Neutral Wind Response Dependence on Geomagnetic Forcing

The thermosphere acts as a sink to energy deposited in the upper atmosphere during geomagnetically active periods by redistributing the mass, energy and momentum. One important mechanism of this redistribution is the neutral wind. Through ion-neutral collisions, momentum is transferred from the ionosphere to the thermosphere and the resulting wind pattern typically resembles that of the ionospheric two-cell convection pattern. However, because the neutral wind is dependent on additional forces, such as the pressure-gradient and coriolis forces, its behavior is not well understood. The neutral wind response time, which indicates the strength of ion-neutral coupling, is reported to have time scales ranging from tens of minutes to hours. The dominant forces of the thermospheric winds also have some contradicting results. Both the ion-neutral coupling time scale and the dominant forcing of the neutral wind are crucial for understanding thermosphere dynamics. We present two studies targeted towards these questions. First, using ground based observations of the neutral wind and plasma flow, we perform a statistical analysis of the neutral wind response time to determine the time scale. A superposed epoch analysis is performed on the geomagnetic and ionospheric conditions of these events in order to characterize the controlling factors of the ion-neutral coupling time scale. Second, we drive TIE-GCM using various high-latitude inputs to study the effects of high-latitude driving on simulated neutral winds. The simulation results are compared with observed neutral winds to determine the accuracy of the model and the various high-latitude inputs. Individual thermospheric forcing terms are analyzed in order to determine the dominant forces of the thermosphere.