement, although the network can struggle when using more extreme, out of sample simulations. The results are very encouraging and result on a time saving of about 5 orders of magnitude; running typical-sized problems for a model hydrogen atom takes less than three hours on a consumer GPU.

Speaker: Tiago Pereira (Rosseland Centre for Solar Physics, University of Oslo)
• 196
The interplay between slow waves and heating/cooling mechanisms in the solar corona

In the corona, the (apparent) thermal equilibrium arises from a delicate balance between radiative losses and some unknown heating mechanism(s). The effect of these heating and cooling mechanisms varies with the plasma parameters, such as density, temperature and potentially magnetic field strength. Meanwhile, slow magnetoacoustic waves can perturb the density and temperature of the hosting plasma enough to affect these heating and cooling mechanisms, to the extent that the waves themselves are impacted in a measurable way. Thus there is great potential in exploiting these slow waves, which are commonplace in the corona, as probes of the local thermal equilibrium.

In this talk I will outline how, by measuring the properties of slow waves as they perturb the coronal equilibrium, we may place some constraints on the poorly understood coronal heating function. I will show that if the magnetic field is sufficiently strong, even if the heating mechanism depends strongly on magnetic field strength, the slow waves are insensitive to this (magnetic) dependence. This holds true in the corona so long as the magnetic field strength is greater than approximately 10G for quiescent loops and plumes, and 100G for hot and dense loops.

Speaker: Tim Duckenfield (KU Leuven)
• 197
Large-amplitude longitudinal oscillations in solar prominences derived from high-resolution simulations

We aimed to study the damping mechanism of the large-amplitude longitudinal oscillations (LALOs) in solar prominences hidden by numerical dissipation. We performed the numerical simulations of LALOs using the 2D magnetic configuration that contains the dipped region. After the prominence mass loading in the magnetic dips, we triggered LALOs perturbing the prominence mass along the magnetic field. We used the same numerical setup gradually increasing spatial resolution. We obtained a good agreement of the period of LALOs with the pendulum model for the simulation with the highest spatial resolution. The analysis of motions revealed that the damping time is similar in the two experiments with the finest grid-scale, indicating that the further improvement of the spatial resolution does not change the damping time of LALOs in the central and bottom prominence region. This indicates that the physical mechanism is responsible for the attenuation of LALOs in those prominence regions. At the prominence top, the oscillations are amplified in the first minutes and then slowly attenuated. The characteristic time suggests more significant amplification in the experiments with high spatial resolution. We have found that energy losses at the bottom prominence region are caused by wave leakage and the energy exchange between the bottom and top prominence regions. We concluded that the high-resolution simulations are crucial for studying the periods and the damping mechanism of LALOs. Furthermore, numerical diffusion can hide the important physical mechanisms as the amplification of oscillations.

Speaker: Ms Valeriia Liakh (Instituto de Astrofísica de Canarias)
• 198
Settling motions in 1D stratified models of the solar corona

We aim at setting and driving 3D magneto-hydrodynamic simulations of the Sun with photospheric magnetogram observations to get a better understanding of the heating mechanism and energy dissipation in the solar corona and thus aiming to move a step closer towards the long-standing solar coronal heating problem. There are already some models that study coronal heating using different heating mechanisms. The working mechanism for our model is the field-line braiding mechanism generating an upward Poynting flux, where plasma motions advect the field. This flux travels as magnetic perturbations into the corona indicating actual magnetic-energy transport. These perturbations can then induce electric currents in the solar corona which are then dissipated due to DC heating. We start the computation with an initial atmospheric stratification that is usually not in hydrodynamic equilibrium. Because settling the initial inequilibrium is costly in large-scale 3D models, instead, we use a 1D model that spans from the solar interior to the corona for finding the numerical equilibrium that exactly fits to the simulation parameters. This new atmospheric stratification we can now use as the initial condition for large simulation runs. Also, we implement and employ an artificial heating function that compensates for a lack of heating in the early phase of the model, where perturbations have not yet reached the corona. We like to start the 3D model with the most realistic physics and less vertical settling motions. This procedure finally allows us to compare our model output with actually observed Doppler shifts in the corona.

