A minimal coherence time for the solar tides in the middle atmosphere

The time scale of the variability in solar tides gives a measure of the time interval over which data will need to be averaged in order to remove local, transient effects. Hence, it gives a measure of the averaging interval required to study global tidal components. It is therefore important to establish some characteristic time scale for the variability of the solar tides and how this may change with the data length. Previous studies have centred on modelling the atmosphere and its response to forcing at tidal frequencies. This allows particular tidal modes to be separated and the times for the establishment of stationarity to be measured. This article presents a technique for measuring the effective tidal coherence time for the real atmosphere, with all its vagaries and complex interactions. Results, using wind data between 78 and 98 km altitude from the MF radar at Adelaide (35 degrees S, 138 degrees E), are presented. (C) 1997 Elsevier Science Ltd.