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

Analysis of RHESSI 3-10 keV spectra for 27 solar flares is reported. This energy range includes thermal free-free and free-bound continua and two line features, at ~6.7 and ~8 keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at ~6.7 keV to derive the electron temperature Te, 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 Te. To minimize instrumental problems with high count rates and effects associated with multitemperature 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 Te = 12 MK; the observed N(Fe XXV)/N(Fe) values appear to be displaced from the most recent theoretical values by between 1 and 3 MK.