The Solar Flare Iron Abundance

TL;DR

This study measures iron abundance in solar flare X-ray spectra, finding it lower than previous coronal estimates but higher than photospheric levels, with implications for solar plasma composition and radiation loss models.

Contribution

It provides a new, more accurate iron abundance measurement during solar flares using improved spectral analysis methods and a large dataset.

Findings

Iron abundance is lower than previous coronal values.

Iron abundance is higher than photospheric values.

No significant variation in iron abundance across different flares.

Abstract

The abundance of iron is measured from emission line complexes at 6.65 keV (Fe line) and 8 keV (Fe/Ni line) in {\em RHESSI} X-ray spectra during solar flares. Spectra during long-duration flares with steady declines were selected, with an isothermal assumption and improved data analysis methods over previous work. Two spectral fitting models give comparable results, viz. an iron abundance that is lower than previous coronal values but higher than photospheric values. In the preferred method, the estimated Fe abundance is $A({\rm Fe}) = 7.91 \pm 0.10$ (on a logarithmic scale, with $A({\rm H}) = 12$), or $2.6 \pm 0.6$ times the photospheric Fe abundance. Our estimate is based on a detailed analysis of 1,898 spectra taken during 20 flares. No variation from flare to flare is indicated. This argues for a fractionation mechanism similar to quiet-Sun plasma. The new value of $A({\rm Fe})$ has important implications for radiation loss curves, which are estimated.