Speaker: Ms Vartika Pandey (University of Graz)
• 199
New insights from umbra modelling and new questions for cycle 25

Despite the recent lull in solar activity, the community has seen new observational discoveries and new advances in the modelling of the umbra. The presence of coronal and transition region downflows have been repeatedly reaffirmed, their role in the typical umbral upper chromosphere remaining unconstrained. A connection between small-scale dynamic fibrils and umbral flashes has been established using semi-empirical inversions that leave open questions regarding the top-most chromospheric temperatures. In parallel ALMA has confirmed the presence of increased radio emission from the chromospheric umbra of unknown source and at unconstrained formation heights. It is not clear how, if at all, these three pieces of the puzzle fit together. We review these advances together as the new open questions were not obvious before. Using synthetic observables from different inversions set in such a way that they lead to the very different models found in the literature, we present how some of the open questions can be answered either in the optical or, even more promisingly, in mm wavelengths. In the process we settle the formation heights for multiple ALMA passbands in the umbra and provide approaches to solve non-LTE inversion degeneracies in the optical. High quality SST data is very briefly presented in the context of dynamic fine-structures and how such features might impact the also recently discovered resonant mode signatures.

Speaker: Vasco Manuel de Jorge Henriques (Rosseland Centre for Solar Physics, University of Oslo, Norway)
• 200
Simulation of prominence oscillations triggered by a shock wave

Solar prominences are cold, dense structures nested in the hot and tenuous solar corona, made up of thread-like fibrils. Embedded in the magnetically dominated, dynamic corona, prominences also exhibit oscillatory behavior [1]. Understanding the interplay of mechanisms that cause prominence and thread oscillations provides important insight into the solar corona. Numerical simulations have been conducted [2, 3, 4] in order to analyze the exact mechanisms governing prominence behavior. To date, all studies on oscillations in prominences ignored their finer structure. The goal of this work is to study causal relations between a localized energy release and a remote prominence oscillation where the prominence has a realistic thread structure. In our setup, we notice coupled transverse and longitudinal oscillations. We use an adiabatic, 2D numerical model with an open-source MHD simulation code, MPI-AMRVAC [5]. We exploit the advantages of adaptive mesh refinement (AMR) to investigate how multiple threads react to a realistic source perturbation. The grid we employ consists effectively of 4160 × 800 cells, which allows us to resolve lengths of 36×7.5 km. This exceeds the resolution limits of observations.

Speaker: Ms Veronika Jercic (Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, B-3001 Leuven, Belgium)
• Poster Session 5.4
Convener: Elena Khomenko (Instituto de Astrofisica de Canarias)
• 201
Numerical study of MHD modes in coronal loops

In order to infer the properties of the coronal plasma, coronal seismology combines the measurement of temporal and spatial signals of oscillations and magnetohydrodynamic waves of different magnetic structures with their theoretical modeling. In the particular case of coronal loops, fast sausage modes and standing slow modes are the most studied oscillation modes, because their compressibility makes them susceptible to being observed. By performing magnetohydrodynamic numerical simulations, we analyze the capability of different types of disturbances, associated with typical solar corona energy fluctuations, to generate these types of modes. We found that confined energy deposition excites slow modes, while global perturbations, capable of instantly modifying the loop temperature, excite fast sausage modes.

Speaker: Mariana Cécere (Instituto de Astronomía Teórica y Experimental (CONICET/UNC))
• 202
Stability of solar atmospheric structures harboring standing slow waves: an analytical model in a compressible plasma

In the context of the solar coronal heating problem, one possible explanation for the high coronal temperature is the release of energy by magnetohydrodynamic (MHD) waves. The energy transfer is believed to be possible, among others, by the development of the Kelvin-Helmholtz instability (KHI) in coronal loops. Our aim is to determine if standing slow waves in solar atmospheric structures such as coronal loops, and also prominence threads, sunspots, and pores, can trigger the KHI due to the oscillating shear flow at the structure’s boundary. We used linearized nonstationary MHD to work out an analytical model in a cartesian reference frame. The model describes a compressible plasma near a discontinuous interface separating two regions of homogeneous plasma, each harboring an oscillating velocity field with a constant amplitude which is parallel to the background magnetic field and aligned with the interface. The obtained analytical results were then used to determine the stability of said interface, both in coronal and photospheric conditions. We find that the stability of the interface is determined by a Mathieu equation. In function of the parameters of this equation, the interface can either be stable or unstable. For coronal as well as photospheric conditions, we find that the interface is stable with respect to the KHI. Theoretically, it can, however, be unstable with respect to a parametric resonance instability, although it seems physically unlikely. We conclude that, in this simplified setup, a standing slow wave does not trigger the KHI without the involvement of additional physical processes.

