Seasonal variation of dayside ionospheric compositions coupled with neutral upper atmosphere on Mars

Abstract:

The Martian thermosphere-ionosphere is a source for atmospheric escape, which is affected by both the lower-middle atmosphere and external forces. Recent studies suggest a significant impact of the lower-middle atmosphere on upper atmosphere. However, its effect on ionospheric compositions and compositions of escape ions is not well understood.

Seasonal variations of atmospheric compositions in the dayside thermosphere and ionosphere have been investigated using data from December 2014 to March 2018 obtained by Neutral Gas and Ion Mass Spectrometer (NGIMS) on Mars Atmosphere and Volatile EvolutioN (MAVEN). In the thermosphere, densities of CO₂, N₂, and O show seasonal sinusoidal variations. Higher values appear during perihelion and lower values during aphelion. CO₂ at 200 km altitude varies in the range from 3.36×10⁶ to 1.74×10⁸ cm^-3. This can be explained by inflation/contraction of the lower atmosphere (Yoshida et al., under review). In the ionosphere, the seasonal sinusoidal trend is also found in the CO₂⁺ number density, associated with the variation of neutral CO₂ in the thermosphere. On the other hand, N⁺ density shows the opposite sinusoidal trend to CO₂⁺. This can be explained by the loss process of N⁺, which reacts with CO₂ by the charge exchange. The vertical structures of ion species between 150 and 250 km altitude show a clear seasonal variation in the whole altitude range for CO₂⁺ and O₂⁺. However, it is noteworthy that the seasonal variation of vertical profiles of O⁺ is not obvious. Under the photochemical equilibrium of the ionospheric layer below ~200 km, we have evaluated our results discussed above. We find that the significant variation of CO₂ associated with atmospheric inflation/contraction can explain the seasonal variations of ionospheric species, CO₂⁺, O₂⁺, and O⁺. On the other hand, our result also shows a strong depletion of O₂⁺ and O⁺ between 150 and 250 km altitude during a dust storm event (Ls ~300 in MY33). Our result reveals that different behavior of ionospheric species during the dust storm can be explained by a significant decrease of neutral O density in the thermosphere.