Hard X-ray imaging of the solar flare on 1981 May 13 with the Hinotori spacecraft

Hard X-ray images and X-ray spectra of an intense solar flare that occurred at E58N09 on 1981 May 13 are presented and discussed. The observation was made with Japanese solar X-ray spacecraft Hinotori. An unusual hard X-ray source, observed at 14-38 keV, had a steady spatial displacement of approx.1' toward the limb from the two-ribbon Hα flare during the 16 minutes of hard X-ray observation, including the time of maximum flux. This suggests that the source was located near the top of a coronal loop structure connecting the two ribbons, at an estimated altitude of approx.4 x 104 km above the photosphere. The soft X-ray (5-10 keV) source nearly coincided in position and size with the hard X-ray source. Near the peak of hard X-ray time profile, approx.40% of the total count rate of the har X-ray image is estimated to come from a power-law component, as observed with the hard X-ray spectrometer. The parameters of the thermal plasma near the loop top were determined to be eta = 3 x 1010 cm-3, T = 2 x 107 K, and β = 16πetakT/B2roughly-equal1.0. Intense heat conduction from the thermal plasma near the loop top to the transition region appears to be equilibrium with the continuous energy release near the loop top