The Solar Flare Sulphur Abundance from RESIK Observations

TL;DR

This study uses RESIK X-ray observations during solar flares to accurately measure sulphur abundance, finding it consistent with photospheric and meteoritic values, suggesting minimal FIP-related fractionation.

Contribution

First precise sulphur abundance measurement from RESIK flare data showing no significant FIP effect, aligning coronal and photospheric abundances.

Findings

Sulphur abundance estimated at A(S) ≈ 7.16

Results consistent with photospheric and meteoritic values

No evidence of FIP-related fractionation for sulphur

Abstract

The RESIK instrument on {\em CORONAS-F} spacecraft observed several sulphur X-ray lines in three of its four channels covering the wavelength range 3.8-6.1 \AA\ during solar flares. The fluxes are analyzed to give the sulphur abundance. Data are chosen for when the instrument parameters were optimized. The measured fluxes of the \ion{S}{15} $1s^2-1s4p$ ($w4$) line at 4.089 \AA\ gives $A({\rm S}) = 7.16 \pm 0.17$ (abundances on a logarithmic scale with $A({\rm H}) = 12$) which we consider to be the most reliable. Estimates from other lines range from 7.13 to 7.24. The preferred S abundance estimate is very close to recent photospheric abundance estimates and to quiet-Sun solar wind and meteoritic abundances. This implies no fractionation of sulphur by processes tending to enhance the coronal abundance from the photospheric that depend on the first ionization potential (FIP), or that sulphur, though its FIP has an intermediate value of 10.36 eV, acts like a "high-FIP" element.