Comparison of MHD Simulations of Isolated and Storm Time Substorms

The Lyon-Fedder-Mobarry global magnetohydrodynamic simulation has been used to model the magnetosphere during a variety of solar wind conditions. Early work with the simulation was concentrated on modeling isolated substorms and the global characteristics of magnetic storms. In this paper we will compare the results from simulations of isolated substorms to those occurring during magnetic storms. The substorm which occurred on August 27, 2001 was a fairly typical isolated substorm which had extensive set of satellite and ground based observations. The LFM simulation of the interval agrees with some of these observations. The simulation results also show that thin current sheets develop during the growth phase of substorms and are disrupted, in part, by flows originating the in mid-tail region prior to ionospheric signatures of substorm onset. Furthermore, detailed analysis of the energy partitioning in the magnetotail shows the transfer of energy from the lobes into the plasma sheet occurs a few minutes before substorm onset. The magnetic storm which began on March 31, 2001 was one of the largest magnetic storms during this solar cycle. In this storm-time interval a very large substorm occurred with an onset at 0630 UT. Results from this simulation agree with some observations made by the constellation of spacecraft present during this substorm. Comparison between the simulation results for these two events indicates that storm-time and isolated substorms share the same essential features, development of intense thin current sheets during growth phase, disruption of this current sheet linked to activity in the mid-tail region, and rapid recovery phases.