Pitch angle distributions of low‐energy ions in the near‐Earth magnetosphere

Abstract

Low‐energy (<1 keV) ions have been frequently observed to have some field alignment in their pitch angle distributions in the magnetosphere at geosynchronous orbit or near the equatorial plane. Using data from the energetic ion composition spectrometer on Dynamics Explorer 1, acquired from September 1981 to March 1985, we have examined the occurrence probabilities of various forms of ion pitch angle distributions, including field‐aligned, conical, and equatorially trapped ions. For both H+ and O+, bidirectional field‐aligned distributions at low energy (<1 keV), with pitch angle peaks parallel and antiparallel to the field line direction, were most common at L shells less than about 6 in local times from postmidnight to dawn, except for H+ near the equator, where the equatorially trapped (pancake) distribution had a comparable or dominant occurrence probability. The presence of O+ suggests that these bidirectional ions have a terrestrial origin. Upflowing ion events, which are believed to be a major source of magnetospheric hot ions, were observed most frequently at higher L shells and earlier local times than the bidirectional ions. Bidirectional H+ ions with local minima near the field line (bidirectional conies) were observed to dominate their field‐aligned counterparts in the morning local times. This is attributed to the charge exchange loss of the field‐aligned ions with atmospheric hydrogen in their course of eastward drift. Bidirectional ions were rarely observed in the afternoon local times. This is interpreted as a loss of ions whose drift paths crossed the magnetopause into the magnetosheath.