Climatology of neutral winds in the lower thermosphere over Millstone Hill (42.6°N) observed from ground and from space

Neutral winds in the lower thermosphere at altitudes of 94-130 km were measured in 1987-2000 by the ground-based incoherent scatter radar (ISR) at Millstone Hill (42.6°N, 288.5°E) and in 1992-1997 by the space-based Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The data from the ISR and WINDII have been separately grouped into four seasons. The daytime wind measurements from both instruments are found to be in good agreement, with similar structures and similar magnitudes. Winds from both instruments show an annual variation with a winter minimum. Winds in spring and summer are generally about a factor of two larger than those in fall and winter. The semidiurnal winds are dominant, but diurnal tides, mainly the in situ thermospheric diurnal tide, are comparable. The tidal characteristics and the background winds (the diurnal mean) derived from WINDII data show that the amplitude of the semidiurnal tide is generally 10 ms^-1 larger than that of the in situ thermospheric diurnal tide, and the zonal and meridional background winds are less than 40 ms^-1 and 15 ms^_1 in all seasons, respectively. The TIME-GCM simulations of wind fields at 42.5°N for the March equinox and the December solstice are in reasonable agreement with the observations, but the comparisons for the June solstice are less satisfactory. More research needs to be done to understand the cause of disagreement at the June solstice and the September equinox and to properly model the tidal effects in the lower thermosphere during those periods.