Before revision: At thermospheric heighs , dissipation of atmospheric waves becomes significant so that at above about 150 km altitude, all wave modes gradually become external waves, and the Hough functions degenerate to spherical harmonics; e.g., mode (1, -2) develops to the spherical harmonic P11(θ), mode (2, 2) becomes P22(θ), with θ the co-latitude, etc.Volland, H., "Atmospheric Tidal and Planetary Waves", Kluwer Publ., Dordrecht, 1988 Migrating Solar Tides The fundamental solar diurnal tidal mode that optimally matches the solar heat input configuration and thus is most strongly excited is the (1, -2) - mode.
After revision: At thermospheric heights , dissipation of atmospheric waves becomes significant so that at above about 150 km altitude, all wave modes gradually become external waves, and the Hough functions degenerate to spherical harmonics; e.g., mode (1, -2) develops to the spherical harmonic P11(θ), mode (2, 2) becomes P22(θ), with θ the co-latitude, etc.Volland, H., "Atmospheric Tidal and Planetary Waves", Kluwer Publ., Dordrecht, 1988 Migrating Solar Tides The fundamental solar diurnal tidal mode that optimally matches the solar heat input configuration and thus is most strongly excited is the (1, -2) - mode.