Gravity Wave Activity in the Martian Atmosphere at Altitudes 20-160 km From ACS/TGO Occultation Measurements

The paper presents observations of gravity wave-induced temperature disturbances in the Martian atmosphere obtained with the mid-infrared (MIR) spectrometer, a channel of the Atmospheric Chemistry Suite instrument on board the Trace Gas Orbiter (ACS/TGO). Solar occultation measurements of a CO2 absorption band at 2.7 mu m were used for retrieving density and temperature profiles between heights of 20 and 160 km with vertical resolution sufficient for deriving small-scale structures associated with gravity waves. Several techniques for distinguishing disturbances from the background temperature have been explored and compared. Instantaneous temperature profiles, amplitudes of wave packets and potential energy have been determined. Horizontal momentum fluxes and associated wave drag have been estimated. The analyzed data set of 144 profiles encompasses the measurements made over the second half of Martian Year 34, from the Solar longitude 165 degrees through 355 degrees. We observe enhanced gravity wave dissipation/breaking in the mesopause region of 100-130 km. Our analysis shows no direct correlation between the wave amplitude and Brunt-Vaisala frequency. It may indicate that convective instability may not be the main mechanism limiting gravity wave growth in the middle atmosphere of Mars. Plain Language Summary Gravity waves (GWs) of lower atmospheric origin continuously disturb the Martian atmosphere. While propagating upward, their amplitudes grow and eventually GWs break up or dissipate. The deposited momentum and energy are the major mechanisms driving the circulation in the thermosphere above 100 km. Since spatial scales of GWs are relatively small, they are difficult to measure. Atmospheric Chemistry Suite (ACS) instrument on board the ExoMars Trace Gas Orbiter allows for extracting altitude profiles of density and temperature from the troposphere to the thermosphere (20-160 km) with high vertical resolution, around 2 km. The instrument measures the solar spectrum occulted by the atmosphere with the carbon dioxide absorption in the middle infrared wavelength range. The observations provide latitudinal and seasonal coverage of the GW activity and its characterization on Mars. Our results allow for the first observational validation of model predictions, quantifying dynamical effects of GWs and constraining Martian general circulation models.