Iron Fluorescence in X-class Solar Flares: Aditya-L1/SoLEXS Observations

TL;DR

This study analyzes iron Kα fluorescence in 47 X-class solar flares observed by India's Aditya-L1 mission, revealing relationships with flare geometry and potential diagnostics for coronal source heights.

Contribution

First comprehensive analysis of iron fluorescence in solar flares using Aditya-L1/SoLEXS data, establishing relationships with flare geometry and source height diagnostics.

Findings

Fluorescence efficiencies show center-to-limb dependence consistent with models.

Mean fluorescence efficiency during flare peaks constrains coronal source height.

SDDs enable new diagnostics despite lower spectral resolution.

Abstract

Iron fluorescence is produced by the irradiation of the solar photosphere by coronal X-rays during flares. This study presents the first comprehensive analysis of iron K$\alpha$ fluorescence characteristics in 47 X-class flares observed during the inaugural year of the Solar Low Energy X-ray Spectrometer (SoLEXS) on board India's Aditya-L1 mission. Leveraging the capability of modern silicon drift detectors (SDDs) for simultaneous broadband continuum and line measurements, the Fe K$\alpha$ flux and the exciting flux ($F_{>7.11 \text{ keV}}$) are quantified for each event, establishing a well-determined relationship between them across the sample. The derived fluorescence efficiencies exhibit a center-to-limb dependence consistent with theoretical models, offering a potential diagnostic to probe coronal source heights and viewing geometries. While statistical uncertainties currently limit the ability to track rapid height variations on short timescales, the mean fluorescence efficiency during the flare peak provides a potential constraint on the effective coronal source height. However, this derivation remains subject to a fundamental degeneracy, as the estimated source height cannot be uniquely determined without assuming a specific value for the photospheric iron abundance. These findings demonstrate that SDDs, despite having lower spectral resolution than traditional crystal spectrometers, provide a new diagnostic for the solar iron fluorescence observations.