THE PRECIPITATION OF RELATIVISTIC ELECTRONS NEAR THE TRAPPING BOUNDARY

Highly relativistic electrons are known to be present at synchronous satellite altitude in time-varying and sometimes large intensities; it is therefore important to consider the fluxes and locations of relativistic electrons precipitating into the atmosphere. Here we present measurements from the low-altitude three-axis-stabilized satellite S81-1 of trapped and precipitating electrons from 6 keV to above 1 MeV. Significant fluxes of precipitating relativistic electrons above 1 MeV within the bounce loss cone are much more often observed near midnight than noon and generally in narrow spikes < 100 km in width typically at L values between 4 and 6 near the radiation belt boundary. The tendency for many of the relativistic spikes to be near the trapping boundary is consistent with the general pattern at lower energies confirmed by the measurements presented here of intensity and threshold energy for isotropy versus L shell. A trend was observed for the higher-energy precipitating electron fluxes to peak at somewhat lower L values. The precipitation of > 1-MeV electrons has been measured to occur at intensities and in locations that are widely variable within a few minutes superposed on longer-term variations. On one of the days of strongest precipitation the total nightime input to the atmosphere during 12 hours from > 1-MeV electrons within the bounce loss cone near the trapping boundary was approximately 10(19) ergs, which was an order of magnitude less than the loss rates estimated by Baker et al. from high-altitude measurements, suggesting that precipitation in the drift loss cone may be significant.