Modeling of inner plasma sheet and ring current during substorms

Abstract

The evolution of the inner plasma sheet and the ring current during substorm dipolarizations is simulated. A substorm cycle is treated by stretching and dipolarizing the magnetosphere according to the Tsyganenko 89 model. In order to clarify the relative influences of steady convection and induction electric field on ring current development, the inductive electric field is superposed on two baseline convective states: a nonstorm state using a weak electric field, and a storm‐time state using a stronger electric field. Ion distributions on the nightside at 12 Earth radii (RE) during these two substorms are obtained using our single‐particle code to trace particle trajectories backward in time to source regions assumed to have steady characteristics. The subsequent acceleration and transport of these boundary ions into the inner magnetosphere is modeled by our kinetic model of the ring current. The simulation generates many frequently observed features of substorm injections, including the sudden appearance of hot plasma tailward of a sharply defined “injection boundary,” the earthward motion of an “injection front,” the azimuthal and tailward expansion of this enhanced region, and the creation of characteristic ion dispersion patterns near geosynchronous orbit. Comparison of the nonstorm and storm cases suggests that substorms occurring without a convection enhancement produce mainly an enhancement of the cross‐tail current but little change in the ring current. With strong convection, the role of substorms is to enable the convection enhancement to create robust ring current in the inner magnetosphere.