Solar-wind electron precipitation on weakly magnetized bodies: The planet Mercury

Rocky objects in the Solar System (such as planets, asteroids, moons, and comets) undergo a complex interaction with the flow of magnetized, supersonic plasma emitted from the Sun called solar wind. We address the interaction of such a flow with the planet Mercury, considered here as the archetype of a weakly magnetized, airless, telluric body immersed in the solar wind. Due to the lack of dense atmosphere, a considerable fraction of solar-wind particles precipitate on Mercury. The interaction processes between precipitating electrons and other nonionized parts of the system remain poorly understood. Shading light on such processes is the goal of this work. Using a 3D fully kinetic self-consistent plasma model, we show for the first time that solar-wind electron precipitation drives (i) efficient ionization of multiple neutral exosphere species and (ii) emission of X-rays from the surface of the planet. We conclude that, compared to photoionization, electron-impact ionization should not be considered a secondary process for the H, He, O, and Mn exosphere. Moreover, we provide the first, independent evidence of X-ray aurora-like emission on Mercury using a numerical approach.