Evanescent Acoustic-Gravity Wave Modes in the Nonisothermal Atmosphere

The propagation of evanescent acoustic-gravity waves in the atmosphere with an arbitrary altitude temperature profile is investigated. The possibility of the existence of two types of evanescent wave modes in a vertically nonisothermal atmosphere is shown. The first type is the f-mode in which the dispersion does not depend on the altitude inhomogeneity of temperature and, therefore, is carried out at any altitude level of the nonisothermal atmosphere. The second type is a recently discovered \(\gamma \)‑mode in which the dispersion depends on the altitude temperature gradient and can be fulfilled only at certain altitude intervals. The possibility of realizing the f- and \(\gamma \)- modes in the Earth’s atmosphere is considered, taking into account the model altitude temperature profile. It is shown that these modes can exist at the heights of local temperature extremes in the atmosphere. Moreover, they are realized only in a narrow range of spectral parameters for which the conditions for a decrease in the wave energy above and below the level of their propagation are satisfied. For the f-mode, this energy condition is fulfilled at the altitudes of the local temperature minima, while that for the \(\gamma \)-mode is at the altitudes of the local maxima. Recommendations are given regarding the possibility of observing these modes in the atmosphere of the Earth and the Sun. In the Earth’s atmosphere, the f-mode can presumably be observed near the mesopause with the characteristic wavelength \({{\lambda }_{x}} \approx 75\,{\text{km}}\) and in the solar atmosphere at the heights of the chromospheres with \({{\lambda }_{x}} \approx 1600\,{\text{km}}\). The period of the f‑mode propagating in the region of the temperature minimum slightly exceeds the Brent-Väisälä period at this altitude. In the Earth’s atmosphere, the \(\gamma \)-mode can be realized in the regions of maximum temperature, for example, at the height of the stratopause with \({{\lambda }_{x}} \approx 100\,{\text{km}}\) and a period slightly larger than the Brent-Väisälä period at the altitude of its propagation.