Narrow spikes in the selective precipitation of relativistic electrons at mid-latitudes

Abstract

Relativistic electron precipitation spikes have been observed in the quasi-trapped population at mid-latitudes (L ≈ 3.5–4.5). The spike electrons are not in the local loss cone but precipitate into the atmosphere upon drift in longitude. In these spikes, often as narrow as 0.3° in invariant latitude, the fluxes of electrons ≳4 MeV may undergo pronounced enhancements, whereas the intensities of electrons in the few hundred keV range may not be significantly affected. This energy selectivity in the formation of the spikes is perhaps more suggestive of wave-particle interactions, such as the Thorne and Kennel (1971) mechanism for the scattering of relativistic electrons by Doppler-shifted ion cyclotron waves, rather than of changes in the magnetic field topology. The data were taken from the low-altitude (∼750 km) polar-orbiting satellite (1972–076B) with an array of several electron detectors having widely differing geometric factors and energy ranges of response. Energy spectra and pitch angle distributions were measured over the energy range 160 keV to 2.4 MeV with a combination of three spectrometers. Fluxes of energetic (>4 MeV) electrons were measured with a high sensitivity in each of two large (∼3.4 kg) plastic scintillator anticoincidence counters surrounding a germanium spectrometer.