A global MHD simulation of an isolated substorm that occurred on March 9, 1995 is presented. The simulation, driven with solar wind data provided by the Wind satellite, reproduced to a surprising degree the evolution of substorm activity. The onset of the expansion phase was coincident with the penetration of an electric field spike into the near-Earth region. This impulse launched a tailward propagating signal (rarefaction wave) that enhanced reconnection in the mid tail. Substorm intensification was correlated with the enhancement of the reconnection rate at the preexisting reconnection region located at 30 RE. The importance of the electric field spike in correlating ionospheric and magnetospheric aspects of the substorm is emphasized. ¿ 1998 American Geophysical Union