The Electrodynamics of Substorm Injection in the Inner Magnetosphere

During substorms, large transient electric fields have been observed by spacecraft in the inner magnetosphere [Aggson et al, 1977;1983]. These fields are 10-100 times the background electric field, last from seconds to minutes, and occur as the local magnetic field returns to a more dipolar configuration, usually in association with dramatic increases in the local energetic particle flux. In most substorm models, these electric fields are created by the inductive collapse of the magnetic field as it dipolarizes. They are assumed to be spatially extensive, which has consequences for both the amount of energization they can provide, as well as the particle populations they most strongly affect. Recent CRRES observations [Rowland, 2002] suggest that these fields have a strong component normal to the propagating front of particle energization, a result which is inconsistent with an inductive source, yet consistent with a pressure gradient source. Using Cluster, Polar, and CRRES observations of electric and magnetic fields during substorm injections in the inner magnetosphere, we will provide estimates of the importance of each term in the generalized Ohm's law at the substorm injection front / dipolarization region. We will show how the size of these terms can be used to infer the spatial extent and large-scale temporal behavior of the electric field during substorm injections. We will model plasma transport due to these electric fields in a test-particle simulation and compare with previous work which assumed simpler electric field models, as well as in situ energetic particle observations.