Intercalibration of the hard X-ray spectrometers on the PVO and ICE (ISEE-3) spacecraft

The energetic photon spectrometers aboard the ICE and PVO (Pioneer Venus Orbiter) are described briefly, and the procedure for their in-flight calibrations is discussed. Successful intercalibration of these two instruments led to stereoscopic observations of 100 keV-2 MeV photon sources in solar flares and the study of the directivity and height structure of these sources. The impulsive hard X-ray source is found to extend from the chromosphere to the corona, the brightness of the source decreasing rapidly with increase in height above the chromosphere. The analysis so far indicates no systematic directivity for the hard X-ray source. The observations are consistent with energetic electrons accelerated in the corona propagating downward toward the chromosphere. However, when avaraged over the duration of an impulsive hard X-ray flare, the 'beaming' of electrons is found to be small in most flares.