Description
The SPICE instrument on board Solar Orbiter observes in the (extreme-)UV, including the Lyβ and Lyγ lines. Forming in the upper chromosphere, they are very sensitive to energy input and the ionisation stratification, offering important diagnostic information of solar flare energetics. We report here on the first high-cadence (5s) flare observations from SPICE, presenting an overview of the dataset, but focusing our analysis on the Lyman lines. Τhis M-class flare was observed during Solar Orbiter’s Major Flare Watch SOOP on 23rd March 2024 ~2348UT. We measure the Lyman decrement (intensity ratio R = Lyβ/Lyγ) as a function of time in an isolated flare footpoint. R decreases impulsively, returning to pre-flare values rapidly, despite the fact that the intensity of each of the Lyman lines (and other EUV spectral lines) returns to pre-flare over a longer duration. In a nearby, weaker, flare ribbon the Lyman decrement shows no meaningful change in response to the flare. A series of field-aligned radiation hydrodynamic simulations were performed, revealing that in electron beam driven flares the synthetic Lyman decrement is very consistent with observations. However, in a flare driven solely by thermal conduction, the Lyman decrement does not exhibit the observed sharp decrease. We conclude that the very strong flare footprint was produced by intense particle precipitation, whereas the weaker conjugate footpoint was more consistent with flare energy transport dominated by thermal conduction. Future analysis will focus on other spectra observed by SPICE, aiming to confirm this picture of differing dominance of energy transport mechanisms.
