MF Radar Observations of Nonlinear Interactions Between Tides in the Mid-latitude Winter Lower Thermosphere

Data obtained from Wuhan MF radar in winter of 2001 are used to study the quadratic nonlinear interactions between tides in lower thermosphere. It is observed that diurnal, semidiurnal and terdiurnal tides are the prominent perturbations in the meridional wind component near mid-latitude winter mesopause region, and the 6 h tide is clear. By bicoherence spectrum analysis, it is revealed that most prominent bicoherence peaks stand for phase correlation between tidal harmonics or self-coherence of a single tidal wave. By examining the vertical wavelength time variations, a significant correlation is found between the vertical wavelength of the observed 8 h tide and that of the supposed nonlinearly generated one by the interaction between 12 and 24 h tides. In the interval of 94.0~98.0 km, there exist not only a certain phase correlation and vertical wavenumber correlation but also a strong amplitude correlation of the oscillatory amplitudes equivalent and oscillatory phases synchronous or reversed between the prominent tides, indicating a wave-wave quadratic interaction has occurred. However, below 94.0 km, the various correlations between tides grow weaker and weaker with descending height and hence the tidal quadratic interaction is more likely a local and temporary phenomenon.