Solar hard X-ray bursts and electron acceleration down to 8 kev

In 2 days that were typical of solar maximum (1991 October 1-2), the CGRO/BATSE Spectroscopy Detectors observed more than five solar X-ray bursts per hour above 8 keV [8-13 keV flux greater than 0.4 (cm(2) s keV)(-1)]. Only approximately one-third of these were detected above 25 keV [25-59 keV flux greater than 1.7 x 10(-3) (cm(2) s keV)(-1)] by the BATSE Large Area Detectors (LADs). Most of the >8 keV bursts are impulsive and similar to solar hard X-ray bursts observed above 20 keV-the bursts occurred on the most rapid rise of the GOES thermal soft X-ray burst, and the ratio of 8-13 keV to greater than 13 keV flux for the impulsive bursts fitted a power-law spectrum [dJ/d(h nu) similar to (h nu)(-gamma)] with gamma = 3-7. More than half of the bursts that should be detected by the LADs if the power law extends to high energies are not detected, indicating that the spectrum steepens or cuts off above similar to 25 keV. These findings suggest that the acceleration of electrons occurs for essentially all impulsive energy releases but may only be detectable at low energies in smaller releases. The burst occurrence frequency vanes with energy TV(>8 keV) in accelerated electrons as dn/dW similar to W-1.6 above W similar to 10(29) ergs, with an average rate of energy deposition of similar to 10(26) ergs s(-1), about 100 times that above 20 keV. Comparison with the occurrence frequency of the energy content in active region transient brightenings (ARTBs) detected by the Yohkoh soft X-ray telescope suggests that accelerated electrons may provide a significant fraction of the total energy in ARTBs.