NONLINEAR-WAVE WAVE INTERACTIONS RELATED TO GRAVITY-WAVE REFLECTION IN THE AURORAL UPPER F-REGION OBSERVED WITH THE EISCAT RADAR

Some observational evidences of nonlinear wave-wave interactions associated with TIDs in the quiet daytime auroral F-region are presented. The dynamic frequency spectra in the upper F-region and the one-dimensional wavenumber spectra of TIDs show energy cascading from longer periods (larger scales) to shorter ones (smaller scales). The bispectra and the rates of change of the action density indicate that a chain of nonlinear resonant triad interactions take place. The TID-inducing AGW's wavenumbers along the magnetic field k(xi) = k(xi)r +jk(xi)j are calculated with an inversion method developed in this paper by using simultaneous measurements of N(e) and V(ixi). The k(xi)r obtained as a function of height and period show a nearly non-dispersive character at greater heights, The observed nonlinear wave-wave interactions in the higher F-region are accompanied by negative values of -k(xi)3 and wave amplitude growth (k(xi)i > 1/2H), which probably imply an over-reflection. This reflection is supported by sharp meridional neutral wind shear and large incident wavelength. The weakness of the dispersion together with the reflection may enable the resonant conditions to be satisfied.