Molecular Ion Contribution to the Polar Plume from Mars: Effects of Solar Wind Parameters and Crustal Magnetic Fields

Polar plume is one of the ion escape mechanisms from Mars [e.g., Dong et al., JGR, 2017]. The polar plume ions are accelerated mainly in the direction of the solar wind convection electric field and characterized by the bulk flow roughly escape to positive E hemisphere of the Mars-Sun-Electric field (MSE) coordinates. While characteristics of the O⁺ plume and its contribution to the ion escape has been investigated, molecular ions contributions to the polar plume are far from understood. In order to investigate properties of the molecular ion (O₂⁺ and CO₂⁺) plumes and their dependence on solar wind conditions, we conducted a statistical study. We analyzed data obtained by STATIC, MAG, and SWIA onboard the MAVEN spacecraft from Nov. 28, 2014 to Oct. 11, 2019. Effects of crustal magnetic fields are also investigated by comparing the plume events in the northern and southern hemispheres.

In some polar plume events, CO₂⁺ ions have larger flux than those of O⁺ and O₂⁺ ions. Results of ion trajectory tracings in global electromagnetic fields obtained by multispecies MHD simulation under a high solar wind dynamic pressure (P_dyn) condition suggest a scenario that the CO₂⁺ plume is caused by deep penetration of the solar wind convection electric field into the low-altitude ionosphere due to high P_dyn. Statistical analysis results show that CO₂⁺ plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. It is consistent with the above scenario. While the O₂⁺ plume events also show similar tendency, the dependences are weaker than those of CO₂⁺. The weaker dependence is consistent with the fact that O₂⁺ ions are abundant at high-altitude ionosphere near the ionopause and the O₂⁺ plumes can be formed even when the solar wind dynamic pressure and convection electric field are in the normal conditions. The statistical results also show that crustal magnetic field affects the composition and escape rate of the polar plume. The total escape rate tends to decrease due to the crustal magnetic fields. This tendency becomes more pronounced under strong solar wind driver conditions, under which CO₂⁺ plume contribution increases.