During periods of geomagnetic disturbances, VLF emissions and enhancements of low‐energy electrons are simultaneously observed by the equatorial orbiting S³‐A (Explorer 45) satellite. These events are characterized by the following features. (1) The VLF emissions occur outside the plasmasphere in the nightside of the magnetosphere. (2) The VLF emissions consist of two frequency regimes, one below the local electron gyrofrequency fg and the other above fg. (3) The VLF emissions below fg are relatively broadband whistler mode waves characteristic of chorus and frequently have a conspicuous band of ‘missing emissions’ near fg/2. (4) The emissions above fg are electrostatic and typically have components near 3fg/2. Occasionally, higher‐frequency components are also observed. (5) The onset of the emissions coincides with abrupt increases outside the plasmasphere (L ≳ 4) in 1‐ to 10‐keV electrons to intensities of the order of 108 el cm−2 s−1 sr−1 keV−1. Less pronounced enhancements sometimes occur for electrons with energies as high as 70 keV. (6) The cessation of the emissions coincides with a drop in the electron intensities to their preenhancement levels, which are of the order of 106 el cm−2 s−1 sr−1 keV−1 or less. This drop in low‐energy electron intensities occurs before or when the satellite crosses the plasmapause back into the plasmasphere. These observed features indicate that the VLF emissions are produced by low‐energy (1‐ to 10‐keV) electrons which penetrate into the dusk‐midnight sector of the magnetosphere from the geomagnetic tail during magnetic storms and substorms and drift eastward outside the plasmasphere. In this paper, events observed during geomagnetically disturbed periods in December 1971 and January 1972 are discussed.
Abstract.The penetration, diffusion and slowing down of electrons in a semiinfinite air medium has been studied by the Monte Carlo method. The results are applicable to the atmosphere at altitudes up to ~ 300 km.Most of the results pertain to monoenergetic electron beams injected into the atmosphere at a height of 300 km, either vertically downwards or with a pitch-angle distribution isotropic over the downward hemisphere. Some results were also obtained for various initial pitch angles between 00 and 900. Information has been generated concerning the following topics: (a) the backscattering of electrons from the atmosphere, expressed in terms of backscattering coefficients, angular distributions and energy spectra of reflected electrons, for incident energies T 0 between 2 keV and 2 MeV; (b) energy deposition by electrons as a function of the altitude, down to ~ 80 kr,, for T between 2 keV and 2 'MeV; (c) the correspondingAn O km; (!) rhP.evolution of the electron flux spectrum as function of the atmospheric depth, for T between 2 keV and 20 keV. As far as possible, the results have been expressed in a scaled forn which reduces the explicit dependence on, and permits interpolation with respect to, T o . Energy deposition :cesults are given for incident electron beams with exponential and power-exponential spectra.
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