SEASONAL-VARIATIONS OF ENERGETIC ELECTRON-PRECIPITATION BY CYCLOTRON-RESONANCE WITH VLF WAVES FROM A GROUND-BASED TRANSMITTER

We report here for the first time the detection of a seasonal variation in the occurrence of peaks in the spectra of energetic approximately 100-keV electrons precipitating from the inner radiation belt as the result of cyclotron resonance between the waves and the electrons. The spectra reported here were observed from the low-altitude polar orbiting satellite P78-1 at midnight local time. The data, acquired over the 5-year period from March 1979 to July 1984, compose the first multiyear data set used to study cyclotron resonance peaks. The geographical distribution of the events is strongly clustered around the 22.3-kHz VLF transmitter NWC in Australia. Binning the data into seasons, we find that the events occur 2.5 times more often during the 6 months from day 80 to 263 than during the remainder of the year and that the seasonal difference is statistically significant at a high level. Since the observed change is in the frequency of occurrence of the peaks and not in the mean energy of the peaks, we attribute the effect to a seasonal change in ionospheric transmission of VLF waves rather than a seasonal change in the equatorial cold plasma density.