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Free keywords:
Mars; Solar system terrestrial planets; Internal waves; Solar activity; Thermosphere; Remote sensing; Planetary science; Solar cycle; Quiet sun
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 the Neutral Gas and Ions Mass Spectrometer on board the Mars Atmosphere and Volatile EvolutioN. 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% and 25%, reaching a maximum at ∼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, the 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.
