Particle-In-Cell Modeling of Martian Magnetic Cusps and Their Role in Enhancing Nightside Ionospheric Ion Escape

Amongst various escape channels, ion outflow is a major contributor to atmospheric loss at Mars over geologic time. On Mars' nightside, observations have indicated that cusp regions within crustal magnetic fields are associated with phenomena such as accelerated particle populations, discrete auroral emissions, and ionospheric outflow; however, the kinetic physics occurring within crustal magnetic cusps is poorly understood. Here, we present 1.5-dimensional particle-in-cell simulations of magnetospheric-ionospheric interactions within martian crustal magnetic cusp regions of varying strength. Simulation results demonstrate the formation of quasi-static, field-aligned potentials pointing away from Mars that accelerate electrons into the martian atmosphere while accelerating ions away, thereby enhancing ionospheric escape. Escaping ionospheric flux scales with crustal field strength, with 160 nT crustal fields yielding >2x the ion escape flux than in the case with no crustal fields. We discuss these results and conclude that magnetic cusp regions may be significant sources of ion loss at Mars. Plain Language Summary Over 4.5 billion years, Mars' atmosphere has gone from warm and dense to cold and thin. An enduring scientific question is how and why Mars lost nearly its entire atmosphere. One possible mechanism for atmospheric loss at Mars is ionospheric escape. Neutral atoms in Mars atmosphere become ionized by gaining a charge after which they are subject to strong electromagnetic forces, which may be sufficient to overcome Mars' gravity and allow the ion to escape Mars completely. Here, we examine a particular environment at Mars, namely, regions on the nightside of Mars that possess strong crustal magnetic fields that are open, meaning one end of the field line connects to Mars while the other is open to space. We use a particle-in-cell model that tracks the behavior of individual ions and electrons as they interact with both magnetic and electric fields. Overall, the model predicts that in regions of strong open crustal magnetic fields, electromagnetic forces stimulate excess ionospheric escape, more than twice the escape in cases with no crustal magnetic fields. These findings are significant as they suggest that regions of open magnetic fields may have contributed significantly to the loss of Mars' atmosphere over geologic time.

Automated summary

Ion outflow is a significant factor contributing to atmospheric loss at Mars over geologic time. Studies have shown that regions within crustal magnetic fields on Mars' nightside may accelerate electrons into the atmosphere and enhance ionospheric escape. The presence of strong open crustal magnetic fields could lead to more than double the ionospheric escape flux compared to regions with no crustal fields, indicating their potential significant contribution to Mars' atmospheric loss.