Time variation of the differential emission measure of hot flare plasma

Time evolution of the flare differential emission measure (DEM) distribution in the temperature range 10-50 MK using high-resolution X-ray spectra recorded by the Bent Crystal Spectrometer aboard the Solar Maximum Mission satellite was investigated. It was found that for big flares the local maximum in the DEM distribution is systematically broader during the phase of flare increase than during the phase of decay. It is shown that this systematic effect is a result of development of a hot 'wing' of the DEM local maximum during flare increase and its disappearance during flare decay. It turned out to be convenient to display the DEM evolution in a two-temperature diagram, T1 vs. T2, where T1 and T2 are the values of the electron temperature calculated from the line ratio in the Fe XXV and Ca XIX spectrum, resp. (author)