ION SOURCES AND ACCELERATION MECHANISMS INFERRED FROM LOCAL-DISTRIBUTION FUNCTIONS

This study investigates the sources of the ions making up the complex and nonisotropic Hf velocity distribution functions observed by the Ge otail spacecraft on May 23, 1995, in the near-Earth magnetotail region and recently reported by Frank et al. [1996]. A distribution function observed by Geotail at similar to 10 R-E downtail is used as input fo r the large scale kinetic (LSK) technique to follow the trajectories o f approximately 90,000 H+ ions backward in time. Time-dependent magnet ic and electric fields are taken from a global magnetohydrodynamic (MH D) simulation of the magnetosphere and its interactions with appropria te solar wind and IMF conditions. The ion population described by the Geotail distribution function was found to consist of a mixture of par ticles originating from three distinct sources: the ionosphere, the lo w latitude boundary layer (LLBL), and the high latitude plasma mantle. Ionospheric particles had direct access along field lines to Geotail, and LLBL ions convected adiabatically to the Geotail location. Plasma mantle ions, on the other hand, exhibited two distinct types of behav ior. Most near-Earth mantle ions reached Geotail on adiabatic orbits, while distant mantle ions interacted with the current sheet tailward o f Geotail and had mostly nonadiabatic orbits. Ions from the ionosphere , the LLBL, and the near-Earth mantle were directly responsible for th e well-separated, low energy structures easily discernible in the obse rved and modeled distribution functions. Distant mantle ions formed th e higher energy portion of the Geotail distribution. Thus, we have bee n successful in extracting useful information about particle sources, their relative contribution to the measured distribution and the accel eration processes that affected particle transport during this time.