Coronal Elemental Abundances during A-class Solar Flares Observed by Chandrayaan-2 XSM

TL;DR

This study uses Chandrayaan-2 XSM data to analyze elemental abundances during A-class solar flares, revealing how FIP bias varies with flare intensity and evolves during different flare phases.

Contribution

It provides the first detailed time-resolved analysis of elemental abundance variations in A-class flares observed by Chandrayaan-2 XSM, linking FIP bias changes to flare dynamics.

Findings

FIP bias is intermediate in lower A-flares and near unity in higher A-flares.

Abundances decrease to photospheric levels during impulsive phase and recover afterward.

Chromospheric evaporation injects unfractionated material into the corona.

Abstract

The abundances of low First Ionisation Potential (FIP) elements are three to four times higher (FIP bias) in the closed loop active corona than in the photosphere, known as the FIP effect. Observations suggest that the abundances vary in different coronal structures. Here, we use the soft X-ray spectroscopic measurements from the Solar X-ray Monitor (XSM) on board the Chandrayaan-2 orbiter to study the FIP effect in multiple A-class flares observed during the minimum of solar cycle 24. Using time-integrated spectral analysis, we derive the average temperature, emission measure, and the abundances of four elements - Mg, Al, Si, and S. We find that the temperature and emission measure scales with the flares sub-class while the measured abundances show an intermediate FIP bias for the lower A-flares (e.g., A1), while for the higher A-flares, the FIP bias is near unity. To investigate it further, we perform a time-resolved spectral analysis for a sample of the A-class flares and examine the evolution of temperature, emission measure, and abundances. We find that the abundances drop from the coronal values towards their photospheric values in the impulsive phase of the flares, and after the impulsive phase, they quickly return to the usual coronal values. The transition of the abundances from the coronal to photospheric values in the impulsive phase of the flares indicates the injection of fresh unfractionated material from the lower solar atmosphere to the corona due to chromospheric evaporation. However, explaining the quick recovery of the abundances from the photospheric to coronal values in the decay phase of the flare is challenging.