Departure of high temperature iron lines from the equilibrium state in flaring solar plasmas

TL;DR

This study investigates whether ionization equilibrium and Maxwellian electron distributions hold in flaring solar plasmas, finding significant deviations during flare events that challenge common assumptions in solar physics.

Contribution

It provides observational evidence that ionization equilibrium and Maxwellian electron distributions can be violated in high-temperature solar flare plasma around 10 MK.

Findings

Temperatures from Fe line ratios show discrepancies during flare phases.

Deviations suggest non-equilibrium conditions in flaring plasma.

Maxwellian assumption may not hold in evaporating solar plasma.

Abstract

The aim of this study is to clarify if the assumption of ionization equilibrium and a Maxwellian electron energy distribution is valid in flaring solar plasmas. We analyze the 2014 December 20 X1.8 flare, in which the \ion{Fe}{xxi} 187~\AA, \ion{Fe}{xxii} 253~\AA, \ion{Fe}{xxiii} 263~\AA\ and \ion{Fe}{xxiv} 255~\AA\ emission lines were simultaneously observed by the EUV Imaging Spectrometer onboard the Hinode satellite. Intensity ratios among these high temperature Fe lines are compared and departures from isothermal conditions and ionization equilibrium examined. Temperatures derived from intensity ratios involving these four lines show significant discrepancies at the flare footpoints in the impulsive phase, and at the looptop in the gradual phase. Among these, the temperature derived from the \ion{Fe}{xxii}/\ion{Fe}{xxiv} intensity ratio is the lowest, which cannot be explained if we assume a Maxwellian electron distribution and ionization equilibrium, even in the case of a multi-thermal structure. This result suggests that the assumption of ionization equilibrium and/or a Maxwellian electron energy distribution can be violated in evaporating solar plasma around 10~MK.