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). This represents 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 (hmin < 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.