The response of the ionosphere to a representative auroral substorm was simulated. The response was found to be significant at all altitudes in a large spatial region near midnight magnetic local time. In this midnight region, there were Te and Ti hot spots, substantial O+→NO+ composition changes, non-Maxwellian velocity distributions, transient ion upwellings, a large-scale lowering of the F-layer, ionization peaks that occur in the E-region, and sharp horizontal gradients. Also, during the expansion phase, the E-region densities increase due to auroral precipitation, while the plasma densities above 300 km decrease due to the overall lowering of the ionosphere. The net result is that the temporal morphologies of the plasma densities at high and low altitudes are opposite during this part of the substorm. These complex features indicate that care must be exercised when interpreting plasma measurements from both ground-based and space-based instruments.¿ 1997 American Geophysical Union