Speaker: Michaël Geeraerts
• 203
Public version of Mancha: a non-ideal MHD code for realistic simulations of the solar atmosphere.

The MANCHA code development was started in 2006 to study wave dynamics in sunspots at the Instituto de Astrofísica de Canarias.
The code can be used for a large variety of problems from linear wave propagation and classical hydrodynamic instabilities to highly realistic simulations of convection-driven solar and stellar atmosphere.

The code solves the MHD equations including nonlinear terms for the ambipolar and Hall diffusion, the Ohm's heating and the Biermann battery.
The radiative energy exchange is implemented by the short-characteristics method.
The equation of state is either ideal (computed on the fly) or realistic (interpolated from pre-computed lookup tables provided by the user).
The code uses uniform Cartesian grid with spatial discretization up to 6th order; the Runge-Kutta scheme is used for the time integration.
A super-time-stepping technique is implemented to overcome timestep limitation due to diffusion processes.
Hyper-diffusion and filtering are available to stabilize the high-frequency numerical noise.
A perfectly-matched-layer is implemented as an optional non-reflecting boundary condition.
MANCHA is written in Fortran and parallelized with MPI, including parallel I/O using the HDF library.

In order to solve a new problem, users will need to find their own optimal set of parameters in an input control file.
In the case of non-periodic boundaries, users are expected to create their own boundary condition module.
A set of samples together with the manual provide basic instructions on how to do that.

Speaker: Mikhail Modestov (Instituto de Astrofísica de Canarias)
• 204
The optical properties of Hydrogen plasma described in the frame of the fully quantum method based on a cut-off Coulomb model potential

The absorption coefficients of hydrogen plasma, calculated within the frame of cut-off Coulomb potential model, for the wide area of electron densities and temperatures observed within the solar atmosphere are presented here. The optical parameter of hydrogen plasma of mid and moderately high nonideality parameter could be described successfully, thus enabling the modeling of optical properties, especially the calculation of plasma opacity. The model was proven in both convergence towards normal condition, ideal plasma case, as well as with the help of analysis of the experimental data and further theoretical consideration. The model potential is solvable in entire space and within entire energy spectrum, thus the yielded wave function solutions are a combination of a special functions. The special form of the cut-of Coulomb potential, possesses an unique feature that enables the precise, fully quantum method of calculation of inverse Brehmstrahlung effect. Although the presented method development is still a work in progress the possibility of unifying a mode for both transport and optical properties of plasma within same model is an attractive direction for it’s further development.

• 205
MHD wave propagation in asymmetric solar waveguides

We present our findings on MHD wave propagation and instabilities in asymmetric Cartesian waveguide models. Generalising the classical slab models this way, thanks to the introduction of various sources of asymmetry (background density, magnetic field or flow speed) allows us to more precisely investigate several important features in the richly structured solar atmosphere. By developing solar magneto-seismologic methods from these analytical models, we can provide an efficient tool for obtaining further information about the solar plasma from observations. We offer further detail on the types and parameters of MHD waves expected to propagate and be observable in a number of multi-layered systems of the solar atmosphere (such as e.g. magnetic bright points or light walls) with new, high-resolution technology.

Speaker: Ms Noémi Kinga Zsámberger
• 206
From highly-collisional to collisionless fluid models

The atmosphere of the sun represents a complex physical environment
where two traditional classes of fluid theories come to interplay. While the solar chromosphere is typically modeled with fluid models derived under the highly-collisional assumption, whereas the collisional frequencies in the solar corona can become so small, that collisionless fluid models seem to be more appropriate. We briefly overview these two distinct classes of fluid theories and discuss the major differences between them. We also discuss some preliminary results from a 3rd unifying class of arbitrary-collisional models that are suitable for any regime of collisionality.

Speaker: Dr Peter Hunana (Instituto de Astrofisica de Canarias (IAC))
• 207
Helicity generation and sign reversal above an Active Region

The magnetic and current helicity of the field above active regions is thought to play a role when estimating the probability for coronal mass ejections or strong reconnection events. A deeper understanding of the mechanisms generating the helicity in the corona can be obtained with the help of observationally driven models, in particular magneto-hydrodynamic and ambipolar-diffusion simulations. We aim to study where the helicity may decouple from the Sun to be ejected into inter-planetary space. While the handedness of the helicity in the heliosphere was observed to be exactly reversed as in the solar interior, we expect to find the required sign reversal in the helicity somewhere in between. We employ large magneto-hydrodynamic model to investigate the actual helicity above a really observed active region. In addition, we set up small artificial sunspot groups to observe which handedness of helicity is generated due to the geometric configuration and magnetic polarity of the spots. This leads us to a quasi-scalar formulation for the generated helicity above multipolar sunspot configurations. We find that some configurations create a local helicity in the corona that is opposite to the expected handedness at the solar surface. We conclude that such configurations may be more eruptive than others.

