Gravity waves in the upper atmosphere of Venus revealed by CO2 nonlocal thermodynamic equilibrium emissions

The imaging capabilities of the Visible and Infrared Thermal Imaging Spectrometer-Mapper (VIRTIS-M) onboard Venus Express mission are used to analyze perturbations of CO2 nonlocal thermodynamic equilibrium emissions in the thermosphere of Venus. These emissions with a wavelength of 4.3 mu m originate from the 110-140 km altitude range and are sensitive to density perturbations. They show wave-like perturbations of about 0.5% root-mean-square amplitude of the background signal with horizontal wavelengths in the 90-400 km range. The horizontal phase velocities are similar in magnitude and direction from one orbit to the next, with averages of 70 m/s westward and 30 m/s northward. The geographical wave distribution and the orientation of wavefronts demonstrate that the polar vortex at work in the cloud layer is the source of these gravity waves. The large westward zonal phase velocity in the 0900-1500 local time range argues in favor of the superrotation dynamics within 30 degrees from the south pole to an altitude of at least 115 km. The gravity wave dispersion relation and the geographical distribution of wavefront amplitudes suggest the presence of a solar to antisolar wind close to the south pole. Because the centroid altitude and the sensitivity of these emissions to gravity wave perturbations are only roughly estimated, it is not possible to make a quantitative estimate of the upward energy transfer. However, this study demonstrates the strong influence of the polar vortex on the circulation in the atmosphere of Venus up to the thermosphere.