Photochemical Modeling of CO2++ and Ar+ in the Martian Ionosphere

The composition of Mars' ionosphere has been extensively characterized by MAVEN/NGIMS, and numerous photochemical models have been developed to replicate ion densities in Mars' dayside ionosphere. The molecular dication CO₂⁺⁺ has, as previously reported, been detected and modeled in Mars’ ionosphere; however, previous works have significantly underestimated the observations. In contrast, the noble gas ion Ar⁺ has received limited attention in photochemical modeling studies. In this study, we focus on modeling CO₂⁺⁺ and Ar⁺ and in Mars' dayside ionosphere. For CO₂⁺⁺, an extended lifetime against natural dissociation compared to what is used in earlier modeling works significantly reduces discrepancies between photochemical model predictions and MAVEN observations throughout the altitude range 160-220 km. Our findings indicate that the natural lifetime of CO₂⁺⁺ may require re-evaluation, which is not in conflict with results from a frequently cited experimental investigation. For Ar⁺, we perform both orbital and statistical comparisons between model results and observations and found reasonable model-observation agreement, particularly at altitudes below 200 km, using a higher rate coefficient for the reaction between Ar⁺ and CO₂, and under high solar EUV conditions.