The solar photospheric-to-coronal Fe abundance ratio from X-ray fluorescence lines

The ratio of the Fe abundance in the photosphere to that in coronal flare plasmas is determined by X-ray lines within the complex at 6.7 keV (1.9 angstrom) emitted during flares. The line complex includes the He-like Fe (Fe XXV) resonance line w (6.70 keV) and Fe K alpha lines (6.39 and 6.40 keV), the latter being primarily formed by the fluorescence of photospheric material by X-rays from the hot flare plasma. The ratio of the Fe K alpha lines to the Fe XXV w depends on the ratio of the photospheric to flare Fe abundance, heliocentric angle theta of the flare and the temperature T-e of the flaring plasma. Using high-resolution spectra from X-ray spectrometers on the P78-1 and Solar Maximum Mission spacecraft, the Fe abundance in flares is estimated to be 1.6 +/- 0.5 and 2.0 +/- 0.3 times the photospheric Fe abundance, the P78-1 value being preferred as it is more directly determined. This enhancement is consistent with results from X-ray spectra from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft, but is significantly less than a factor of 4 as in previous work.