YOHKOH OBSERVATIONS OF FLARES WITH FLAT HARD X-RAY-SPECTRA (VOL 320, PG 620, 1997)

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 flares are large (foot-point separation about 3 10(4) km) and ther efore well resolved by the Yohkoh imaging instruments. The time variat ions match the Neupert effect. The hard and soft X-ray images also sho w footpoint brightening and loop filling. The spikes with the flattest spectra have the weakest Neupert-effect signature, but no perceptible time delays between the hard X-ray time profile and the soft X-ray ti me derivative. These events do not produce superhot emission but are p robably microwave-rich. We find no evidence for large-scale magnetic r econnection in the development of these flares. We note two discrepanc ies between the observations and the existing numerical hydrodynamic m odels of flare energetics, and suggest that rapid spike events of this type provide good tests of such models.