Speaker: Philippe-A. Bourdin (University Graz)
• Poster Session 5.5
Convener: Emilia Kilpua (University of Helsinki)
• 208
Comparison of the parameters of coronal mass ejections and type II radio bursts in cycles 23 and 24

Coronal mass ejections (CMEs) are one of the main factors determining space weather. The results of the study of CMEs, type II radio bursts (RBIIs) and CMEs associated with RBIIs (RBII CMEs) observed from 1995 to 2017 are presented.
DH RBIIs are believed to be generated by magnetohydrodynamic shock waves. CMEs are considered to be the main phenomenon of solar activity, which are the sources of these shock waves. The study showed that CMEs associated with RBIIs constitute a separate CME population. CMEs and RBII CMEs behave differently in cycles 23 and 24. While the total number of CMEs increased in cycle 24 compared to cycle 23, the number of RBII CMEs decreased. In cycle 24, not only the number of CMEs increased, but also their parameters and the nature of cycle variations changed. Different types of CMEs behave differently during cycles 23 and 24.
The results indicate that the cycle variations in the number of both CMEs and RBII CMEs, and their parameters do not correspond to the well-known evolution of active regions. The dependence of the rate and parameters of the RBII CMEs and RBIIs on the magnetic field strength and plasma parameters in the solar atmosphere has been revealed. The decrease in both the polar and nonpolar magnetic fields of the Sun observed in cycle 24 led to changes in the coronal and interplanetary plasma, which influenced both the CME parameters and the nature of their propagation, and the conditions for the generation of RBIIs.

Speaker: Dr Irina Bilenko (Sternberg State Astronomical Institute)
• 209
Magnetic fields and turbulent velocities in a limb solar flare by hydrogen, helium and ionized calcium lines

We present simultaneous magnetic field measurements for the limb solar flare of 1981 July 17 using of the Ca II K, H$\delta$, He I 4471.5 Å and H$\beta$ lines (Yakovkin et al., 2021; https://doi.org/10.1016/j.asr.2021.03.036). For two moments during the flare, which differ in time by 16 min, we analyzed Stokes $I \pm V$ and $V$ profiles of these lines from observations made on the Echelle spectrograph of the horizontal solar telescope of the Astronomical Observatory of Taras Shevchenko National University of Kiev. For heights of 10$-$18 Mm above the level of the photosphere, we found that (a) very strong kG magnetic fields (up to about 3 kG) existed at both moments of the flare, (b) the locations with strongest fields, in general, do not coincide for different spectral lines, (c) the polarities of the magnetic field for different spectral lines are in most cases identical, but sometimes they do not coincide. The data obtained indicate a significant inhomogeneity of the magnetic field in the flaring corona and the probable presence of the conditions necessary for magnetic reconnection of field lines. A new indication of the existence of superstrong magnetic fields ( > 5 kG ) follows from a comparison of the kinetic temperatures and turbulent velocities in the flare (Yakovkin & Lozitsky, 2020; https://doi.org/10.18524/1810-4215.2020.33.216453). From analysis of Stokes $I$ profiles, we found a tendency to anti-correlation between temperature and turbulent velocity. Perhaps, this unlikely tendency presents masked presence of very strong magnetic fields of 7$-$8 kG range.

Speaker: Ivan Yakovkin (Astronomical Observatory of the Taras Shevchenko National University of Kyiv)
• 210
Double plasma resonance instability as a source of solar radio zebra emission

Radio zebras are detected as narrow-band stripes in radio observations from Sun, Jupiter, and Crab pulsar. They are Type IV radio fine structures and can help with diagnoses of local plasma properties in the active solar regions. The double plasma resonance model of solar radio zebra assumes dense and cold background isotropic plasma and rare hot component with an unstable loss-cone type of distribution function. The instability generates the Bernstein electrostatic waves that can transform into electromagnetic radiation. We used analytical theory and 3D electromagnetic relativistic Particle-in-Cell simulations and found that increasing the temperature shifts the growth-rate maxima to lower frequencies for the DGH velocity distribution function of hot electrons. Moreover, the maxima are not distinguishable for loss-cone thermal velocities $v_\mathrm{t} \geq 0.3\, c$. We estimated the brightness temperature, energy density, size of the zebra emission source, and conversion rates into electromagnetic waves.

