Yohkoh observations of flares with flat hard X-ray spectra

A series of flares with exceptionally hard spectral indices in the hard X-ray band occurred on 3 October 1993. The non-thermal bremsstrahlung spectra may extend to a few keV in these events, one of which was detectable in the Yohkoh Bragg Crystal Spectrometer at 7 keV as well as by the hard X-ray instruments at higher energies. We present Yohkoh soft and hard X-ray imaging, spectroscopy and energetics analysis of these 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 systematic variations in structure over a period of 3.5 hours except for jet-like ejecta accompanying Type III/V bursts. Unlike other hard events, these Bans are large (footpoint separation about 3 10(4) km) and therefore well resolved by the Yohkoh imaging instruments. The time variations match the Neupert effect. The hard and soft X-ray images also show 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 time derivative. These events do not produce superhot emission but are probably microwave-rich. We find no evidence for large-scale magnetic reconnection in the development of these flares. We note two discrepancies between the observations and the existing numerical hydrodynamic models of flare energetics, and suggest that rapid spike events of this type provide good tests of such models.