Diagnostics of Non-Maxwellian Electron Distributions in Solar Active Regions from Fe xii Lines Observed by the Hinode Extreme Ultraviolet Imaging Spectrometer and Interface Region Imaging Spectrograph

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jats:titleAbstract</jats:title> jats:pWe present joint Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) and Interface Region Imaging Spectrograph (IRIS) observations of Fe jats:scxii</jats:sc> lines in active regions, both on-disk and off-limb. We use an improved calibration for the EIS data, and find that the 192.4 Å/1349 Å observed ratio is consistent with the values predicted by CHIANTI and the coronal approximation in quiescent areas, but not in all active-region observations, where the ratio is often lower than expected by up to a factor of about two. We investigate a number of physical mechanisms that could affect this ratio, such as opacity and absorption from cooler material. We find significant opacity in the EIS Fe jats:scxii</jats:sc> 193 and 195 Å lines, but not in the 192.4 Å line, in agreement with previous findings. As we cannot rule out possible EUV absorption by H, He, and He jats:scii</jats:sc> in the on-disk observations, we focus on an off-limb observation where such absorption is minimal. After considering these, as well as possible nonequilibrium effects, we suggest that the most likely explanation for the observed low Fe jats:scxii</jats:sc> 192.4 Å/1349 Å ratio is the presence of non-Maxwellian electron distributions in the active regions. This is in agreement with previous findings based on EIS and IRIS observations independently.</jats:p>

5101 Astronomical Sciences, 51 Physical Sciences
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Astrophysical Journal
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American Astronomical Society
Science and Technology Facilities Council (ST/P000665/1)
STFC (ST/T000481/1)
STFC (ST/W001896/1)