Substorm associated ion acceleration in the dynamic magnetotail

Ion acceleration and flux increases associated with substorm energetic particle injections are investigated on the basis of geosynchronous observations and test proton orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and depolarization in the magnetotail. The energetic particle flux changes obtained from the test particle orbits agree well with observations that demonstrate rapid ion flux increases at energies above ∼20 keV but little change at lower energies. The "injection region" inferred from the test particles not only has a sharp earthward boundary (the usual injection boundary) but also a sharp but ragged tailward boundary well earthward of the neutral line. The earthward portion of enhanced ion flux can be traced to the enhanced cross-tail electric field associated with the collapse and depolarization of the innner tail, whereas the tailward edge is closely associated with the near-Earth x-type neutral line. Due to the rapid earthward motion of accelerated ions away from the neutral line, however, this boundary is displaced earthward to where the energetic ions become more adiabatic in the stronger dipolar field. Lower-energy ions are not affected by cross-tail acceleration in the strong E_v fields because their earthward ExB drift dominates the cross-tail drift, except very close to the neutral line.