THERMOSPHERIC TIDES SIMULATED BY THE NATIONAL CENTER FOR ATMOSPHERIC RESEARCH THERMOSPHERE-IONOSPHERE GENERAL-CIRCULATION MODEL AT EQUINOX

Simulations of the equinox thermospheric structure by the National Center for Atmospheric Research thermosphere-ionosphere general circulation model (TIGCM) are presented. Calculations were made for both solar cycle minimum and solar cycle maximum under geomagnetically quiet conditions. Emphasis here is on the tidal structures at geographic longitude 70-degrees-W. Comparisons are made with observations and empirical model predictions of the thermospheric tides from the extended mass spectrometer and incoherent scatter radar model (MSIS) (Hedin, 1991) and the harmonic wind model (HWM) (Hedin et al., 1991). In general, the TIGCM tidal simulations are broadly consistent with available data. Agreement is best for the neutral temperatures and worst for the zonal winds. Investigation of the latitudinal structure of the tidal meridional velocities and temperatures near 300 km reveals important differences between the TIGCM and the empirical MSIS and HWM models. For example, the TIGCM predicts nearly a threefold increase in the diurnal temperature amplitude from solar cycle minimum to maximum with little latitudinal variation in the amplitudes from the equator to about 50-degrees. The empirical MSIS model predicts a somewhat smaller variation of the the diurnal amplitudes over the solar cycle with the amplitudes decreasing with latitude. Resolution of the discrepancies requires acquisition of simultaneous observations from widely separated locations. Ideally, observations would be made in both the lower and upper thermosphere in order to help determine the contribution of the tides excited in the lower atmosphere to the thermospheric structure.