[1] Large ionospheric variability is found at low to middle latitudes when a quasi-stationary planetary wave is specified in the winter stratosphere in the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model for solar minimum conditions. The variability includes change of electric field/ion drift, F2 peak density and height, and the total electron content. The electric field/ion drift change is the largest near dawn in the numerical experiments. Analysis of model results suggests that, although the quasi-stationary planetary wave does not propagate deep into the ionosphere or to low latitudes due to the presence of critical layers and strong molecular dissipation, the planetary wave and tidal interaction leads to large changes in tides, which can strongly impact the ionosphere at low and middle latitudes through the E region wind dynamo. Large zonal gradients of zonal and meridional winds from the tidal components and the zonal gradient of electric conductivities at dawn can produce large convergence/ divergence of Hall and Pedersen currents, which in turn produces a polarization electric field. The ionospheric changes are dependent on both the longitude and local time, and are determined by the amplitudes and phases of the superposing wave components. The model results are consistent with observed ionospheric changes at low and middle latitudes during stratospheric sudden warming events, when quasi-stationary planetary waves become large.

[1] TIMED Doppler Interferometer (TIDI) measurements of zonal and meridional winds in the mesosphere/lower thermosphere are analyzed for diurnal nonmigrating tides (June 2002 to June 2005. Climatologies of monthly mean amplitudes and phases for seven tidal components are presented at altitudes between 85 and 105 km and latitudes between 45°S and 45°N (westward propagating wave numbers 2, 3, and 4; the standing diurnal tide; and eastward propagating wave numbers 1, 2, and 3). The observed seasonal variations agree well with 1991-1994 UARS results at 95 km. Comparisons between the TIDI results and global scale wave model (GSWM) and thermosphere-ionospheremesosphere-electrodynamics general circulation model (TIME-GCM) tidal predictions indicate that the large eastward propagating wave number 3 amplitude is driven by tropical tropospheric latent heat release alone. In contrast, latent heating and planetary wave/ migrating tidal interactions are equally important to westward 2 and standing diurnal tidal forcing. There is good quantitative agreement between TIDI and the model predictions during equinox, but the latter tend to underestimate the westward 2 and standing diurnal tide during solstice. Neither model reproduces the observed seasonal variations of the eastward propagating components.

Previous studies have shown that the wind, temperature, and composition structures calculated by the National Center for Atmospheric Research (NCAR) thermospheric general circulation model (TGCM) were in reasonable agreement with measurements made by the Dynamics Explorer 2 satellite over the southern hemisphere polar cap during October 1981. The winds at F region heights follow but generally lag behind the two‐cell ion‐drift pattern of magnetospheric convection. A diagnostic package for the TGCM has been developed to calculate the magnitude and direction of the various terms in the hydrodynamic and thermodynamic equations within the model. The package has been used to decompose the thermospheric momentum equations at each hour of universal time for a 24 hour “steady state” run of the TGCM. Displaced geomagnetic and geographic poles are considered, and a 60‐kV cross‐tail potential is assumed for the magnetospheric convection model. The individual momentum forcing terms for constant‐pressure levels z = 1 (300 km) and z = −4 (120 km) are presented to illustrate the balance of forces acting on the neutral gas at F and E region altitudes over the southern hemisphere polar cap. The results show that at F region altitudes the largest forces in the “steady state” are due to ion drag induced by sunward‐drifting ions on the dawnside and duskside of the auroral oval/polar cap boundary. There is a universal time dependence of thermospheric momentum forcing due to the diurnal oscillation of the convection pattern with respect to the solar terminator. At F region altitudes the basic balance of forces is between ion drag and pressure, whereas at E region altitudes Coriolis and advection forces also have significant contributions. The lower thermosphere cold, low‐pressure cyclonic circulation and the warm, high‐pressure anticyclonic circulation calculated by the TGCM on the dawnside and duskside of the polar cap, respectively, show a near gradient flow balance. The ion‐drag, Coriolis, and pressure forces are in approximate balance on both sides of the polar cap with the advection force enhancing and diminishing the strength of the dawn and dusk vortices, respectively.