Solar Flare Composition and Thermodynamics from RESIK X-ray Spectra
  Previous estimates of the solar flare abundances of Si, S, Cl, Ar, and K from the RESIK X-ray crystal spectrometer on board the CORONAS-F spacecraft were made on the assumption of isothermal X-ray emission. We investigate the effect on these estimates by relaxing this assumption and instead determining the differential emission measure (DEM) or thermal structure of the emitting plasma by re-analyzing RESIK data for a GOES class M1.0 flare on 2002 November~14 (SOL2002-11-14T22:26) for which there was good data coverage. The analysis method uses a maximum-likelihood (Withbroe--Sylwester) routine for evaluating the DEM. In a first step, called here AbuOpt, an optimized set of abundances of Si, S, Ar, and K is found that is consistent with the observed spectra. With these abundances, the differential emission measure evolution during the flare is found. The abundance optimization leads to revised abundances of silicon and sulfur in the flare plasma: $A({\rm S}) = 6.94 \pm 0.06$ and $A({\rm Si}) = 7.56 \pm 0.08$ (on a logarithmic scale with $A({\rm H}) = 12$). Previously determined abundances of Ar, K, and Cl from an isothermal assumption are still the preferred values. During the flare's maximum phase, the X-ray-emitting plasma has a basically two-temperature structure, with the cooler plasma with approximately constant temperature (3--6~MK) and a hotter plasma with temperature $16-21$~MK. Using imaging data from the RHESSI hard X-ray spacecraft, the emission volume of the hot plasma is deduced from which lower limits of the electron density $N_e$ and the thermal content of the plasma are given. 
B. Sylwester, J. Sylwester, K. J. H. Phillips, A. Kepa, and T. Mrosek
astro-ph.SR