IONOSPHERIC RESPONSE TO AN AURORAL SUBSTORM

The response of the ionosphere to a representative auroral substorm wa s 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 T-e and T-i hot spots, substantial O+ --> NO+ composition changes, non-Maxwellian velocity distributions, transi ent 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 a uroral 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 alt itudes are opposite during this part of the substorm. These complex fe atures indicate that care must be exercised when interpreting plasma m easurements from both ground-based and space-based instruments.