A three‐dimensional general circulation model of the thermosphere

The global neutral gas temperature distribution and circulation of the thermosphere are calculated for equinox and solstice conditions by using NCAR's thermospheric general circulation model (TGCM). The variables are determined on a 5° grid in latitude and longitude at 24 constant pressure surface layers in the vertical from about 90 to 500 km. Global empirical models of electron density and neutral composition are used to specify ion drag and the neutral gas background properties. The energy sources that drive the thermosphere include heating caused by the absorption of solar EUV and UV radiation and a high‐latitude heat source associated with auroral processes. Starting from a global average state, the TGCM reached a diurnal reproducible pattern in approximately 5 days and showed no apparent instabilities in the basic flow. The calculated motion patterns indicate that the observed day‐to‐day variability in the thermosphere is likely because of variations in the heat and momentum sources such as the result of neutral plasma interactions and waves coming from the lower atmosphere. The calculated horizontal structure of wind and temperature is qualitatively different between the upper and lower thermosphere. This difference occurs because the diurnal and semidiurnal forcing are in phase in the optically thick lower thermosphere but out of phase in the optically thin upper thermosphere. Using the latest calculated heating efficiencies and measured fluxes of EUV, the calculated diurnal temperature amplitude is 250 K, compared to 320 K predicted by the MSIS empirical model, indicating that solar heating is primarily responsible for maintaining the structure of the thermosphere. The zonally averaged properties of the TGCM essentially agree with previous results obtained from a two‐dimensional model of the zonally symmetric thermosphere. However, for reasons discussed, the latitudinal variations of temperatures is somewhat larger than previously calculated. Our previously estimated auroral heating terms need to be reduced by 30%–40% to get best agreement with observations.