COLLISIONAL LOSSES OF RING CURRENT IONS

The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly d eveloped kinetic model for H+, He+ and O+ ions which includes nonequat orially mirroring particles. The bounce-averaged distribution function is defined for variables that are accessible to direct measurement, a nd some useful formulas for calculating the total energy and number de nsity of the ring current are derived. The bounce-averaged kinetic equ ation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift pa ths. Time-dependent magnetospheric electric fields and anisotropic ini tial pitch angle distributions are considered. The generation of ion p recipitating fluxes is addressed, a process that is still not complete ly understood. It is shown that both the decrease of the distribution function due to charge exchange losses and the buildup of a low-energy population caused by Coulomb collisions proceed faster for particles with smaller pitch angles. The maximum of the equatorial precipitating fluxes occurs on the nightside during the early recovery phase and is found to be of the order of 10(4) - 10(5) cm(-2)sr(-1)s(-1)keV(-1). T he mechanisms considered in this paper indicate that magnetospheric co nvection plays the predominant role in causing ion precipitation; Coul omb scattering contributes significantly to the low-energy ion precipi tation inside the plasmasphere.