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Jeffrey M. Forbes
and
Henry B. Garrett

Abstract

The solar diurnal tide in the thermosphere excited by in situ absorption of EUV and UV solar radiation for equinox conditions is determined by numerically integrating the linearized tidal equations for a spherical, rotating, viscous atmosphere. The model takes into account eddy viscosity, Newtonian cooling, molecular viscosity and conductivity, Coriolis acceleration and anisotropic ion drag. Owing to the inseparability of the mathematical system in height and the latitude, vertical structures of all tidal fields are found to vary with latitude; or equivalently, the horizontal structures vary with height, contrary to classical inviscid tidal theory. Tidal structures also vary with the level of solar activity, since the altitudes where diffusion and hydromagnetic ion drag dominate the momentum balance depend upon the background temperature and ionospheric structures. Increased ion drag (i.e., associated with more active solar conditions) inhibits acceleration of the neutral winds via transfer of momentum to the (denser) ionospheric plasma; however, the concomitant suppression of subsidence heating, which is nearly in antiphase with the EUV heat source, gives rise to increased temperature amplitudes. A factor of 2 increase in the integrated solar heat input from minimum to maximum sunspot conditions thus leads to a factor of 3 increase in the diurnal thermospheric temperature oscillation, but relatively little difference in the velocity fields. Solar heat inputs of 0.4 erg cm−2 s−1 at sunspot minimum and 0.8 erg cm−2 s−1 at sunspot maximum are found to yield thermospheric tides which are consistent with incoherent scatter measurements and satellite density data. Consistency is also maintained with Hinteregger's (1970) solar flux values for medium solar activity combined with a heating efficiency of 30%. In addition we demonstrate the ability of an equivalent gravity wave f-plane formalism to locally approximate our three-dimensional tidal solutions.

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Jeffrey M. Forbes
and
Henry B. Garrett

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.

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