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 100-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 mechanisms, we have studied energetic electron data from the Burst and Transient Source Experiment (BATSE) on NASA's Compton Gamma Ray Observatory (GRO) satellite. Wave-particle interactions are detected when the satellite traverses a cloud of energetic electrons which have been scattered into quasi-trapped trajectories by VLF waves. The previously reported seasonal variation in the frequency of occurrence of cyclotron resonance interaction is confirmed with the continuous coverage provided by GRO. These data also provide the first detailed measurements of the frequency of occurrence of the quasi-trapped electron fluxes as a function 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 variations are governed by the transmission of VLF waves through the ionosphere.