MAVEN NGIMS observations of atmospheric gravity waves in the Martian thermosphere

Gravity waves have a significant impact on both the dynamics and energy budget of the Martian thermosphere. Strong density variations of spatial scales indicative of gravity waves have previously been identified in this region by using in situ observations. Here we use observations from the Neutral Gas and Ion Mass Spectrometer (NGIMS) mass spectrometer on Mars Atmosphere and Volatile EvolutioN Mission to identify such waves in the observations of different atmospheric species. The wave signatures seen in CO₂ and Ar are almost identical, whereas the wave signature seen in N₂, which is lighter and has a larger scale height, is generally smaller in amplitude and slightly out of phase with those seen in CO₂ and Ar. Examination of the observed wave properties in these three species suggests that relatively long vertical wavelength atmospheric gravity waves are the likely source of the waves seen by NGIMS in the upper thermosphere. A two-fluid linear model of the wave perturbations in CO₂ and N₂ has been used to find the best fit intrinsic wave parameters that match the observed features in these two species. We report the first observationally based estimate of the heating and cooling rates of the Martian thermosphere created by the waves observed in this region. The observed wave density amplitudes are anticorrelated with the background atmospheric temperature. The estimated heating rates show a weak positive correlation with the wave amplitude, whereas the cooling rates show a clearer negative correlation with the wave amplitude. Our estimates support previous model-based findings that atmospheric gravity waves are a significant source of both heating and cooling.