LOCAL TIME AND SEASONAL-VARIATIONS IN THE PRECIPITATION OF ENERGETIC ELECTRONS FROM THE INNER RADIATION BELT BY CYCLOTRON-RESONANCE WITH WAVES FROM POWERFUL VLF TRANSMITTERS

Powerful ground-based VLF transmitters resonantly scatter similar to 1 00-keV electrons from the inner radiation belt onto trajectories from which they precipitate into the atmosphere as they drift eastward. To obtain a better understanding of the underlying wave-particle mechanis ms, we have studied energetic electron data from the Burst and Transie nt Source Experiment (BATSE) on NASA's Compton Gamma Ray Observatory ( GRO) satellite. Wave-particle interactions are detected when the satel lite traverses a cloud of energetic electrons which have been scattere d into quasi-trapped trajectories by VLF waves. The previously reporte d seasonal variation in the frequency of occurrence of cyclotron reson ance interaction is confirmed with the continuous coverage provided by GRO. These data also provide the first detailed measurements of the f requency of occurrence of the quasi-trapped electron fluxes as a funct ion of local time at the transmitter. The variations are more complex than just a day-night effect, and the precipitation is most often seen in the predawn hours. As with the seasonal changes, the local time va riations are governed by the transmission of VLF waves through the ion osphere.