Theoretical effects of geomagnetic activity on thermospheric tides

Abstract

The theoretical effects of auroral activity on thermospheric tides during equinox solar cycle minimum are investigated using simulations from the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model. One set of model runs examined the effects of increasing levels of geomagnetic activity on the neutral horizontal winds and temperatures. A second set of model runs examined the generation of diurnal and semidiurnal waves in the neutral horizontal winds and temperatures by solar forcing, auroral forcing, and waves propagating vertically from the lower atmosphere. The model simulations were made for four levels of geomagnetic activity, parameterized principally by the total hemispheric power index and the potential drop across the polar cap. The resulting neutral horizontal wind and temperature fields were examined at geographic latitudes of 17.5°N, 42.5°N, and 67.5°N at 70°W longitude. The modeled response to the level of geomagnetic activity varies with altitude and latitude: the effects tend to maximize at high altitudes and high latitudes and penetrate lower in altitude as geomagnetic activity increases. The simulated mean temperatures increase and the mean winds become more southward and westward at all latitudes with increasing auroral activity. In the upper thermosphere, the model diurnal temperature amplitudes decrease with increasing activity, while the diurnal meridional wind amplitudes increase. The modeled semidiurnal winds are strongly affected by the level of geomagnetic activity, while the semidiurnal temperatures are not. Small but discernible effects are predicted at low latitudes. Analysis of the second set of model simulations focusing on the generation of the tidal waves indicates that the tidal response to auroral activity is largely determined by the interference between the waves due to upward propagating tides and in situ solar forcing and those generated by the auroral momentum and energy sources. Depending on location, the waves forced by these sources may have comparable amplitudes but different phases or may be dominated by one source. Thus at high latitudes the wave response is largely driven by the auroral sources; at low latitudes the upward propagating tides dominate. At midlatitudes, complex tidal structures are predicted, since all sources contribute.