Escape probability of Martian atmospheric ions: Controlling effects of the electromagnetic fields

Abstract

This study quantifies several factors controlling the probability of a pickup oxygen ion to escape from the Mars upper atmosphere. It is commonly presumed that ions with sufficient kinetic energy are able to escape to space. To test the validity of this simple assumption, we examined results from our Monte Carlo model, which monitors the motion of billions of test particles due to gravity and the Lorentz force through the electromagnetic fields of a magnetohydrodynamic model solution. It is shown that the electromagnetic fields are the dominant factor, surpassing the deceleration of gravity, in controlling ion transport and thus determine whether particles ultimately escape Mars or return to the planet. The particle kinetic energy and the local time of the crustal fields are also important factors greatly influencing the escape probability. In a simulation case in which the strongest crustal fields face the Sun at nominal solar minimum conditions, on average, only 45% of isotropically distributed newborn particles at ∼400 km altitude are able to escape, even with a sufficiently high initial energy of ∼10 eV. Furthermore, there is a distinct hemispheric asymmetry in the escape probability distribution, as defined by the upstream convection electric field direction (Esw). In the above case, the particles produced in the −Esw hemisphere have a much smaller chance to escape, on average, about 17%. These findings imply that one has to be careful when using satellite periapsis measurements to estimate atmospheric loss, where ion densities are high but escape chances may be very low.