The Solar Flare 3.8–10 keV X‐Ray Spectrum

Abstract

The 3.8-10 keV solar flare spectrum includes lines of highly strippedCa, Fe, and Ni ions, as well as a continuum steeply falling with energy.Groups of lines at ~7 and ~8 keV, observed during flares by thebroadband RHESSI spectrometer and called here the Fe line and Fe/Ni linefeatures, are formed mostly of Fe lines but with Ni lines contributingto the ~8 keV feature. Possible temperature indicators of these linefeatures are discussed: the peak or centroid energies of the Fe linefeature, the line ratio of the Fe line to the Fe/Ni line features, andthe equivalent width of the Fe line feature. The equivalent width is byfar the most sensitive to temperature. However, results will be confusedif, as is commonly believed, the abundance of Fe varies from flare toflare, even during the course of a single flare. With temperaturedetermined from the thermal continuum, the Fe line feature becomes adiagnostic of the Fe abundance in flare plasmas. These results are ofinterest for other hot plasmas in coronal ionization equilibrium such asstellar flare plasmas, hot gas in galaxies, and older supernovaremnants.