Acoustic Gravity Waves with Height-Independent Amplitude in the Isothermal Atmosphere

Acoustic gravity wave modes in the Earth’s thermosphere, the amplitude of which does not depend on height, are theoretically investigated. These studies are stimulated by satellite observations, according to which the amplitudes of acoustic gravity waves in the polar thermosphere do not show dependence on height in the altitude range of 250–450 km. It is shown that the propagation of acoustic gravity wave modes with the height-independent amplitude should be considered as an oscillatory process that occurs simultaneously at two natural frequencies. The dispersion equation for these waves is obtained. According to the frequency–wave vector diagnostic diagram, the dispersion dependence of waves with the constant amplitude is in the region that is prohibited for free propagation. It separates the waves propagating horizontally, in which the amplitude in the vertical direction increases from waves with the amplitude decreasing in the vertical direction. Solutions are found for the perturbed quantities in the two-frequency mode of oscillations. It is noted that the superposition of a few of such modes can lead to the emergence of complex resulting motions close to turbulent ones. It is shown that there is a selected quasi-harmonic mode with the constant amplitude, which is characterized by a fixed frequency and wavelength. It is concluded that this kind of wave mode with the height-independent amplitude of the perturbed values prevails in the observations in the Earth’s polar thermosphere.