RHESSI Observations of the Solar Flare Iron-line Feature at 6.7 keV

Abstract

Analysis of RHESSI 3--10 keV spectra for 27 solar flares is reported. This energy range includes thermal free--free and free--bound continuum and two line features, at 6.7keV and 8keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at 6.7keV to derive the electron temperature T_e, the emission measure, and the Fe-line equivalent width as functions of time in each flare. The Fe/H abundance ratio in each flare is derived from the Fe-line equivalent width as a function of T_e. To minimize instrumental problems with high count rates and effects associated with multi-temperature and nonthermal spectral components, spectra are presented mostly during the flare decay phase, when the emission measure and temperature were smoothly varying. We found flare Fe/H abundance ratios that are consistent with the coronal abundance of Fe (i.e. 4 times the photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7 flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We find evidence that the Fe XXV ion fractions are less than the theoretically predicted values by up to 60% at T_e=25 MK appear to be displaced from the most recent theoretical values by between 1 and 3 MK.