Gravity wave climatology and trends in the mesosphere/lower thermosphere region deduced from low-frequency drift measurements 1984-2003 (52.1°N, 13.2°E)

Fluctuations of lower ionospheric drifts in the virtual height range 85-110 km at 52.1°N, 13.2°E during 1984-2003 are presented. These fluctuations may be partly owing to neutral atmosphere gravity waves in the period window 0.7-3 h. The results show maximum wave activity in the mesosphere in summer, with a shift to equinoxes at higher altitudes. Maximum gravity wave amplitudes are found near the regions of strongest vertical mean wind shear. The propagation direction is generally close to E-W, however, during winter at lower heights a more South-Easterly direction is preferred, while at greater heights during summer a North-Easterly direction is visible. Time series of seasonal (3-monthly) mean zonal drift variances show maximum amplitudes around years 1989-1991 and 2000-2002, when maxima of solar activity within the 11-year solar cycle occurred. We interpret the results in terms of gravity wave activity although additional influences like ionospheric perturbations also may lead to possible wind fluctuations correlated with solar activity. Therefore, the conclusions are partly qualitative and require further experimental verification.