Solar Semidiurnal Tide in the Thermosphere

Abstract

A three-dimensional model is developed to study the behavior of the solar semidiurnal tide in the thermosphere. In this model, we include viscosity, thermal conductivity, Coriolis effects, the sphericity of the earth, and ion drag. Sources of excitation are absorption of solar radiation by H2O and O2 below the mesopause, and by O2 in the Schumann-Runge continuum, and O, O2, N2 in the extreme ultraviolet, in the thermosphere. The effects of mean wind below the thermosphere are obtained by joining our present results to the relevant solutions obtained by Lindzen and Hong (1974) at 100 km, modified, however, by the use of improved and corrected heating functions. Our calculations provide detailed predictions for all meteorological fields as functions of season, solar cycle, and other parameters. Among our findings are the following: In the present calculations, the semidiurnal tide between 100 and 130 km is dominated by the 2,4 mode excited below the thermosphere. Since the 2,4 mode decays more rapidly than the 2,2 mode above 120 km, the 2,2 mode emerges as the dominant mode above 130?200 km. During sunspot minimum conditions, the thermospheric tidal fields are driven by forcing from below, but at sunspot maximum the upward propagating tides are so severely attenuated in the lower thermosphere that thermospheric in situ forcing is of comparable importance. The thermospheric tidal fields are larger at sunspot minimum than at sunspot maximum. Ion drag is more effective in dissipating the semidiurnal tidal modes than concluded by Lindzen (1971). The omission of either ion drag or viscosity in the model leads to unrealistic results. Comparisons are made between our results and tidal observations at 45° latitude. In general, observed upper thermospheric amplitudes and phases are compatible with calculations. However, such comparisons are difficult below 200 km because of the importance of the 2,4 and 2,3 modes which are dependent on variable mean winds below 100 km. Indeed, we have found choices of mean wind which even lead to significant amplitudes for the 2,5 and higher order modes.