Hard X-Radiation from a Fast Coronal Ejection

We have observed a high-speed coronal ejection in hard X-rays, detectable to an altitude of some 2 × 10 5 km in the Yohkoh 23-53 keV energy bands. Simultaneous imaging at 17 and 34 GHz from the Nobeyama radioheliograph shows complex moving features simultaneous with the ejection, including a compact source that we identify with the rapid X-ray source motion. The hard X-ray and microwave observations agree on ejection velocities in the vicinity of 1000 km s -1 . The hard X-ray sources also corresponded in position angle to a bright coronal mass ejection (CME) detected about 15 minutes later and temporally to both fast-drift and slow-drift radio bursts in the decimeter-meter bands. Other components of coronal hard X-ray emission were also detected, including an extended long-duration event with a nonthermal spectrum. We suggest that a major eruptive flare occurred in NOAA Active Region 9415, approximately 26° beyond the west limb at the time of the event. Estimating a source density of 4 × 10 9 cm -3 from the compact source observed at 17 GHz, we find a total electron number (>20 keV) of approximately N 20 ~ 1.3 × 10 36 for the compact part of the source. We infer that these electrons were trapped in expanding loops forming a part of the CME and may have contributed substantial pressure within these loops.