Evidence for the resonance precipitation of energetic electrons from the slot region of the radiation belts

Abstract

An investigation is made of the energy spectra of energetic electrons precipitating from the slot region between .the inner and outer radiation zones (L • 2 to •3.5). Thins represen'ts an extension of a previous study in which we reported the discovery of peaks (having strongly L-dependent central energies) often occurring in the spectra of electrons precipitating from the outer edge (L -• 1.4 to • 1.8) of the inner radiation zone. Quasi-trapped electrons (hmm < 0) are often observed to have exponential spectral shapes in the slot region, but a variety of other spectral shapes can also occur. On certain passes, L-dependent peaks are found in the spectra of the precipitating electrons. However, the peaks are generally broader and less pronounced than those found at the outer edge of the inner belt. In the slot region up to L • 3.5 the dependences of the central energies of the peaks upon L, on a given satellite pass, are generally consistent with the energy variations calculated for first-order cyclotron resonance at the equator with waves of a constant frequency in the range of a few hundred hertz to about 2 kHz propagating parallel to the magnetic field. This frequency band is consistent with the measurements by Thorne et al. (1973) of plasmaspheric hiss occurring over a broad range of L shells throughout the plasmasphere. Within the limits of available data on equatorial electron densities the frequencies required for cyclotron resonance can be put into agreement with the wave band assumed by Lyons et al. (1972) to account for the high electron loss rates in the slot region. ß ß 1 i I I ß . , 69427 SEC ß I ß , L =2.888 .,, ß ß ß ß ©© ß ß lee e

High‐resolution spectral features observed in the inner radiation belt trapped electron population

“…The energies of the peaks decrease slowly with increasing L value at a rate such that the calculated longitude drift periods remain nearly the same over the full L shell range for which they appear: often from ---1.2 to ---1.4 but occasionally up to L shells as high as ---2.2. These peaks are frequently observed at ---600-km altitude in the more durably trapped electron population having minimum drift altitudes of 100 km or greater and are to be contrasted with peaks in the quasi-trapped electrons precipitating from the inner belt at L = (1.5-1.85) that decrease in energy much more rapidly with increasing L value (Imhof et al, 1974(Imhof et al, , 1978Vampola and Kuck, 1978). The spectra at the lower edge of the inner belt often contain sharp features which are continually changing, perhaps as a result of new injections and/or redistributions and the rapid loss of trapped electrons due to atmospheric scattering.…”

Differences between transmitter precipitation peaks and storm injection peaks in low‐altitude energetic electron spectra

“…The energy of the peak followed the relationship expected from first‐order resonance between the electrons and a single VLF transmitter frequency. The peaks were found in the quasi‐trapped region: the electrons are locally trapped but will precipitate as they drift eastward around the Earth [ Imhof et al , 1974; Vampola and Kuck , 1978].…”

A first approach to model the low-frequency wave activity in the plasmasphere

“…VLF transmitter emissions (10-20 kHz) are coherent right-handed polarized waves which are known to interact with low energetic electrons (less than 50 keV) through the cyclotron resonance in the slot region of the radiation belts (see, for example, Imhof et al, 1974Imhof et al, , 1981Vampola, 1977;Inan and Helliwell, 1982). They are observed in narrow bandwidth ( f < 1 kHz).…”