K. J. H. Phillips, B. R. Dennis
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.
View original:
http://arxiv.org/abs/1201.2875
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