Variations of the Martian Thermospheric Gravity Wave Activity during the Recent Solar Minimum as Observed by MAVEN

Abstract: Atmospheric gravity (buoyancy) waves (GWs) are of great importance for the energy and momentum budget of all planetary atmospheres. Propagating upward waves carry energy and momentum from the lower atmosphere to thermospheric altitudes and re-distribute them there. On Mars, GWs dominate the variability of the thermosphere and ionosphere. We provide a comprehensive climatology of Martian thermospheric GW activity at solar minimum (end of Solar Cycle 24) inferred from measurements by Neutral Gas and Ions Mass Spectrometer on board Mars Atmosphere and Volatile EvolutioN (NGIMS/MAVEN). The results are compared and interpreted using a one-dimensional spectral nonlinear GW model. Monthly mean GW activity varies strongly as a function of altitude (150-230 km) between 6-25%, reaching a maximum at $\sim$170 km. GW activity systematically exhibits a local time variability with nighttime values exceeding those during daytime, in accordance with previous studies. The analysis suggests that the day-night difference is primarily caused by a competition between dissipation due to molecular diffusion and wave growth due to decreasing background density. Thus, convective instability mechanism is likely to play a less important role in limiting GW amplitudes in the upper thermosphere, which explains their local time behavior.