Speaker: Dr Jan Benáček (TU Berlin)
• 211
Study of the White-light Emission During the X9.3 Flare on September 06, 2017

Using data from SDO/HMI, Hinode/SOT, and LYRA instruments, we study the white-light continuum emission during the X9.3 solar flare (SOL2017-09-06T11:53). Assuming that the emission is due to hydrogen Balmer and Paschen continua, we estimate the temperature evolution during that solar flare.

Speaker: Jana Kasparova
• 212
What do coronal dimmings tell us about magnetic connectivity and reconfiguration during the early stage of a solar eruption?

Coronal dimmings are temporary regions of strongly reduced emission in extreme-ultraviolet (EUV) and soft X-rays caused by expansion and evacuation of plasma associated with coronal mass ejections (CMEs).
We seek to gain insight into observed features of coronal dimmings (location, dynamics, intensity distribution) as they relate to magnetic reconnection due to the associated flare. We analyze the X2.1 flare/CME event on September 6, 2011 by combining EUV observations of coronal dimmings from SDO/AIA with MHD simulations initiated by a non-force-free magnetic field extrapolation.
The extrapolation captures a 3D null point configuration overlying a highly sheared arcade, co-spatial with an observed sigmoid in 94Å. Prior to the flare, small but distinctly bipolar dimming regions are observed in 211Å logarithmic base-ratio images. These dimmings likely form due to rising magnetic loops reconnecting at the pre-existing 3D null point; the reconnection leads to the expansion and loss of previously confined plasma.
The simulated dynamics show the transfer of twist from the arcade to the overlying loops through reconnection, forming a flux rope. We find that simultaneous reconnections at the 3D null and an X-type geometry, newly formed between the flux rope and lower-lying arcades, can explain the observed flare ribbons.
Finally, reconnection at the 3D null transforms closed inner spine field lines into open field lines of the outer spine. Footpoints of these field lines appear to trace the dome as they correspond to a ring-shaped dimming region. Plasma loss along open field lines can potentially explain dimming regions of strongest intensity decrease.

Speaker: Karin Dissauer (NorthWest Research Associates & Institute of Physics, University of Graz)
• 213
Solar observations with the Nancay Radioheliograph in support of the Solar Orbiter and Parker Solar Probe missions

The Nancay Radioheliograph is dedicated to imaging the solar corona at decimetre-to-metre wavelengths. The imaged structures are the quiet corona, through thermal bremsstrahlung, and bright collective emissions due to electrons accelerated in quiescent, flaring and eruptive active regions. After more than five years of interruption for an instrument upgrade, solar observations were resumed in November 2020. This contribution will give a brief overview of the technical changes and present observations at eight frequencies of solar activity since November 2020. During the perihelion passages of Parker Solar Probe in January and April 2021 several radio events produced by electron beams (type III bursts) were observed, where the radio imaging provides insight into the coronal regions that guide particles to the interplanetary space. We will illustrate these observations with an attempt to reconstruct the 3D structure of the propagation paths. We will furthermore address more complex radio sources associated with coronal mass ejections and solar energetic particle events.

Speaker: Karl-Ludwig Klein (Observatoire de Paris)
• 214
Type III Radio Bursts Observations on 20th August 2017 and 9th September 2017 with LOFAR Bałdy Telescope

We present the observations of two type III solar radio events performed with LOFAR (LOw-Frequency ARray) station in Bałdy (PL612), Poland in single mode. The first event occurred on 20th August 2017 and the second one on 9th September 2017. Solar dynamic spectra were recorded in the 10 MHz up to 90 MHz frequency band. Together with the wide frequency bandwidth LOFAR telescope (with single station used) provides also high frequency and high sensitivity observations. Additionally to LOFAR observations, the data recorded by instruments on boards of the Interface Region Imaging Spectrograph (IRIS) and Solar Dynamics Observatory (SDO) in the UV spectral range complement observations in the radio field. Unfortunately, only the radio event from 9th September 2017 was observed by both satellites. Our study shows that the LOFAR single station observations, in combination with observations at other wavelengths can be very useful for better understanding of the environment in which the type III radio events occur.

