Variability of temperatures and gravity wave activity in the Martian thermosphere during low solar irradiance

In this paper, we study the temperatures and gravity wave (GW) activity in the Martian thermosphere during low solar activity. For this purpose, we extracted the GW amplitudes and thermospheric temperatures from CO2 densities measured in situ by the Neutral Gas and Ion Mass Spectrometer (NGIMS) aboard the Mars Atmo-sphere and Volatile EvolutioN (MAVEN) mission. These observations were obtained during the declining phase of solar activity between solar longitude (Ls) =294 degrees in Mars year (MY) 32 and Ls = 242 degrees in MY 35. The observations of the present study show that the temperatures are lower and GW amplitudes are higher at low solar activity. The response of the thermospheric temperatures to solar irradiance is local time dependent such that the noontime and duskside temperatures show sig-nificant correlation (correlation coefficient, R > 0.8) with the solar irradiance whereas the temperatures on the dawnside show moderate correlation (R = 0.55). Furthermore, the nominal negative correlation between the gravity wave amplitudes and thermospheric temperatures, which was disturbed during the 2018 global dust event, was restored after the subsidence of the event. Interestingly, the correlation between the thermospheric temperatures and GW activity is also local time dependent with moderate correlation at noon (R =-0.65) and weak correlation at other local times. From the results of the present study, it is inferred that the variability of GW amplitudes in the Martian thermosphere are not necessarily controlled by the temperatures of the underlying atmosphere alone. Other factors, such as the variation of GW amplitudes at the source region and/or changes in the circulation of the underlying atmosphere, are also likely to play a significant role, particularly at the terminator and on the nightside.