We present a spectral model in terms of spherical and Fourier harmonics to describe gravity waves in the thermosphere of Venus. This is an extension of a transfer function model for a dissipative multiconstituent atmosphere on Earth which describes quasi three-dimensional perturbations on the globe, including acoustic-gravity waves. Owing to the differences in the vertical temperature distributions on Earth and Venus, there are significant differences in the propagation characteristics of gravity waves. For example, the ducted wave mode that is excited in the Earth's thermosphere through reflections from the surface and the temperature minimum at the mesopause is absent on Venus. On Venus, as is the case on Earth, wave reflection from the surface can occur. For both planets, the most important gravity wave mode is propagating quasi-horizontally, near the local speed of sound. The model is used to interpret density perturbations in CO2, O and He observed with the orbital neutral mass spectrometer on Pioneer Venus [Niemann, 1977; Kasprzak et al., this issue> which leads to the following conclusions: (1) For localized sources (2 km thick) in the lower atmosphere (at 80 km or below) and thermosphere (at 130 km), of the same magnitude day and night, the computed wave amplitudes at satellite altitudes are about a factor of three to five larger during the night than during the day; in substantial agreement with the observations.

The difference is due to the large night to day increase in the thermospheric temperatures. (2) Waves excited in the lower atmosphere propagate more obliquely, and geometric attenuation as well as dissipation are less important. Thus extended wave trains are produced that can simulate the data. (3) Waves excited in the thermosphere (at 130 km) are rapidly attenuated. Although such signatures are also observed in the data, they could also be produced by an impulsive perturbation propagating up from the lower atmosphere. (4) For the wavelengths observed, the wave periods are between 30 and 60 min. (5) Due to wind induced diffusion, the computed wave amplitudes for the three species are of comparable magnitude, that of CO2 being largest; and He varies out of phase with the heavier gases; reproducing the observed behavior. ¿ American Geophysical Union 1988