During most summers, the quasi two-day wave (QTDW) is seen in lower thermospheric winds, thus also in our LF measurements.
The Figure shows the evolution of the wave as it was registered in summer 2000. The wave appears in several bursts of different intensity. This is a regular feature frequently seen in other years, too. Note also the decay of the wave in August towards the end of summer, which is also registered in most years. The zonal component of the wave amplitude is, on average, somewhat stronger than the meridional one. This is often seen in Collm winds, but not a regular feature described in literature, and the reported ratios of zonal/meridional amplitude differ from values below to above unity.
During time intervals of strong amplitude the meridional phase (lowermost panel) heads the zonal one by approximately 90°. This means that, according to theory, the wave is nearly circularly polarised. The phase is given in UT, indicating the time of maximum northward or eastward winds, respectively.
Compared to recent years ( see, for example, the 1999 wave), the wave amplitude (uppermost panel) is relatively weak. This does not support an earlier hypothesis that proposed a 10-12 year oscillation in QTDW. Further measurenments in the following years will be necessary to clarify that point.
28.9.2000, last modification: 3.11.2000