Low-energy electrons in the dark magnetosphere

Abstract

A number of observations on satellite data are presented which pertain to low energy (∼100 key) electrons in the dark magnetosphere and which were obtained with a scintillation counter aboard Explorer 14. It is shown that the trapping region near the magnetic midnight meridian and at low magnetic latitudes extends on quiet days to distances in excess of 12 Re. The extent of this region and the morphology of electrons near its outer boundary are shown to have a pronounced dependence on geomagnetic activity. Electrons trapped in the distant geomagnetic field are shown to have a pitch-angle distribution whose maximum is along the field lines and which undergoes a 90° change through a region of near Isotropic electron fluxes, generally located at about 7 to 8 Re, near the equatorial plane. Evidence is presented to the effect that the distant field as deduced from the anisotropic features of trapped electrons is highly distorted, having an almost radial appearance and showing a tendency to align itself along the earth-sun direction. A case has been found in which trapped electrons in this region displayed a net flow away from the earth over a 3-minute interval. Finally, it is shown that electron fluxes outside the trapping region and specifically in the tail of the magnetosphere are essentially isotropic, that they appear in pulse-like events clustered around the geomagnetic equatorial plane (at least up to 16 Re), and that the frequency of their occurrence decreases with geocentric distance.