Non-linear three dimensional spectral model of the venusian thermosphere with super-rotation—II. Temperature, composition and winds

Abstract

We discuss the results from a three dimensional non-linear spectral model of the Venusian thermosphere with CO 2, O and He. As described in an accompanying paper (Stevens-Rayburn et al., 1989, Planet. Space Sci. 37, 701), an expansion in terms of vector spherical and Fourier harmonics is used to represent the latitude and Local Time dependencies, treated as perturbations of a globally uniform atmosphere taken from the empirical model of Hedin et al. (1983, J. geophys. Res. 88, 73). A rigid shell super-rotation rate, uniform in altitude, is adopted. Standard heating rates with an efficiency of about 20% are taken from the work of Fox (1988, Planet. Space Sci. 36, 37). Based on the results from earlier analyses of Pioneer Venus neutral composition data (Niemann et al., 1980, J. geophys. Res. 85, 7817; von Zahn et al., 1980, J. geophys. Res. 85, 7829), height dependent eddy diffusion coefficients around 3 ×10 7cm 2s −1 are adopted, assuming a Prandtl number of 1. Following Bougher et al. (1986, Icarus 68, 284), wave drag in the form of Rayleigh friction is introduced to slow the horizontal winds and to increase the temperature contrast between day and night. With minimal adjusting of the parameterizations for eddy diffusion and Rayleigh friction, the model is successful in reproducing the major observed thermospheric features: the broad daytime maxima in CO 2 and O, with significantly larger values at dusk than at dawn, and the “saw tooth” like density maximum in He around 05:00 LT. To provide a better understanding of the dynamic conditions influencing the temperature and composition, numerical experiments were carried out. (1) The diurnal variations in He are most sensitive to thermospheric super-rotation, and calculations were performed with different rotation periods from 4 to 8 days. The longer the period is, the larger is the day-night increase in the He density and the shorter is the time delay in the density buildup after midnight. We can fit the data best with a super-rotation period of 6 days. Super-rotation also accounts for the dawn-dusk asymmetries in the major species. (2) Given a globally uniform atmosphere as input, larger heating rates yield larger temperature contrast between day and night. The temperature contrast can also be increased by lowering the rate of energy advection from day to night. Thus, an alternative model was constructed, with lower heating rates (artificially reduced by a factor of 2 from the standard values) and enhanced Rayleigh friction to slow the winds, reproducing the observed variations in the temperature and in the major species CO 2 and O. However, in this case, the winds were too small and produced He variations much smaller than observed. (3) With lower heating rate and enhanced Rayleigh friction, a reduced eddy diffusion coefficient for He increased the day-night variations of this species, but produced also a large decrease in its density from the pole to the equator, which is not observed. From these parametric studies we conclude that lower u.v. heating rates with an efficiency significantly less than 20% cannot reproduce simultaneously the observed diurnal variations in the temperature, in the heavier species and in helium. With this heating efficiency, the Venusian thermosphere is strongly non-linear; our model would not converge if it were much larger.