Equivalent gravity modes - An interim evaluation

The behavior of the main solar semidiurnal tidal mode in a dissipative atmosphere is studied both in a rotating spherical atmosphere and by means of the equivalent gravity mode approximation. Coefficients are chosen to crudely simulate the effects of molecular viscosity and conductivity. Major findings are: (1) Below 130 km, where friction is unimportant, equivalent gravity mode results are, for all practical purposes, identical to those at the equator obtained from a spherical calculation. (2) Above 130 km amplitudes over the equator obtained from the spherical calculation are about 30 percent smaller than those obtained from the equivalent gravity mode calculations. Also, there is a 15 deg (1/2 hour) difference in phase. (3) The amplitude reduction over the equator, is associated with a broadening of the latitude distribution of amplitude for the oscillatory pressure and temperature fields within the thermosphere. There is also significant variation of phase with latitude within the thermosphere. Associated with the above variations are significant changes in the latitude distribution of horizontal velocity within the thermosphere.