Gravity wave driven variability in the Mars ionosphere-thermosphere system

Gravity waves (GWs) have been persistently observed in neutral species in the Martian thermosphere. In this paper, we study characteristics of GWs in O2+ and O+ ions in the Mars ionosphere (-160-200 km) and examine their relation to GWs in the thermosphere. For this purpose, we use O2+, O+, and Ar densities measured by the Neutral Gas and Ion Mass spectrometer (NGIMS) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The amplitudes and (horizontal) wavelengths of the GWs are extracted from the density profiles. The results of the present study show that GWs are ubiquitous in the Martian ionosphere-thermosphere system. We found that the amplitudes of GWs in ions are -2 times larger than those in neutrals. While the amplitudes in ions reach as high as 70% of the background densities, the most probable amplitude 5%. The amplitudes of the GWs in ions are more on the nightside than on the dayside; more during the 2018 global dust storm period than during nominal dust conditions; and increase with decreasing in solar activity. All these results are qualitatively similar to those observed in neutrals. In addition, amplitudes of the GWs in ions show moderate correlation with those neutrals while the wavelengths show good correlation. The amplitudes of GWs in ions and neutrals show moderate negative correlation with thermospheric temperatures. All these results strongly suggest that the GWs in ions are due to those in neutrals and ion-neutral coupling may be strong in the altitude range of interest. The correlation between the amplitudes of GWs in ions and neutral, however, is local time dependent with weak correlation on the nightside. Other factors such as recombination or particle precipitation may be playing an important role in distorting the wavy nature in ions leading to weaker correlation with neutral waves on the night side. Furthermore, the correlation between GW amplitudes in ions and neutrals is weaker in strong crustal magnetic field regions. This implies that the magnetic fields control the motion of the ions leading to distorting in wavy nature in ions.