Geomagnetic activity effects on thermospheric tides: A compendium of theoretical predictions

The theoretical effects of auroral activity on thermospheric tides are investigated using simulations from the National Center for Atmospheric Research thermosphere-ionosphere general circulation model. Simulations for March, June, and December during solar cycle minimum and maximum are presented for three levels of geomagnetic activity representing quiet, moderate, and disturbed conditions. The mean, diurnal, and semidiurnal components of the model neutral temperatures and horizontal winds at geographic longitude 70 degrees W at 17.5 degrees N, 42.5 degrees N, and 67.5 degrees N are examined. The model predicts that the effects of varying geomagnetic activity are larger at solar cycle minimum than solar cycle maximum and increase with increasing latitude and altitude. In general, the predicted mean temperatures increase, the mean zonal winds become more westward and the mean meridional winds more equatorward as geomagnetic activity increases. In the upper thermosphere, the effects of geomagnetic activity on the mean and tidal neutral winds and temperatures are such that diurnal temperature amplitudes decrease, diurnal zonal winds generally decrease at low and middle latitudes and increase at high latitudes, and diurnal meridional winds increase; semidiurnal amplitudes generally increase. At low altitudes in high latitudes, increasing levels of geomagnetic activity cause tidal amplitudes to increase strongly, peaking near 140 km. At middle latitudes, the lower thermosphere diurnal winds increase in March and June and decrease in December. At low latitudes, the effects at low altitudes are generally small and complex. Comparison with observational and other theoretical results is inconclusive because of a lack of data. (C) 1997 Elsevier Science Ltd.