Speaker: Katarzyna Mikuła (Space Radio Diagnostics Research Centre, University of Warmia and Mazury)
• Poster Session 5.6
Convener: Manolis Georgoulis (RCAAM of the Academy of Athens)
• 215
Sun CubE OnE: A Multi-wavelength Synoptic Solar Micro Satellite

The Sun cubE onE (SEE) is a proposed 12U CubeSat mission in LEO that will investigate Gamma and X-ray fluxes and UV solar emission to support studies in Sun-Earth interaction and Space Weather. SEE's primary goal is to monitor the emission from soft-X to Gamma ray energy range and the solar activity using full disk images in the Mg II doublet at 280 nm.
The Gamma and X-ray fluxes will be studied with unprecedented temporal resolution and with a multi-wavelength approach thanks to the combined use of silicon photodiode and SiPM based detectors. The flare spectrum will be explored from the KeV to the MeV range of energies by the same payload, and with a cadence > 10 kHz to investigate the sources of the solar flares.
Given its UV imaging capabilities, SEE will be a key space asset to support detailed studies on solar activity, especially in relation to ultraviolet radiation which strongly interacts with the upper layers of the Earth's atmosphere, and in relation to space safety, included in the field of human exploration.
SEE data will be used together with space and ground-based observatories that provide Solar data, high energy particle fluxes and geomagnetic data in a multi-instrument/multi-wavelength/multi-messenger approach. SEE has recently passed the technical assessment by Italian Space Agency and it is currently in the shortlist of mission proposals for which the commission will proceed to the adequacy assestment.

Speaker: Dr Francesco Berrilli
• 216
An Overview of the European Union FLARECAST Project: Where Do We Stand and Potential Future Directions of Research

The European Union’s Flare Likelihood and Region Eruption Forecasting (FLARECAST) project ran between January 2015 and February 2018. It comprised the synergistic, interdisciplinary work of tens of researchers active in the fields of solar physics, space weather forecasting, computer science, big data, infrastructure and science communication. The theme of the project was solar flare prediction and its three primary objectives were to, first, understand the drivers of the flare phenomenon and improve flare forecasting, second, provide a globally accessible flare forecast service scalable enough to facilitate expansion and, third, engage with space weather end users, stakeholders and the public in an effort to raise awareness and educate on the need to predict solar flares and space weather, in general. We attempt an overview of the results of accumulating more than 170 apparent flare predictors extracted by photospheric magnetic field measurements over the past solar cycle and using them as input in 14 supervised and unsupervised machine learning forecast methods. We further brief on project-supported modeling works aiming to highlight potentially promising leads toward forecasting and to link flare activity with coronal mass ejections. We also detail on the recent open availability of the project’s data, codes and infrastructure in hopes of avoiding effort duplication in future forecasting efforts. The current state-of-the-art and potentially meaningful actions for the future are also discussed. The project’s overview paper is available in arXiv (2105.05993). Results shown have received support by the EU Horizon 2020 FLARECAST project (grant agreement no. 640216).

Speaker: Dr Manolis K Georgoulis (RCAAM of the Academy of Athens, 11527 Athens, Greece)
• 217
SunDish Project: Single-Dish Solar Imaging with INAF Radio Telescopes

The SunDish project is devoted to radio imaging and monitoring of the solar atmosphere at high radio frequencies (at present 18-26 GHz, up to 100 GHz in perspective) through single-dish observations with INAF radio telescopes.

Observations of the brightness temperature of the solar atmosphere in the radio band can map plasma processes that produce free-free emission in the local thermodynamic equilibrium and gyromagnetic phenomena, providing a probe of physical conditions in a wide range of atmospheric layers both for quiet and active regions.

During the development and early science phase of the project (2018-2020), we obtained about 170 maps of the entire solar disk in the 18-26 GHz band, filling the observational gap in the field of chromospheric imaging at these frequencies.

In order to ease the multi-wavelength exploitation of our data products, we present a first catalogue of continuum imaging observations including the multi-wavelength identification of active regions and their brightness and spectral characterization.

Through systematic monitoring of the Sun, our system can provide: (1) accurate measurement of the brightness temperature of the radio-quiet Sun component, that has been poorly explored in the 20-26 GHz range to date; (2) characterization of the flux density, spectral properties and long-term evolution of dynamical features (active regions, coronal holes, loop systems, streamers, and the coronal plateau); (3) prediction of powerful flares through the detection of peculiar spectral variations in the active regions, as a valuable forecasting probe for the Space Weather hazard network.

