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.