Solar gradual hard X-ray bursts and associated phenomena

Abstract

From an examination of the white-light coronagraph, Ha, and radio (microwave, decimetric, and metric) data associated with a sample of 10 gradual hard X-ray burst (GHBs) observed with the Hard X-Ray Burst Spectrometer on SMM and the University of California/Berkeley detector on IEEE 3, we found the following: (1) The hard (~> 30 keV) X-ray photon energy spectrum began to harden near the onset of the GHBs, became systematically harder through the peak of the events, and continued to harden (or at least did not become softer) during the decay phase. (2) A coronal mass ejection (CME) was observed, or inferred to have occurred, in association with at least nine of the 10 GHBs, and CMEs appeared to precede the GHBs, bu times ranging from 5 to 60 minutes. (3) The GHBs characteristically occurred in the late phase of major flares; in general, the changes in Ha flare brightness and area during GHBs were small and not commensurate with their associated microwave burst peak intensities. (4) The centimeter wavelength bursts associated with the GHBs had relatively low frequency spectral maxima, and were "microwave-rich" in relation to the observed hard X-ray emission. (5) The associated decimetric bursts typically exhibited significant intensity variations on time scales of from ~ 0.1 to ~> 1 minute, short in comparison with the ~> 10 minute durations of the gradual X-ray/microwave bursts. (6) The GHBs were poorly associated with coincident metric type II bursts and more strongly associated with type IV or continuum events. The data argue against the type II shock hypothesis for GHBs and are consistent with the acceleration and trapping of the raditing electrons in postflare loop systems following CMEs.