Speaker
Description
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.
