Tidal waves in the polar lower thermosphere observed using the EISCAT long run data set obtained in September 2005

Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Tromso (69.6 degrees N, 19.2 degrees E) over similar to 23 days, from September 6 to 29, 2005. The derived semidiurnal amplitude exhibited day-to-day variations (similar to 5-30 ms(-1)) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17-22, 2005. Between September 6 and 16, the vertical wavelengths were estimated to be similar to 58 km and similar to 76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than similar to 24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms(-1)) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5-6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere.