The Evolution of Plasma Composition During a Solar Flare

TL;DR

This study investigates how plasma composition changes during a small solar flare, revealing selective elemental abundance variations and proposing mechanisms involving ionization and magnetic field effects, which challenge static models of solar plasma.

Contribution

It provides new insights into plasma abundance evolution during flares, highlighting the need for dynamic models over static assumptions.

Findings

Increased Ca abundance during the flare for low-FIP elements.

Unchanged Si/S abundance ratio suggests selective ionization effects.

Proposes mechanisms involving ionization and magnetic field influences.

Abstract

We analyse the coronal elemental abundances during a small flare using Hinode/EIS observations. Compared to the pre-flare elemental abundances, we observed a strong increase in coronal abundance of Ca XIV 193.84 {\AA}, an emission line with low first ionisation potential (FIP < 10 eV), as quantified by the ratio Ca/Ar during the flare. This is in contrast to the unchanged abundance ratio observed using Si X 258.38 {\AA}/S X 264.23 {\AA}. We propose two different mechanisms to explain the different composition results. Firstly, the small flare-induced heating could have ionised S, but not the noble gas Ar, so that the flare-driven Alfv\'en waves brought up Si, S and Ca in tandem via the ponderomotive force which acts on ions. Secondly, the location of the flare in strong magnetic fields between two sunspots may suggest fractionation occurred in the low chromosphere, where the background gas is neutral H. In this region, high-FIP S could behave more like a low-FIP than a high-FIP element. The physical interpretations proposed generate new insights into the evolution of plasma abundances in the solar atmosphere during flaring, and suggests that current models must be updated to reflect dynamic rather than just static scenarios.