Energy spectra in relativistic electron precipitation events

Abstract

Two events in August 1967, categorized as relativistic electron precipitation (REP) events by their effect on VHF transmissions propagated via the forward-scatter mode, have been examined with regard to the energy spectra of trapped and precipitated electrons. These two substorm-associated events (11 August and 25 August) differ with respect to the relativistic, trapped electron population at synchronous altitude; in the 25 August event there was a nonadiabatic enhancement of relativistic (>400 keV) electrons while in the 11 August event no relativistic electrons were produced. In both events electron spectra deduced from bremsstrahlung measurements (made on a field line close to that of the satellite) had approximately the same e-folding energies as the trapped electron enhancements. However, the spectrum of electrons in the 26 August event ( E 0 = 100–150 keV) was significantly harder than the spectrum in the event of 11 August ( E 0 = 40–70 keV). The spectrum of precipitated electrons deduced from the ratio of VHF forward-scatter absorption to riometer absorption can be made consistent with the spectrum deduced from the bremsstrahlung measurements if the normal summer daytime scattering height for VHF propagation is assumed to lie between 70 and 75 km. However, in the absence of an independent determination of the scattering height, the spectrum of the precipitating electrons can not be deduced from these VHF measurements alone.