Properties of Acoustic-Gravity Waves at the Boundary of Two Isothermal Media

The properties of evanescent acoustic-gravity waves that can propagate along the interface between two isothermal half-spaces with different temperatures are studied. In such a model, the condition of a simultaneous decrease in the wave energy density below and above the interface between the media is not satisfied for the known surface f mode. This study shows that it is possible to implement evanescent waves in the form of combinations of f modes and pseudo-modes (f_p modes) for both half-spaces at the interface between two isothermal media. The cross-linking of solutions at the interface depends on the wave spectral parameters and the magnitude of the temperature jump. At the interface, the wave properties change with an increase in the wavelength and their dispersion and polarization acquire features characteristic of acoustic-type waves. These differences are manifested not only in the dispersion dependence of the waves but also in the change in their amplitudes with height, polarization, and velocity divergence at the interface between the media. It is also found for large temperature differences between the lower and upper half-spaces that there is a spectral region in which the solutions satisfying the boundary condition cannot simultaneously decrease in energy below and above the interface. In this region of the spectrum, the f_p modes with a decreasing energy in the upper half-space and the f modes with an increasing energy in the lower half-space are joined at the interface. The considered waves at the interface between two media can be observed in the stratified atmosphere at altitudes with a sharp temperature change, for example, in the lower part of the Earth’s thermosphere or in the chromosphere–corona transition region on the Sun.