2022/07/14 by G. Del Zanna, Del Zanna, G., V. Polito +10
Physics and Astronomy · #FOS: Physical sciences #Ionosphere and magnetosphere dynamics #Solar and Space Plasma Dynamics #Solar and Stellar Astrophysics (astro-ph.SR) #Stellar, planetary, and galactic studies #astro-ph.SR
paper · pdf · doi:10.48550/arxiv.2207.07026
Accepted for publication in ApJ
arxiv created 2022/07/14 · openalex publication_date 2022/07/14 · arxiv updated 2022/07/15 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
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.