Observations of the magnetosphere and ionosphere by Dynamics Explorer satellite. IX - Pitch-angle distribuions of magnetospheric low-energy ions observed by DE-1

Using data collected in the 1981-1985 period by the Energetic-Ion Composition Instrument aboard DE-1, the pitch-angle distributions of low-energy ions in the magnetosphere at geosynchronous orbit and near the equatorial plane are examined. Particular consideration is given to the occurrence probabilities of the various forms of ion pitch-angle distribution, including field-aligned, conical, and equatorially trapped ions. The results show that bidirectional field-aligned distributions, with pitch-angle peaks parallel and antiparallel to the field-line direction, are most common for the lowest energy (less than 1 keV) ions at L-shells less than about six, at local times from postmidnight to dawn. Upflowing ion events were observed most frequently at higher-than-six L-shells and at earlier local times. It was found that the bidirectional field-aligned ions, especially H(+), change to bidirectional distributions having local minima near the field line at morning local times.