Wavelet analysis on transient behaviour of tidal amplitude fluctuations observed by meteor radar in the lower thermosphere above Bulgaria

On the basis of bispectral analysis applied to the hourly data set of neutr al wind measured by meteor radar: in the MLT region above Bulgaria it was d emonstrated that nonlinear processes are frequently and regularly acting in the mesopause region. They contribute significantly to the short-term tida l variability and are apparently responsible for the observed complicated b ehavior of the tidal characteristics. A Morlet wavelet transform is propose d as a technique for studying nonstationary signals. By simulated data it. was revealed that the Morlet wavelet transform is especially convenient for analyzing signals with: (I) a wide range of dominant frequencies which are -localized in different time intervals; (2) amplitude and frequency modulat ed spectral components, and (3) singular, wavelike events, observed in the neutral wind of the MLT region and connected mainly with large-scale distur bances propagated from below. By applying a Morlet wavelet transform to the hourly values of the amplitudes of diurnal and semidiurnal tides the basic oscillations with periods of planetary waves (1.5-20 days), as well as the ir development in time, are obtained. A cross-wavelet analysis is used to c larify the relation between the tidal and mean neutral wind variability. Th e results of bispectral analysis indicate: which planetary waves participat ed in the nonlinear coupling with the atmospheric tides, while the results of cross-wavelet analysis outline their time intervals if these interaction s are local.