Atmospheric Research Thermosphere-ionosphere General Circulation Research Articles

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 ° W at 17.5 ° N, 42.5 ° N, and 67.5 ° 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.

Simulations of the September 1987 lower thermospheric tides with the National Center for Atmospheric Research Thermosphere‐Ionosphere General Circulation Model

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) was used to simulate incoherent scatter radar observations of the lower thermosphere tides during the first Lower Thermosphere Coupling Study (LTCS) campaign, September 21–26, 1987. The TIGCM utilized time‐varying histories of the model input fields obtained from the World Data Center for the LTCS period. These model inputs included solar flux, total hemispheric power, solar wind data from which the cross‐polar‐cap potential was derived, and geomagnetic Kp index. Calculations were made for the semidiurnal ion temperatures and horizontal neutral winds at locations representative of Arecibo, Millstone Hill, and Sondrestrom. The diurnal tides at Sondrestrom were also simulated. Tidal inputs to the TIGCM lower boundary were obtained from the middle atmosphere model of Forbes and Vial (1989). The TIGCM tidal structures are in fair general agreement with the observations. The amplitudes tended to be better simulated than the phases, and the mid‐ and high‐latitude locations are simulated better than the low‐latitude thermosphere. This may indicate a need to incorporate coupling of the neutral atmosphere and ionosphere with the E region dynamo in the equatorial region to obtain a better representation of low‐latitude thermospheric tides. The model simulations were used to investigate the daily variability of the tides due to the geomagnetic activity occurring during this period. In general, the ion temperatures were predicted to be affected more than the winds, and the diurnal components more than the semidiurnal. The effects are typically largest at high latitudes and higher altitudes, but discernible differences were produced at low latitudes. Additional observational studies are clearly warranted to test and refine the model predictions.