Morphological study of energetic electron precipitation events using the satellite bremsstrahlung X ray technique

The precipitation of energetic electrons into the atmosphere is investigated with simultaneous measurements of bremsstrahlung X rays emitted from different local time sectors performed from the low-altitude polar-orbiting satellite P78-1 with an array of X ray spectrometers (21–139 keV), each looking in a different direction. From the measurements spanning a broad interval in L, simultaneous profiles in local time are obtained of the intensities and energy spectra of the bremsstrahlung X rays which relate to the corresponding parameters for the precipitating electrons. From ∼0930 to ∼1400 MLT the average X ray intensity was found to decrease with increasing magnetic local time, whereas from ∼2200 to ∼0200 MLT, no clear trend with local time was evident. Within both of these local time intervals the average flux increased with increasing level of geomagnetic activity, and the data acquired near midnight were best organized by the AE index. The correlation between the fluxes of X rays emitted simultaneously at different magnetic local times decreased with increasing separation in local time, but a significant correlation still existed over separations as great as 3 hours. The energy spectrum was harder before midnight than after and harder before midnight than near noon. When a hard energy spectrum was observed in one local time sector, the spectra at other local times within 3 hours were significantly harder than average. From the entire set of data no evidence has been found for a distinct group of relativistic electron precipitation events.