Observations of Thermal Plasma in a Solar Limb Flare

Abstract

All but a small portion of the top of the 1991 October 21 flare was occulted by the solar limb. Thus, both the Bragg crystal spectrometer (BCS) and the soft X-ray telescope (SXT) on Yohkoh observed only a small region of flaring plasma. Using observations from those instruments, we have determined the temporal behavior of the emission measure, temperature, and nonthermal broadening in the loop-top source. We find that for a portion of the decay phase of this flare, the temperatures determined using the Fe XXV, Ca XIX, and S XV BCS channels and SXT filter ratios all agree. Assuming that the plasma is isothermal, we determine the abundances of Fe XXII, Fe XXIII, and Fe XXIV relative to Fe XXV. Comparing the results with recent ionization equilibrium calculations, we find that there are discrepancies between the Fe XXII-to-Fe XXV ratio, the Fe XXIII-to-Fe XXV ratio, and the Fe XXIV-to-Fe XXV ratio and ionization equilibrium calculations, which we attribute to uncertainties in the ionization balance calculations.