Collisional losses of ring current ions

Abstract

The time evolution of the ring current population during the recovery phase of a typical moderate magnetic storm is studied, using a newly developed kinetic model for H+, He+ and O+ ions which includes nonequatorially mirroring particles. The bounce‐averaged distribution function is defined for variables that are accessible to direct measurement, and some useful formulas for calculating the total energy and number density of the ring current are derived. The bounce‐averaged kinetic equation is solved, including losses due to charge exchange with neutral hydrogen and Coulomb collisions with thermal plasma along ion drift paths. Time‐dependent magnetospheric electric fields and anisotropic initial pitch angle distributions are considered. The generation of ion precipitating fluxes is addressed, a process that is still not completely 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 104–105 cm−2sr−1s−1keV−1. The mechanisms considered in this paper indicate that magnetospheric convection plays the predominant role in causing ion precipitation; Coulomb scattering contributes significantly to the low‐energy ion precipitation inside the plasmasphere.