Silicon abundance from RESIK solar flare observations

TL;DR

This study uses RESIK solar flare X-ray spectra to measure silicon abundance in flare plasmas, finding it slightly above photospheric levels and suggesting an increase in Si/S ratio from active regions to flares.

Contribution

First measurement of silicon abundance in solar flare plasmas using RESIK spectra, with implications for solar composition and flare physics.

Findings

Silicon abundance is about 2.6 times the photospheric value.

Silicon abundance varies little across different flares.

Possible increase in Si/S ratio from active regions to flares.

Abstract

The RESIK instrument on the CORONAS-F spacecraft obtained solar flare and active region X-ray spectra in four channels covering the wavelength range 3.8 -- 6.1 \AA in its operational period between 2001 and 2003. Several highly ionized silicon lines were observed within the range of the long-wavelength channel (5.00 -- 6.05 \AA). The fluxes of the \sixiv Ly-$\beta$ line (5.217 \AA) and the \sixiii $1s^2 - 1s3p$ line (5.688 \AA) during 21 flares with optimized pulse-height analyzer settings on RESIK have been analyzed to obtain the silicon abundance relative to hydrogen in flare plasmas. As in previous work, the emitting plasma for each spectrum is assumed to be characterized by a single temperature and emission measure given by the ratio of emission in the two channels of GOES. The silicon abundance is determined to be $A({\rm Si}) = 7.93 \pm .21$ (\sixiv) and $7.89 \pm .13$ (\sixiii) on a logarithmic scale with H = 12. These values, which vary by only very small amounts from flare to flare and times within flares, are $2.6 \pm 1.3$ and $2.4 \pm 0.7$ times the photospheric abundance, and are about a factor of three higher than RESIK measurements during a period of very low activity. There is a suggestion that the Si/S abundance ratio increases from active regions to flares.