Collisionless Electron Dynamics in the Magnetosheath of Mars

Electron velocity distributions in Mars's magnetosheath show a systematic erosion of the energy spectrum with distance downstream from the bow shock. Previous attempts to model this erosion invoked assumptions to promote electron ionization impact collisions with Mars's neutral hydrogen exosphere. We show that the near collision‐free magnetosheath requires a kinetic description; the population of electrons at any location is a convolution of electrons arriving from more distant regions that ultimately map directly to the solar wind. We construct a simple model that captures all the essential physics. The model demonstrates how the erosion of the electron distributions is the result of the trapping, escape, and replacement of electrons that traverse the global bow shock; some are temporarily confined to the expanding cavity formed by the cross‐shock electrostatic potential. The model also has implications for the ability of solar wind electrons to reach altitudes below the pileup boundary. Plain Language Summary All the planets are embedded in a supersonic flow of ionized gas originating from the Sun's hot atmosphere. Thus, upstream of a planet we see shock waves that slow, heat, and divert the flow around the planet. At planets such as Mars, which lack an internal magnetic field, that shock and the sheath of diverted flow are quite close to the top of the atmosphere. Thus, it has been tempting to attribute the evolution of the gas within the sheath by invoking collisions between, for example, the shocked solar wind electrons and neutral constituents from the planet. However, such collisions are actually quite infrequent. We have developed a simple collision‐free model that shows how the apparent reduction in energy of the shocked solar wind electrons is the result of electron escape back into the exterior flow and their effective replacement by less energized electrons. This approach also highlights the ability of exterior electrons to penetrate more deeply into the atmosphere. Key Points This study develops an idealized collisionless model for electrons in Mars's magnetosheath The model reproduces the erosion of higher energy electron phase space within the magnetosheath