The results of a 3D MHD simulation of the January 10--11, 1997 geomagnetic storm are presented. The simulation results agree well with ground-based and geosynchronous observations. The 28 hours modeled by the simulation include the magnetic cloud responsible for the storm, the shock preceding the cloud, and the dense plasma filament following it. The simulation shows that during the period of southward IMF ionospheric activity was strongly correlated to the solar wind density. The arrival of the plasma filament during northward IMF pushed the dayside magnetopause well within geosynchronous orbit, but generated little ionospheric activity. It appears that nsw as well as the orientation of Bsw plays a role in controlling the intensity of ionospheric and magnetospheric activity. ¿ 1998 American Geophysical Union