Using Wind-measured solar wind data, we have simulated the Earth's magnetosphere and ionosphere for the period 1930--2330 UT on November 24, 1996. The simulation model is a global, three-dimensional, MHD formulation. This event is the focus of a Geospace Environment Modeling (GEM) substorm challenge. The event features a strongly northward interplanetary magnetic field (IMF) period, followed by a sudden rotation to steadily southward. About 80 min after the Wind-observed north-south transition a substorm was observed by the Polar-visible imaging system (VIS). There was good data coverage throughout this period, and the modeling community was challenged to simulate the event. During the northward period the simulation produces a small open polar cap at high magnetic latitudes, especially on the dawnside, where a polar arc was observed by the Polar-VIS and was reproduced in the simulation results. The simulation also shows the ionospheric response to the southward transition in the IMF propagate from the dayside to the nightside in only a few minutes, consistent with Super Dual Auroral Radar Network (SuperDARN) data. Later, the simulation also produces a substorm, but it occurs nearly a half hour earlier than was observed. Numerical experiments were performed by altering the solar wind IMF to investigate the sensitivity of substorm onset timing to this parameter. The model substorms occur spontaneously and do not show a strong correlation between an IMF-induced convection reduction and the onset. The initial event in the expansion onset of the simulation substorm is the beginning of fast magnetic reconnection at ~20RE in the stressed magnetotail. ¿ 2001 American Geophysical Union