An Analysis of Bandgaps in the Spectrum of Acoustic-Gravity Waves in an Isothermal Atmosphere

The entire spectrum of acoustic-gravity waves (AGWs), which can exist in an infinite isothermal atmosphere, is analyzed. The main attention in the study has been paid to those regions of the spectrum that are bandgaps for freely propagating waves. However, other types of waves that differ from the freely propagating AGWs in the way of propagation and in properties still may exist in these regions. Different types of bandgaps in the acoustic-gravity wave spectrum, which are found from the analysis of the dispersion equation obtained in the model of the infinite isothermal atmosphere, are studied. Classification of the types of bandgap regions in the AGW spectrum is proposed. The structure and the localization of the bandgaps relative to the regions of freely propagating waves and special points in the bandgaps of the AGW spectrum are studied using the corresponding spectral diagrams. In the bandgap region of type I, which separates the acoustic and gravity bands of freely propagating AGWs, horizontal waves with a purely imaginary value of the vertical wavenumber can exist. In the AGW spectrum, the possibility of the existence of special acoustic-gravity modes for which one of the perturbed quantities is zero has been considered and it is shown that they can exist only in the spectral bandgap of type I. A spectral bandgap in which vertical acoustic-gravity waves with a purely imaginary value of the horizontal wavenumber can exist was also analyzed. A spectral region in which the existence of acoustic-gravity waves is impossible but atmospheric oscillations may occur is also taken into consideration in this study. The properties of wave solutions in various types of spectral bandgaps, including the peculiarities of polarization ratios, are also analyzed. The theoretical analysis of spectral bandgap regions of AGWs can be used for the experimental search of new types of wave solutions in the atmosphere.