Physics of Magnetic Storms

Magnetic storms are defined by the presence of a ring current which comes into existence through the acceleration of particles during episodes of strongly enhanced input of energy from the solar wind into the magnetosphere. The main phase of a storm, during which the primary ring current growth takes place is typically accompanied by sustained substorm expansive phase activity, leading to the suggestion that substorm perturbations play a role in ring current growth. In this paper we shall show that substorm expansive phases do indeed play an important role in energization of ring current particles; however they contribute only a small portion of the energy which is typical for ring current particles. It appears that the dynamical changes in the near-Earth tail magnetic field which occur during substorm expansive phases are not effective in ring current generation regardless of the proximity of the locale of the initiation of the expansive phase with respect to the Earth. Rather, the substorm expansive phase involves a breakdown of the shielding electric field and, in this way, substorms cause the locale of future expansive phases to migrate further earthward. This, in turn, permits plasma sheet ions to penetrate closer to the Earth and become energized adiabatically to the rather high energies typical of the particles that contribute significantly to the ring current. Cyclical stretching and dipolarization of the near-Earth tail magnetic field can energize plasma sheet ions to the extent that the incremental energy provided by the convection electric field may lead to energies in excess of 100 keV for ring current particles and enhance the lifetime of the ring current itself.