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

Abstract

Abstract: We present joint Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) and Interface Region Imaging Spectrograph (IRIS) observations of Fe xii 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 xii 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 ii 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 xii 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.