Diagnostics of non-Maxwellian electron distributions in solar active regions from Fe XII lines observed by Hinode/EIS and IRIS

TL;DR

This study combines Hinode/EIS and IRIS observations to analyze Fe XII line ratios in solar active regions, suggesting non-Maxwellian electron distributions as a key factor affecting spectral diagnostics.

Contribution

It provides evidence that non-Maxwellian electron distributions influence Fe XII line ratios, improving understanding of plasma conditions in solar active regions.

Findings

Fe XII 192.4 A / 1349 A ratio often lower than predicted in active regions

Significant opacity found in Fe XII 193 and 195 A lines, but not in 192.4 A

Non-Maxwellian electron distributions likely explain the observed line ratio discrepancies

Abstract

We present joint Hinode/EIS and 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 A / 1349 A 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 A lines, but not in the 192.4 A 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 non-equilibrium effects, we suggest that the most likely explanation for the observed low Fe XII 192.4 A / 1349 A 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.