Logo: to the web site of Uppsala University

The molecular dication CO₂⁺⁺ has, as previously reported, been detected in the Martian ionosphere by the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution (MAVEN) mission. Photochemical models have also been developed to reproduce the CO₂⁺⁺ density in the Martian dayside ionosphere but underestimate significantly the observations. In this study, we examine the influence of the CO₂⁺⁺ natural lifetime against spontaneous dissociation on its modeled density. We show that extending the assumed CO₂⁺⁺ lifetime significantly reduces the discrepancy between the photochemical model predictions and MAVEN observations. Specifically, when treating CO₂⁺⁺ as stable against natural dissociation, instead of invoking a lifetime of 4 s as done in previous studies, the data-to-model ratio comes close to unity throughout the altitude range 160–220 km. We argue that stability of CO₂⁺⁺ against natural dissociation does not necessarily conflict with results from a frequently cited experimental investigation. Our study provides new insights for advancing photochemical modeling of the Martian ionosphere and underscores the need for further laboratory measurements targeting fundamental properties of doubly charged ions.

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 CO₂ and/or (ii) reduced solar irradiance. At altitudes above 200 km, Ar⁺loss via reactions with H₂ 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.

We conduct a statistical analysis of multiple bow shock (BS) crossings at Mars. Data from the magnetometer (MAG) and Solar Wind Ion Analyzer (SWIA) onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft from its first 2 years in orbit is used to identify flapping events. These are interpreted as the bow shock moving toward and away from Mars. 9% of all MAVEN passes occur when the BS is flapping. Such events are more common on the flanks than on the ramside and more common at southern dayside latitudes than in the north. The probability of flapping increases with increased dynamic pressure and decreased Mach number. The distribution of shock velocity and shock jump differ from the single-BS cases. The shock moves in a swaying motion on the time scale of minutes, and such dynamics could influence other processes at Mars, such as plasma waves, wave-particle interaction, and ion acceleration.