Iron Kα-fluorescence in solar flares: A probe of the photospheric iron abundance

The X-ray line at 6.4 keV has been observed from solar flares. It is found that Kα-fluorescence of neutral iron in the photosphere due to 'thermal' (T approximately 107K) X-rays of the gradual phase is its dominant production mechanism. For a given flux and energy spectrum of incident X-rays, the flux at 1 AU of iron Kα-photons depends on the photospheric iron abundance, the height of the X-ray source, and the heliocentric angle between the flare and the observer. Therefore, the flux of iron Kα-photons, when measured simultaneously with the flux and energy spectrum of the X-ray continuum and the flare location, can give information on the height of the X-ray source and the photospheric iron abundance. Monte Carlo calculations are presented of iron Kα-fluorescence efficiencies, so that they might be useful for interpretations of future measurements of the 6.4 keV line. (Auth.)