Low-Energy Electrons in the Magnetosphere as Observed by OGO-1 and OGO-3

Abstract

Although the first observation of low energy (a few hundred eV to a few keV) electrons in the magnetosphere dates back to the early days of space exploration (Gringauz et al., 1960), detailed studies of these particles have been undertaken only in the last few years. The launch of Vela 2 in July 1964 initiated the first extensive survey of the low-energy electron population. Observations with electrostatic analyzers aboard the Vela satellites (Bame et al., 1966, 1967) showed that the low-energy electrons form part of a plasma sheet stretching across the tail of the magnetosphere, with a broad, quasi-thermal, non-Maxwellian energy spectrum peaked in the vicinity of 1 keV. However, the orbits of the Vela satellites are circular and hence all their observations are confined to geocentric radial distances near 17 R E (earth radii). A complementary survey of low-energy electrons, to study their distribution in radial distance, first became possible a few months after Vela 2, with the launch of OGO-1 into a highly eccentric orbit on September 5, 1964; this satellite carries, among its many experiments, a Faraday cup to detect electrons in the 100 eV – 2 keV range. I shall be reporting results obtained with this detector and a similar one flown two years later on OGO-3.