Data from the Grahamstown (33.3 • S, 26.5 • E) meteor radar have been used to study the short-term variations of the mean flow at ∼ 90 km altitude. The results show considerable variation characterised by a superposition of fluctuations on different planetary time scales. Wavelet multi-resolution and spectral techniques reveal that the quasi-16-day oscillation dominates the wave spectrum in the ∼ 2-20-day period range. This quasi-16-day oscillation, which is thought to be related to a similar

more »

... cillation in the lower atmosphere, is found to be dominant in winter and the equinoxes. However, it is sometimes significant in summer, which could be due to cross-equatorial ducting and the selective transmissivity of gravity waves.