COULOMB LIFETIME OF THE RING CURRENT IONS WITH TIME-VARYING PLASMASPHERE

We have developed a time-dependent model of the plasmasphere to evalua te the spatial variation of the Coulomb lifetime of ring current ions. Coulomb collision has been considered to be one of major loss process es of the ring current ions interacted with the thermal plasma in the plasmasphere. The distribution of plasmaspheric density is derived by a continuity equation under the hydrostatic assumption. The protons su pplied from both conjugate ionospheres are drifted by a time-dependent convection field and a corotation electric field. Calculated profiles of the number density and the relative motion of the plasmasphere are in fairly good agreement with the observational results by EXOS-B sat ellite. We traced the energetic ions during a storm on June 4-8, 1991 and calculated the differential flux and the pressure to examine the l oss effects on the pressure due to the both loss processes. We found t hat(1) the Coulomb collision loss restrictively affects at L less than or equal to 3 because the plasmasphere drastically shrank due to the strong convection, and that (2) there is no significant change in the ion composition ratio during the initial rapid recovery of Dst, i.e., the rapid recovery of Dst is not caused by the short charge exchange l ifetime of O+ ions for this particular storm.