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 variatio n 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 orbit ing satellite P78-1 at midnight local time. The data, acquired over th e 5-year period from March 1979 to July 1984, compose the first multiy ear 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 fi nd 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 seas onal difference is statistically significant at a high level. Since th e observed change is in the frequency of occurrence of the peaks and n ot in the mean energy of the peaks, we attribute the effect to a seaso nal change in ionospheric transmission of VLF waves rather than a seas onal change in the equatorial cold plasma density.