We have used our Global Geospace Circulation Model (GGCM) to simulate two time intervals that were proposed as the Geospace Environment Modeling (GEM) Grand Challenge for modelers to investigate to what extent and accuracy models can predict the ionosphere's response to the solar wind and interplanetary magnetic field. In this paper we present comparisons of our GGCM with the comprehensive experimental study by Lyons [this issue> (which provided synoptic maps of the polar cap electrodynamics and particle precipitation) for the two time intervals, January 27, 1992, 1325--1715 UT and 1730--1930 UT. We find a very good agreement between the potential patterns predicted by our model and those obtained by the assimilative mapping of ionospheric electrodynamics (AMIE) procedure. We also find that the separatrix and cusp locations predicted by our model generally compare well with those obtained from particle precipitation data. The soft electron zone of ionospheric precipitation, as defined by Lyons, lies almost entirely in the region for which our model predicts open field lines. However, the model predicts cross polar cap potential drops that are roughly a factor of 2 larger than those predicted by AMIE. ¿ 1998 American Geophysical Union