The upper atmosphere of Venus during morning conditions

Abstract

The structure and composition of the Venus upper atmosphere between 130‐ and 650‐km altitude were measured for a solar zenith angle of approximately 60° by the neutral gas mass spectrometer on board the Pioneer Venus multiprobe bus. Below 180 km a wavelike structure is quite evident in the CO2 and He number density profiles. For altitudes above 100 km a one‐dimensional model of the Venus upper atmosphere during morningside conditions (MS model) is presented. Number densities at 150‐km altitude are as follows: CO2= 4.9 × 109, N2=1.1 × 109, CO=2.8 × 109, and He=4.8 × 106 cm−3. The exospheric temperature is 275 K. The vertical compositional structure is best described by an eddy coefficient K=1.4 × 1013na−1/2 cm²/s, where na is the total number density in units per cubic centimeter. The homopause altitudes for N2 and He are at 136 and 130 km, respectively.