Initial observations of low-energy electrons in the Earth's magnetosphere with OGO 3

Abstract

Initial observations of electrons over the energy range extending from ∼100 ev to 50 kev at geocentric radial distances 8–20 RE in the dark hemisphere of the earth's magnetosphere with electrostatic analyzers borne on OGO 3 are presented for June 12–13, 1966. The electron differential energy spectrums typically are characterized by a single peak in intensities occurring in the energy range ∼0.8–10 kev and at lower energies with increasing geocentric radial distance, by broader widths with decreasing radial distance, and by greater slopes for electron energies Ee ≳ 5 kev with increasing radial distance. The radial profiles of unidirectional and omnidirectional, integral and differential intensities, and energy densities of electrons within the above energy range are characterized by catastrophic variations in magnitude that are presumably reflections of both temporal and spatial variations in intensities. Beyond ∼13 RE many electron spectrums are ‘monoenergetic’ to the extent that ≳75% of the energy flux is shared among electrons in the energy range 1-3 kev, as an example, although measurable electron intensities are observed over the entire energy range ∼400 ev to 50 kev. In contrast with the presistent softening of the electron spectrums with increasing radial distance between 8 and 20 RE, the electron energy densities in the peaks of intensities do not show a marked radial dependence beyond ∼13 RE. The observed electron (Ee > 280 ev) energy densities in the peaks of the radial profiles almost always rise to ∼10−9 erg/cm³, an effect which may be indicative of an instability or ‘saturation’ of the local magnetic field, and are significant in substantially distorting the geomagnetic field beyond ∼8 RE. Typical values of the ratios of intensities J(Ee > 610 ev)/J(Ee > 45 kev) are 104 in the magnetospheric tail. The maximum temporal resolution of the apparatus is ∼100 msec: temporal variations of low-energy electron intensities by factors ≳2 occurred usually in periods ∼seconds to several minutes.