Injection of Energetic Ions During the 31 March 0630 Substorm

The geomagnetic storm beginning on March 31, 2001 was characterized by high solar wind speeds and long intervals of strongly-southward interplanetary magnetic field. These conditions led to high levels of magnetospheric convection and significant distortion in the inner magnetosphere. During a period of particularly intense driving by the solar wind, an ~0630 UT substorm onset was observed to inject energetic particles into relatively low L-values in the premidnight regions of the inner magnetosphere. In this work we present the results of MHD/particle simulations of the storm, focusing on the period immediately surrounding the 0630 substorm. We analyze the characteristics of the global field prior to onset, and the evolution and effect of the subsequent dipolarization on the keV protons injected from the plasma sheet into the inner magnetosphere. We find good qualitative agreement between global observations of the injection event and the results of the MHD/particle simulations, and suggest that a weak field region in the near-Earth tail may have served as a source region for the more energetic particles injected in the premidnight regions of the magnetosphere.