Satellite observations of thermospheric tides: Results from the wind imaging interferometer on UARS

Thermospheric winds measured by the Wind Imaging Interferometer (WINDII) on the upper atmosphere research satellite are analyzed for migrating solar tides. The data cover a 2-year period commencing February 1992 and are obtained from the atomic oxygen O(S-1) 557.7-nm emission, which provides observations of the 90- to 200-km altitude range during daytime and the 90- to 110-km range at night. The subtropical lower thermosphere is dominated by the diurnal propagating tide which exhibits a vertical wavelength of approximately 22 km, grows in amplitude up to 95 km, and decays rapidly above where molecular diffusion greatly reduces the vertical shears. Although the phase remains fairly uniform throughout the year, a pronounced semiannual oscillation is observed in the diurnal tide amplitude. At both 20 degrees N and 20 degrees S the meridional and zonal wind components attain their maximum values at equinox of approximately 70 and 40 m/s, respectively, while the solstitial minima are nearly a factor of 2 smaller. At 35 degrees N the diurnal tide semiannual amplitude oscillation is still present in the lower thermosphere, but above 100 km it is replaced by an annual cycle with a maximum in July and August. This contrasts with 35 degrees S where the July/August peak is absent and the semiannual oscillation extends to 110 km. At midlatitudes the zonal and meridional winds are of similar magnitude, and no significant hemispheric asymmetries in amplitudes are observed. In the lower thermosphere the semidiurnal tide amplitude exhibits an annual oscillation, with maximum values of 30 to 40 m/s occurring in June/July near 100 km at 35 degrees N, 35 degrees S, and the equator. A bimodal structure in the seasonal variation of the semidiurnal phase is observed. This feature is characterized by rapid equinoctial transitions and is particularly well defined at the equator. Examination of the equatorial middle thermosphere indicates that the semidiurnal tide attains its maximum amplitude at 140 km and exhibits a vertical wavelength of approximately 60 km. These findings indicate the predominance of the antisymmetric (2,3) Hough mode in the tropics.