Mapping and energization in the magnetotail: 2. Particle acceleration

Abstract

The selection of magnetic and electric field models specifies the rate at which charged particles must be accelerated throughout the magnetosphere during steady state conditions. This rate is estimated in the mid‐tail region (−60 RE < xsm < −10 RE) using the Tsyganenko (1989) magnetic field model and the assumption that E is approximately uniform in the plasma sheet. The uniform E assumption is found to be consistent with a projection of the Heppner and Maynard (1987) electric field model, and with observations in the magnetotail. The resulting energization rate decreases from 4 and 50 GW/RE at xsm = −10 RE to 0.8 and 5 GW/RE at xsm = −60 RE during quiet and disturbed times, respectively. The total energization rate throughout this entire tail region varies from 100 GW when Kp = 0 to 700 GW when Kp = 5. Ions carry most of the cross‐tail current, and therefore gain most of the kinetic energy within the plasma sheet. Electrons, which are responsible for most auroral phenomena, are primarily accelerated at lower altitudes. For quasi‐static conditions, the electromagnetic energy flow to low altitudes is controlled by the magnitude and location of Birkeland currents, even when the current‐carrying particles have negligible kinetic energy. The typical power deposited in the entire nightside auroral zone during steady conditions is approximately 15% to 20% of the Poynting flux entering the plasma sheet. Induced electric fields are known to produce most of the energetic ring current and radiation belt particles during substorm injection events. The energization rate goes well above 1000 GW during brief intervals. A simple injection model is used to compare the structure of induced and potential fields. This model shows plasma drifting earthward, equatorward, and toward midnight during a localized injection event.