Seasonal-Latitudinal Structure of the Diurnal Thermospheric Tide

Abstract

The seasonal-latitudinal structure of the diurnal thermospheric tide is investigated for minimum and maximum levels of solar activity by numerically solving the linearized inseparable tidal equations for a spherical, rotating, viscous atmosphere with anisotropic ion drag. Variations in F-region ionospheric structure and the solar zenith angle dependence of the EUV heat source are major factors controlling the seasonal-latitudinal structure of the diurnal thermospheric tide. The combined interaction of the solar-cycle dependent background temperature profile (which controls the altitude of diffusion dominance) and variations in the seasonal structure of the ionospheric plasma with sunspot activity leads to a solar-cycle variation of the seasonal-latitudinal morphology of the diurnal tide. For instance, summer-winter differences in tidal winds at a given height are more pronounced during SSMAX as opposed to SSMIN. At a given level of solar activity, westerly winds, vertical winds and temperatures are generally larger in the summer hemisphere, whereas the amplitude of the northerly wind is greatest during winter. There also exist summer-winter phase differences in the tidal fields ranging from 1 to 6 h, depending upon height, latitude and sunspot activity. Computations of diurnal oscillations in O and N₂ are presented which similarly demonstrate a complex dependence of tidal effects on height, latitude, season and solar activity. In particular, the temperature-atomic oxygen phase difference (the so-called “phase anomaly”) at 300 km varies from about 7 to −2 h at SSMAX and 2 to −2 h at SSMIN, between 80°S and 80°N, respectively, at December solstice. The above results suggest that related seasonal-latitudinal and solar-cycle variations exist in midlatitude ionospheric structure, the F-region equatorial anomaly, the tidal distributions of O₂, Ar, He and H, and magnetic and electric fields generated by the E-region dynamo mechanism. Finally, it is concluded that static diffusion models based on families of empirical temperature profiles cannot simultaneously yield realistic diurnal variations in O, N₂ and temperature below 200 km for SSMIN and 300 km for SSMAX.