For realistic predictions of global thermospheric dynamics using thermospheric general circulation models (TGCMs), one of the main problems lies in the specification of the three-dimensional, time-dependent forcing fields for any given day. The September 18--19, 1984, Equinox Transition Study (ETS) interval was simulated using the National Center for Atmospheric Research TGCM with inputs guided by observations. During the ETS campaign, large quantities of high-quality data were available from high latitude, and the high-latitude forcings for the TGCM could therefore be defined with precision for this period. The importance of upward propagating semidiurnal tides is also emphasized. This is the first realistic time-dependent simulation to incorporate upward propagating tides. Magnetically quiet conditions prevailed for several days prior to the magnetic storm which began about 0930 UT on September 19, 1984. The storm generated major disturbances in the global circulation, temperature, and composition of the thermosphere. A description of the simulated quiet time thermospheric structure for September 18 is contrasted with the storm time behavior on September 19. Several features of the storm response are described, including the generation of large-scale equatorward propagating disturbances. Longer-lived perturbations of the neutral temperature and composition are also discussed. A new feature has been discovered in the model predictions for altitudes around 200 km. This feature comprises a quasi-fixed four-cell pattern of high- and low-density regions in the polar cap. The exact location of the feature varies with UT, and the magnitude of the density perturbations depends on the strength of the high-latitude convection. ¿ American Geophysical Union 1989