Venus Thermospheric Circulation.

Abstract

A variety of Pioneer Venus observations suggest a global scale, day-to-night Venus thermospheric circulation. The two-dimensional hydrodynamic model of Dickinson and Ridley (1977) correctly predicted the gross characteristics of this largely symmetric circulation. However, it failed to calculate the observed cold nightside temperatures, and the exact phases and densities of the neutral constituents. This thesis presents model studies of the dynamics and energetics of the Venus thermosphere, in order to address new driving, mixing and cooling mechanisms for an improved model simulation. The adopted approach has been to re-examine the circulation by first using the previous two-dimensional code to quantify those physical processes which can be inferred from the Pioneer Venus observations. Specifically, the model was used to perform sensitivity studies to determine the degree to which eddy cooling, eddy or wave drag, eddy diffusion and 15 (mu)m radiational cooling are necessary to bring the model temperature and composition fields into agreement with observations. Three EUV heating cases were isolated for study. Global temperature and composition fields in good agreement with Pioneer data were obtained. Large scale horizontal winds (LESSTHEQ)20 m/s were found to be consistent with the observed cold nightside temperatures and dayside bulges of O, CO and CO(,2). Fine tuning required that an eddy coefficient (LESSTHEQ)20% of previous one-dimensional models be used for nightside diffusion (K = 7.5 x 10('6) cm('2)/s). Very little eddy diffusion was required for the dayside (K (LESSTHEQ) 4 x 10('6) cm('2)/s). Observed dayside temperatures were obtained by using a 7-19% EUV heating efficiency profile. The enhanced 15(mu)m cooling needed for thermal balance is obtained using the best rate coefficient (K(,CO(,2)-0) = 5 x 10('-13) cm('3)/s) available for atomic O collisional excitation of CO(,2)(0,1,0). Eddy conduction was not found to be a viable cooling mechanism due to the weakened global circulation. The strong 15(mu)m damping and low EUV efficiency imply a very weak dependence of the general circulation to solar cycle variability. Finally, the NCAR terrestrial thermospheric general circulation model (TGCM) was adapted for Venus inputs using the above two-dimensional model parameters, to give a three-dimensional benchmark for future Venus modelling work.