QUANTITATIVE-EVALUATION OF THE SOLAR-FLARE MORPHOLOGY IN HARD X-RAYS USING THE YOHKOH HXT

To quantitfy the hard X-ray morphology of solar flares observed with the Yohkoh HXT (Hard X-ray Telescope), a quantity named Configurational Difference Measure (CDM) was introduced. In terms of a single scalar value, CDM measures the difference between two X-ray brightness distributions specified by two data sets from the same flare, separated either in time or in energy. It has been shown that CDM can sensitively quantify the time evolution and energy dependence of the spatial structure of flare hard X-ray sources, with several advantages over the Maximum Entropy Method (MEM). An analysis of a sample of eleven flares employing CDM confirmed a close relationship among the light curve, spectrum, and morphology of flare hard X-rays. The early phase of a flare is generally characterized by impulsive lightcurves, hard power-law spectra, and multiple bright spots suggestive of loop footpoints. In the late phase, a flare tends to show gradual lightcurves, softer spectra, and a loop-like morphology. These properties as revealed with CDM are discussed in terms of thermal and nonthermal mechanisms.