2 TYPES OF ION ENERGY DISPERSIONS OBSERVED IN THE NIGHTSIDE AURORAL REGIONS DURING GEOMAGNETICALLY DISTURBED PERIODS

The Akebono satellite has observed two types of energy dispersion sign atures of discrete ion precipitation event in the nightside auroral re gions during active geomagnetic conditions. The charged particle exper iments and electric and magnetic field detectors on board Akebono prov ide us with essential clues to characterize the source regions and acc eleration and/or injection processes associated with these two types o f ion. signatures. The magnetic field data obtained simultaneously by the geosynchronous GOES 6 and 7 satellites and the ground magnetograms are useful to examine their relationships with geomagnetic activity. Mass composition data and pitch angle distributions show that differen t sources and processes should be attributed to two types (Types I and II) of energy dispersion phenomena, Type I consists of multiple bounc ing ion clusters constituted by H+. These H+ clusters tend to be detec ted at the expansion phase of substorms and have characteristic multip le energy-dispersed signatures. Type II consists of O+ energy dispersi on(s), which is often observed at the recovery phase. It is reasonable to consider that the H+ clusters of Type I are accelerated by dipolar ization at the equator, are injected in the field-aligned direction, a nd bounce on closed field lines after the substorm onset. We interpret these multiple energy dispersion events as mainly due to the time-of- flight (TOF) effect, although the convection may influence the energy- dispersed traces. Based on the TOF model, we estimate the source dista nce to be 20-30 R(E) along the field lines. On the other hand, the Oenergy dispersion of Type II is a consequence of reprecipitation of te rrestrial ions ejected as an upward flowing ion (UFI) beam from the up per ionosphere by a parallel electrostatic potential difference. The O + energy dispersion is induced by the E x B drift during the field-ali gned transport from the source region to the observation point.