Measurements of solar hard x-ray bursts with high spectral resolution and sensitivity

We review here the first observations of solar hard X-ray bursts obtained with a combination of a high spectral resolution, < or approx. =1 keV, germanium spectrometer and a large area, low background, NaI/CsI phoswich scintillation detector. In observations of a large flare, the Ge spectrometer was able to identify for the first time a superhot, > or approx. =3 x 1070K thermal component whose spectrum is too steep for measurement by scintillation detectors. With the high sensitivity of this detector complement, a total of approx.25 solar hard X-ray bursts with peak fluxes approx.10-102 times smaller than in solar flare hard X-ray bursts were observed in 141 minutes of observation. These hard X-ray microflares last a few to a few tens of seconds. The integral number of bursts varies as the inverse of the peak flux down to the sensitivity limit of the measurements. These observations suggest that even small transient releases of energy by the Sun may be non-thermal in character