Speaker: Dr Marco Marongiu (Istituto Nazionale di Astrofisica (INAF))
• 218
Machine and deep learning in solar flare forecasting

Solar flare forecasting can be realized by means of either the numerical solution of model equations or the analysis of magnetic data by means of artificial intelligence techniques. Here we consider this second approach and discuss how neural network optimization and forecasting assessment by means of skill scores are deeply intertwined. A special focus is also given to the interpretation of data descriptors in terms of their impact on the prediction effectiveness. Applications will be concerned with magnetograms recorded by SDO/HMI, in the case of both point-in-time images and time series of magnetogram frames.

Speaker: Dr Sabrina Guastavino (MIDA, Dipartimento di Matematica, Università di Genova)
• 219
On the source of the anomalous June 23, 2020 Pc5 waves detected at both ground and satellite data.

We examine the characteristics of ULF waves observed in the magnetosphere and at ground on June 23, 2020 during super solar quiet geomagnetic conditions. A highly monochromatic and large-amplitude wave at f = 1.67 mHz was observed between 6:10 and 7:00 UT at CSES LEO satellite orbit and across a wide longitudinal range of ground stations from low to high latitudes. We found the possible driver of such global ULF wave activity in the impact of a Tangential Discontinuity intersecting the bow shock quasi-parallel to it. As a consequence, a foreshock bubble (FB) was generated, producing pressure pulses which in turn generates Pc5 waves in magnetosphere. The combination of the long period of these pulsations, their extended duration, the latitude-independent frequency, and the small azimuthal wave number suggests the occurrence of a global magnetospheric waveguide mode. The amplitude and cross-phase analysis of the wave activity on the ground and the polarization pattern indicate that the waveguide mode coupled to the field line resonance (FLR) occurs at different latitudes at different local times. Such FLRs occurred at latitudes smaller than usually observed at the same frequency, suggesting a significant reduction of the local field line eigenfrequencies as compared with usual values. A model estimation of such eigenfrequencies suggests a change in the magnetospheric field geometry characterized by more stretched field lines than those for usual conditions probably caused by the compression of the magnetosphere driven by the TD coupled with the stable northward orientation of the interplanetary magnetic field.

Speaker: Mirko Piersanti (Istituto Nazionale di Astrofisica (INAF))
• 220
Isolated low-frequency radio type II bursts

The appearance of type II radio bursts is mostly associated with flares and coronal mass ejections and other transient features. Propagating shock waves to the interplanetary medium can also be the cause to have a variety of radio emissions simultaneously. We analyzed well-separated (isolated) Decameter-Hectometer (DH) type II bursts which appear in the dynamic spectra without any earlier radio signatures. We studied 26 isolated DH type II bursts observed in 1998-2016. One explanation for the isolation of the DH type II bursts and delay in their observation time could be the occurrence of the flare-CME launch on the far side of the Sun. It could cause the emission to be observed only when the source gets high enough in the solar atmosphere. We found that only 30% of the bursts originated from the far side of the Sun, but also that no bursts originated from the central region of the Sun. Our analysis suggests that for the majority of isolated DH type II bursts a CME bow shock is more probable than a shock near the CME flanks. For 12 events we had observations from different viewing angles, from the instruments onboard Wind and STEREO satellites. Only five of them showed delays in the radio emission onset. Our analysis shows that in three events occultation could have been the cause of the delay in the emission onset. We also suggest that isolated type II radio burst needs specific conditions to form and be visible in the dynamic spectrum.

Speaker: Nasrin Talebpour Sheshvan (University of Turku)
• 221
Connecting solar flare hard X-ray spectra to in-situ electron spectra using RHESSI and STEREO/SEPT

The energy spectrum of impulsive solar energetic electron events is believed to carry the imprint of the flare acceleration process which can be studied by analyzing the hard X-ray (HXR) spectrum of the flare. We have identified 17 energetic electron events well-observed with the SEPT instrument aboard STEREO which show a clear association with a hard X-ray (HXR) flare observed with the RHESSI spacecraft. We compare the spectral indices of the RHESSI HXR spectra with those of the interplanetary electrons. Because of the frequent double-power-law shape of the in-situ electron spectra, we pay special attention to the choice of the spectral index used for comparison.
We find a significant correlation of about 0.8 between the spectral indices of the HXR flare and the in-situ electrons. The correlations increase if only events with significant anisotropy are considered. This suggests that transport effects can alter the injected spectra leading to a strongly reduced imprint of the flare acceleration.
Although our results suggest a clear imprint of flare acceleration for the analyzed event sample, most of the studied events occur with onset delays at the spacecraft and in temporal coincidence with coronal mass ejections (CMEs). Therefore, a secondary acceleration, which would account for the observed onset delays, could be involved. However, the limited and variable pitch-angle coverage of SEPT could also be the reason for the observed delays.

