The Solar Flare Chlorine Abundance from RESIK X-ray Spectra

TL;DR

This study measures the chlorine abundance in the solar atmosphere during flares using RESIK X-ray spectra, providing a precise estimate that is higher than previous measurements and consistent across various flare intensities.

Contribution

It presents the most accurate solar chlorine abundance measurement from X-ray spectra, demonstrating its constancy across different flare intensities and comparing favorably with prior estimates.

Findings

Chlorine abundance is estimated as 5.75 ± 0.26 on a logarithmic scale.

The measured abundance is 1.8 to 2.7 times higher than previous infrared and H II region estimates.

The abundance remains consistent across a wide range of solar flare intensities.

Abstract

The abundance of chlorine is determined from X-ray spectra obtained with the RESIK instrument on {\em CORONAS-F} during solar flares between 2002 and 2003. Using weak lines of He-like Cl, \ion{Cl}{16}, between 4.44 and 4.50 \AA, and with temperatures and emission measures from {\em GOES} on an isothermal assumption, we obtained $A({\rm Cl}) = 5.75 \pm 0.26$ on a scale $A({\rm H}) = 12$. The uncertainty reflects an approximately factor 2 scatter in measured line fluxes. Nevertheless our value represents what is probably the best solar determination yet obtained. It is higher by factors of 1.8 and 2.7 than Cl abundance estimates from an infrared sunspot spectrum and nearby \ion{H}{2} regions. The constancy of the RESIK abundance values over a large range of flares ({\em GOES} class from below C1 to X1) argues for any fractionation that may be present in the low solar atmosphere to be independent of the degree of solar activity.