Photochemical modeling of Ar⁺ ions in the Martian dayside ionosphere: Implications for ionospheric modeling on Mars

The Martian dayside ionosphere has been widely modeled using photochemical equilibrium calculations. These efforts have mostly focused on dominant ion species in order to make comparisons with orbital observations and on displaying non-negligible model-observation discrepancies. In this study, we investigate Ar + ions in the Martian dayside ionosphere, an ion species with a relatively simple chemistry, and perform both case-by-case orbital comparisons and a statistical comparison over five years of observations by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission. Statistically, the ratio of modeled to observed Ar + densities increases from ∼1 near 130 km to ∼4 at 220 km, with notable variations as a function of the solar zenith angle. Pressure-dependent discrepancies show a weaker correlation with the solar zenith angle. Model performance improves when incorporating (i) a higher reaction rate coefficient for the charge transfer between Ar + and CO 2 and/or (ii) reduced solar irradiance. At altitudes above 200 km, Ar + loss via reactions with H 2 becomes increasingly important. However, we find that model-observation agreement varies between orbits: Some show strong consistency, particularly during Deep Dip campaigns, while others exhibit systematic deviations or significant discrepancies. We suggest that while systematic adjustments to reaction rate coefficients, ionization cross sections, solar irradiance, or background neutral densities may improve model fidelity for certain orbits, capturing the dynamic and time-varying nature of the Martian ionosphere requires further comprehensive investigations.