Signatures of Helium continuum in cool flare loops observed by SDO/AIA

TL;DR

This study analyzes cool flare loops observed by SDO/AIA, identifying helium continuum signatures and their relation to hot faint loops, revealing limitations of standard absorption models in explaining dark-loop brightness.

Contribution

It provides the first detailed analysis of helium continuum signatures in off-limb cool flare loops using SDO/AIA data and semi-empirical modeling.

Findings

Helium continuum signatures are consistent with observations in the AIA 211 Å channel.

Surplus EUV emission is attributed to hot faint loops in front of cool loops.

Standard absorption models fail to fully explain dark-loop brightness.

Abstract

We present an analysis of off-limb cool flare loops observed by SDO/AIA during the gradual phase of SOL2017-09-10T16:06 X8.2-class flare. In the EUV channels starting from the 335 {\AA} one, cool loops appear as dark structures against the bright loop arcade. These dark structures were precisely coaligned (spatially and temporally) with loops observed by SST in emission lines of hydrogen and ionized calcium. Recently published semi-empirical model of cool loops based on SST observations serves us to predict the level of hydrogen and helium recombination continua. The continua were synthesized using an approximate non-LTE approach and theoretical spectra were then transformed to AIA signals. Comparison with signals detected inside the dark loops shows that only in AIA 211 {\AA} channel the computed level of recombination continua is consistent with observations for some models, while in all other channels which are more distant from the continua edges the synthetic continuum is far too low. In analogy with on-disk observations of flares we interpret the surplus emission as due to numerous EUV lines emitted from hot but faint loops in front of the cool ones. Finally we briefly comment on failure of the standard absorption model when used for analysis of the dark-loop brightness.