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

Abstract

Hard X-ray images and X-ray spectra of an intense solar flare that occurred at E58N09 on May 13, 1981 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 approximately 1 arcmin toward the limb from the two-ribbon H-alpha 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 approximately 40,000 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, approximately 40 percent of the total count rate of the hard 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 n = 3 x 10 to the 10th per cu cm, T = 2 x 10 to the 7th K, and beta = 16(pi) n k T/(B squared) approximately 1.0. Intense heat conduction from the thermal plasma near the loop top to the transition region appears to be in equilibrium with the continuous energy release near the loop top.