YOHKOH OBSERVATIONS OF FLARES WITH FLAT HARD X-RAY-SPECTRA

A series of flares with exceptionally hard spectral indices in the har d X-ray band occurred on 3 October 1993. The non-thermal bremsstrahlun g spectra may extend to a few keV in these events, one of which was de tectable in the Yohkoh Bragg Crystal Spectrometer at 7 keV as well as by the hard X-ray instruments at higher energies. We present Yohkoh so ft and hard X-ray imaging, spectroscopy and energetics analysis of the se events, with the idea that flares with such flat spectra (power-law as hard as 1.98 below 33 keV) might differ appreciably from ordinary flares. The series of events is strongly homologous, with no systemati c variations in structure over a period of 3.5 hours except for jet-li ke ejecta accompanying Type III/V bursts. Unlike other hard events, th ese Bans are large (footpoint separation about 3 10(4) km) and therefo re well resolved by the Yohkoh imaging instruments. The time variation s match the Neupert effect. The hard and soft X-ray images also show f ootpoint brightening and loop filling. The spikes with the flattest sp ectra have the weakest Neupert-effect signature, but no perceptible ti me delays between the hard X-ray time profile and the soft X-ray time derivative. These events do not produce superhot emission but are prob ably microwave-rich. We find no evidence for large-scale magnetic reco nnection in the development of these flares. We note two discrepancies between the observations and the existing numerical hydrodynamic mode ls of flare energetics, and suggest that rapid spike events of this ty pe provide good tests of such models.