The Solar Flare 3.8–10 keV X‐Ray Spectrum

The 3.8-10 keV solar flare spectrum includes lines of highly stripped Ca, 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 the broadband RHESSI spectrometer and called here the Fe line and Fe/Ni line features, are formed mostly of Fe lines but with Ni lines contributing to the ~8 keV feature. Possible temperature indicators of these line features are discussed: the peak or centroid energies of the Fe line feature, the line ratio of the Fe line to the Fe/Ni line features, and the equivalent width of the Fe line feature. The equivalent width is by far the most sensitive to temperature. However, results will be confused if, as is commonly believed, the abundance of Fe varies from flare to flare, even during the course of a single flare. With temperature determined from the thermal continuum, the Fe line feature becomes a diagnostic of the Fe abundance in flare plasmas. These results are of interest for other hot plasmas in coronal ionization equilibrium such as stellar flare plasmas, hot gas in galaxies, and older supernova remnants.