Broad Non-Gaussian Fe XXIV Line Profiles in the Impulsive Phase of the 2017 September 10 X8.3 class Flare Observed by Hinode/EIS

TL;DR

This study reveals broad, non-Gaussian Fe XXIV line profiles during a solar flare, indicating particle acceleration, turbulence, and non-Maxwellian plasma conditions, with implications for high-temperature diagnostics.

Contribution

It demonstrates the use of $$ distributions to model line profiles and reveals higher formation temperatures, advancing understanding of flare plasma dynamics.

Findings

Fe XXIV lines are best fitted with $$ distributions with $$ between 1.7 and 3.

Regions with non-Gaussian profiles coincide with high-energy HXR sources, indicating particle acceleration.

Non-thermal broadening suggests turbulence rather than chromospheric evaporation.

Abstract

We analyze the spectra of high temperature \fexxiv~lines observed by \emph{Hinode}/EIS during the impulsive phase of the X8.3--class flare on September 10, 2017. The line profiles are broad, show pronounced wings, and clearly depart from a single Gaussian shape. The lines can be well fitted with $\kappa$ distributions, with values of $\kappa$ varying between~$\approx$1.7 to 3. The regions where we observe the non-Gaussian profiles coincide with the location of high-energy ($\approx$100--300 keV) HXR sources observed by \emph{RHESSI}, suggesting the presence of particle acceleration or turbulence, also confirmed by the observations of a non-thermal microwave sources with the \emph{Expanded Owens Valley Solar Array} (EOVSA) at and above the HXR looptop source. We also investigate the effect of taking into account $\kappa$ distributions in the temperature diagnostics based on the ratio of the \fexxiii~263.76~\AA~and \fexxiv~255.1~\AA~EIS lines. We found that these lines can be formed at much higher temperatures than expected (up to log($T$\,[K])\,$\approx$\,7.8), if departures from Maxwellian distributions are taken into account. Although larger line widths are expected because of these higher formation temperatures, the observed line widths still imply non-thermal broadening in excess of ~200\,\kps. The non-thermal broadening related to HXR emission is better interpreted by turbulence rather than chromospheric evaporation.