Speaker: Nina Dresing
• 10:31 AM
Coffee break
• Poster Session 6.1
Convener: Etienne Pariat (LESIA, Observatoire de Paris)
• 222
Slow magnetoacoustic waves in gravitationally stratified two-fluid plasmas in strongly ionized limit

The plasma dynamics at frequencies comparable with collisional frequency between various species has to be described in multifluid framework, where collisional interaction between particles is an important ingredient. In our study, we will assume that charged particles are strongly coupled, meaning that they form a single fluid that interacts with neutrals, therefore we will employ a two-fluid model. Here, we aim to investigate the evolutionary equation of slow sausage waves propagating in a gravitationally stratified flux tube in the two-fluid solar atmosphere in a strongly ionized limit using an initial value analysis. Due to the collisional interaction between massive particles (ions and neutrals), the governing equations are coupled. Solutions are sought in the strongly ionized limit and the density ratio between neutrals and charged particles is a small parameter. This limit is relevant to the upper part of the chromosphere. Our results show that slow sausage waves associated with charged particles propagate such that their possible frequency is affected by a cut-off due to the gravitational stratification. In contrast, for neutral acoustic waves the cut-off value applies on their wavelength and only small wavelength waves are able to propagate. Slow modes associated with neutrals are driven by the collisional coupling with ions.

Speaker: Mr Abdulaziz Alharbi (The University of Sheffield)
• 223
Relaxing the assumption of subsonic flows in Enthalpy Based Thermal Evolution of Loops

Field aligned 1-D simulations of coronal loops show that density, temperature, and pressure do not vary much across the coronal loop. Consequently, such quantities averaged over the coronal part of the loop, can be used to characterize the loop's coronal part and its thermodynamic evolution. This is referred to as 0-D description of coronal loops, which provides an efficient method for conducting an approximate but quick study of the thermodynamic evolution of coronal loops. EBTEL2 is a commonly used 0-D code, which models the mass and energy exchange between the corona and the transition region. It solves for density, temperature, and pressure, averaged over the coronal part, velocity at the coronal base, and instantaneous differential emission measure distribution in the transition region. EBTEL2 assumes that the flows are subsonic at all stages, hence, neglects the kinetic energy term. However, in some cases, simulations using EBTEL2, show supersonic flows during the impulsive phase, while 1-D simulations by HYDRAD show subsonic speeds. This is a potential limitation to EBTEL2's usage. Therefore, it becomes important to have the solutions of the full energy equation. Here, we upgrade EBTEL2 by adding kinetic energy (referred to as EBTEL3). We compare the new solutions with those obtained using EBTEL2 and 1-D hydrodynamic code HYDRAD. EBTEL3 guarantees subsonic flows in cases where HYDRAD shows subsonic flows. However, when HYDRAD produces supersonic flows, EBTEL3 produces unreliable Mach numbers, and velocities, despite other quantities being reliable. A simple criterion for predicting situations where this happens, is also discussed.

Speaker: Mr Abhishek Rajhans (IUCAA, Pune, Maharashtra, India)
• 224
Formation of the IRIS OI and CI lines in a flare

The OI 135.56 nm line and CI 135.58 nm line are weak lines that are covered by NASA's Interface Region Imaging Spectrograph (IRIS) mission which studies how the solar atmosphere is energized. The emission in the OI 135.56 nm line is dominated by a recombination cascade. This line is optically thin and provides powerful diagnostics of unresolved velocity fields in the chromosphere. The CI line is optically thick and is dominated by a recombination cascade. Normally, the oxygen line is stronger than the carbon line but in flares, the ratio is opposite. The line intensities of oxygen and carbon peak at different times. The change in the ratio of CI/OI is due to an increase in the electron density in flares. We study the formation of the OI and CI lines in a 1D RADYN simulation of a flare to find out the cause of the change in ratio. For studying the line formation in a 1D atmosphere, we use the radiative transfer code RH to get a non-LTE solution with hydrogen, carbon and oxygen solved simultaneously.

Speaker: Aditi Bhatnagar (Rosseland Centre for Solar Physics, ITA, UiO)