The solar flare soft X-ray differential emission measure and the Neupert effect at different temperatures

Data from the Yohkoh Soft X-ray Telescope (SXT) and Bragg Crystal Spectrometer (BCS) are used to obtain the differential emission measure (DEM) of solar flare soft X-ray plasma. The maximum entropy method is used to obtain the DEM for 80 flares observed by SXT and BCS from 1991 October to 1992 November. We find that it is often difficult to describe the DEM using a monotonically decreasing function; functions with a hump at high temperature are occasionally more useful. From the time history of the DEM, we show that higher temperature plasma peaks earlier and cools faster. We also show that high-temperature plasma (>16.5 MK) is more likely than low-temperature plasma to exhibit the so-called Neupert effect, in which the time derivative of the soft X-ray emission measure is similar to the light curve of the impulsive hard X-ray emission for the flare.