Seasonal and Dust‐Related Variations in the Dayside Thermospheric and Ionospheric Compositions of Mars Observed by MAVEN/NGIMS

Abstract

Abstract We report seasonal and dust‐related variations in neutral and ion species (CO 2 , O, and N 2 , and CO 2 + , O 2 + , O + , and N + , respectively) in the dayside Martian upper atmosphere between altitudes of ∼150 and ∼250 km observed by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile Evolution spacecraft. The sinusoidal seasonal variations in CO 2 + and O 2 + densities are clearly identified, while that of O + is less discernible. These observed variations in ion densities are well reproduced by a photochemical equilibrium model for CO 2 + and O + densities when we combine them with solar cycle variations. Furthermore, we find a decrease in O, O + , and O 2 + densities in the whole altitude range at L s = 342–346 in MY 33 during a regional dust event. The decrease in O density would lead to decreases in O + and O 2 + densities in the ionosphere through ion‐neutral reactions. Observed variations in ion and neutral species associated with the season and a regional dust storm are also confirmed in pressure coordinates. Observations show that the CO 2 + /O + ratio at a given pressure level in the ionosphere varies by a factor of ∼3, which can modify the composition of ion outflow from the Martian atmosphere.