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 disper sions observed in the low-altitude central plasma sheet by two polar-o rbiting satellites: Akebono and DMSP F8. In general, the typical energ y decreases with decreasing latitude. Their main ion composition frequ ently consists of singly charged oxygen, suggesting that the energy di spersing ions are of ionospheric origin. We conclude that the energy-d ispersed signature represents a spatial structure, produced mainly by the global plasma convection driven by the E x B drift on the closed f ield lines. A probable source of these DFI energy dispersions is the u pward flowing ion (UFI) beam accelerated by parallel electrostatic pot ential at low altitudes. Some case studies show that the energy ratios of the multiple (overlapped) energy dispersions and the pitch angle d istributions have some important characteristics consistent with featu res expected from this model. Clear DFI energy dispersions are observe d frequently in stable eastward convection in the postmidnight region, rather than in the premidnight sector. Although the occurrence freque ncy is. different from the local time dependence of the UFIs as report ed 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 frequ ently seen in the premidnight sector.