DIFFERENTIAL EMISSION MEASURE AND IRON-TO-CALCIUM ABUNDANCE IN SOLAR-FLARE PLASMAS

The temperature,distribution of the coronal plasma during solar flares is studied for a set of energetic events of class M and X observed with the bent crystal spectrometer (BCS) flown on the Solar Maximum Mission (SMM) satellite. From the analysis of Ca XIX and Fe XXV spectra, emitted in the regions from 3.165 to 3.231 Angstrom and from 1.840 to 1.894 Angstrom, respectively, we can derive different indicators of the thermal conditions of the flare plasma: the temperature T-Ca,(Fe), derived from the ratio of the intensities of the Ca XIX and Fe XXV resonance lines, and the temperatures T-Ca and T-Fe obtained from the ratio of the dielectronic satellites to the resonance line in the Ca XIX and Fe XXV spectrum. In most of the analyzed flares, the temperature indicators T-Ca,T-Fe, T-Ca, and T-Fe, are considerably different. Our study shows that a difference in T-Ca, and T-Fe is a signature of departure from quasi-isothermal conditions and the variability from flare to flare of the quantity T-Ca,T-Fe is a signature of the variability of the iron to calcium relative abundance. We develop a model of plasma differential emission measure which allows us to account for the enhancements of T-Fe with respect to T-Ca by assuming the presence of a ''superhot'' component at temperature greater than 20 x 10(6) K. The difference of the observed value of T-Ca,T-Fe and that derived from the model can be accounted for by varying the relative abundance of iron to calcium. By applying this model to the observations in a few relevant cases, we infer that the ''superhot'' component in the flare plasma can be at temperatures within 7.3-8.0 in log T, and that the iron to calcium relative abundance with respect to the photospheric Fe/Ca abundance ratio is within 0.4 and 2.3. Hence, on the basis of their chemical abundances we can identify Fe-rich flares and Ca-rich flares.