Gravity waves in Venus upper atmosphere revealed by CO2 Non Local Thermodynamic Equilibrium emissions

The imaging capabilities of VIRTIS-M infrared spectrometer on board Venus Express mission are used to analyze perturbations of CO2 non local thermodynamic equilibrium emissions in the thermosphere of Venus. These emissions at 4.3µm wavelength originate from the 110-140 km altitude range and are sensitive to density perturbations. They present wave like perturbations of about 0.5% root mean square amplitude of background signal with wavelengths in the 90-400 km range. The horizontal phase velocities are similar in magnitude and direction from one orbit to the other, 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 9h-15h local time range favours the super-rotation dynamics within 30 degrees from south pole, and at least up to 115 km altitude. Because the maximum altitude and sensitivity of these emissions to density perturbations is not quantified, a quantitative estimate of the upward energy transfer and its effect on the atmosphere circulation cannot be done. However, this study demonstrates the strong influence of the polar vortex on the Venus atmosphere circulation up to the thermosphere.