Characteristics of downward flowing ion energy dispersions observed in the low-altitude central plasma sheet by Akebono and DMSP

We present characteristics of downward flowing ion (DFI) energy dispersions observed in the low-altitude central plasma sheet by two polar-orbiting satellites: Akebono and DMSP F8. In general, the typical energy decreases with decreasing latitude. Their main ion composition frequently consists of singly charged oxygen, suggesting that the energy dispersing ions are of ionospheric origin. We conclude that the energy-dispersed signature represents a spatial structure, produced mainly by the global plasma convection driven by the E x B drift on the closed field lines. A probable source of these DFI energy dispersions is the upward flowing ion (UFI) beam accelerated by parallel electrostatic potential at low altitudes. Some case studies show that the energy ratios of the multiple (overlapped) energy dispersions and the pitch angle distributions have some important characteristics consistent with features expected from this model. Clear DFI energy dispersions are observed frequently in stable eastward convection in the postmidnight region, rather than in the premidnight sector. Although the occurrence frequency is. different from the local time dependence of the UFIs as reported previously, this discrepancy is due probably to some differences of the convection patterns in the premidnight and postmidnight sectors. The clear energy dispersions might be smeared out by the distorted or complicated convection pattern, irregularities, and disturbances frequently seen in the premidnight sector.