  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.
