Pitch angle distributions of energetic ions in the lobes of the distant geomagnetic tail

Abstract

Analysis of energetic ( > 35 keV) ion data from the ISEE-3 spacecraft obtained during 1982–1983, when the spacecraft made a series of traversais of the distant geomagnetic tail ( X GSE > −230 R E), indicates that the pitch angle distribution of energetic ions in the distant tail lobes is usually highly anisotropic, being peaked closely perpendicular to the magnetic field direction, but with a small net flow in the antisunward direction. In this paper we present a model, based on the motion of single particles into and within the tail lobes, which accounts for these observed distributions. This model assumes that the lobe ions originate in the magnetosheath, where the energetic ion population consists of two components; a spatially uniform “solar” population, and a population of “terrestrial” origin, which decreases in strength with downtail distance. The density of energetic ions in the magnetosheath is therefore high close to the Earth, and falls with increasing distance down the tail. The ions are assumed to enter the lobes across the open magnetopause boundary, and the speed and point of entry as a function of pitch angle are determined from consideration of the subsequent motion of the ions within the lobe, including mirroring close to the Earth. The pitch angle distribution at any point within the lobe may then be constructed, assuming that the value of the distribution function along the particle trajectory is conserved. In general, those ions with a large field-aligned component to their motion enter the lobes in the deep tail, where the “terrestrial” source is weak, whilst those moving closely perpendicular to the field enter the lobes at positions much closer to the Earth, where the source is strong. The fluxes of these latter ions are therefore much enhanced above the rest of the pitch angle distribution, and are shown to account for the form of the observed distributions. It is further shown that at a given point in the tail, the degree to which the ion distribution function is localized to large pitch angles depends on the spatial scale with which the magnetosheath source population decreases away from the Earth. The distributions broaden as the spatial scale gets larger, and show only modest variations with pitch angle in the limit that the source is uniform. The model thus also accounts for the more isotropic ion population observed in the lobe during solar particle events, when the “terrestrial” component of the magnetosheath source may be considered negligible in comparison to the enhanced “solar” component.