Solar Diurnal Tide in the Thermosphere

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.