The significance of VLF transmitters in the precipitation of inner belt electrons

Abstract

From the low‐altitude satellite P78‐1, narrow and sometimes multiple peaks have been observed in the energy spectra of electrons from 68 to 1120 keV precipitating in the drift loss cone from the inner radiation belt. Simultaneously, measurements of the plasma wave environment at higher altitudes on the same field lines have been made with the ISEE plasma wave experiment in many cases. The peaks in electron energy are sometimes extremely narrow, being of the order of 20‐keV wide after the instrument resolution is unfolded. The plasma wave observations in these cases reveal the frequent presence of single and sometimes multiple monochromatic frequencies in the 10 kHz to 25‐kHz range that are attributed to ground‐based VLF transmitters. The plasma wave experiment also provides electron plasma density profiles. The variations of the peak energies with L value and the nearly simultaneous and spatially coordinated narrow band wave frequencies strongly support the conclusion that the narrow peaks in the electron spectra result from cyclotron resonance interactions with the waves produced by ground‐based VLF transmitters and that the effects of two or more transmitters are sometimes observed simultaneously.