(Dated: January 2023) ABSTRACT Mercury is the closest planet to the Sun, possesses a weak intrinsic magnetic field and has only a very tenuous atmosphere (exosphere). These three conditions result in a direct coupling between the plasma emitted from the Sun (namely the solar wind) and Mercury’s surface. The planet’s magnetic field leads to a non-trivial pattern of plasma precipitation onto the surface, that is expected to contribute to the alteration of the regolith over geological time scales. The goal of this work is to study the solar wind plasma precipitation onto the surface of Mercury from a geographical perspective, as opposed to the local-time-of-day approach of previous precipitation modeling studies. We employ solar wind precipitation maps for protons and electrons from two fully-kinetic numerical simulations of Mercury’s plasma environment. These maps are then integrated over two full Mercury orbits (176 Earth days). We found that the plasma precipitation pattern at the surface is most strongly affected by the upstream solar wind conditions, particularly by the interplanetary magnetic field direction, and less by Mercury’s 3:2 spin-orbit resonance. We also found that Mercury’s magnetic field is able to shield the surface from roughly 90% of the incoming solar wind flux. At the surface, protons have a broad energy distribution from below 500 eV to more than 1.5 keV; while electrons are mostly found in the range 0.1-4 keV. These results will help to better constrain space weathering and exosphere source processes at Mercury, as well as to interpret observations by the ongoing ESA/JAXA BepiColombo mission.
1. INTRODUCTION Mercury is the only telluric solar system planet, other than Earth, with an intrinsic magnetic field (Ogilvie et al. 1974). Mercury’s magnetic field shapes the interaction between the planet’s surface and the surrounding solar wind — a turbulent, supersonic, and magnetized plasma flowing outward from the Sun (Meyer-Vernet 2007). The interaction between Mercury’s magnetic field and the solar wind is a crucial part of the global Hermean environment, both shape and evolution. This is a result of the planet’s proximity to the Sun (0.31 AU at perihelion and 0.47 AU at aphelion) and its relatively weak magnetic field (around 200 nT at the surface). The interaction between the solar wind and Mercury’s magnetic field determines the precipitation of solar wind protons and electrons onto the planet’s surface, through a complex series of coupled local processes. On the dayside, the presence of a bow shock in front of the planet along with magnetic reconnection in the magnetopause determines the pattern and energy of precipitating plasma; while on the nightside, the precipitating plasma is mostly affected by magnetic reconnection and the magnetic field configuration in the tail. On the dayside, downstream of the bow shock (in the so-called magnetosheat), the plasma density, temperature and magnetic field increases while the plasma bulk velocity decreases. This slowdown leads to an order of magnitude reduction in the proton kinetic energy, which still dominates over the thermal energy component in the total energy of the protons. This differs with respect to the 2 slowdown in the electron velocity, which does not alter significantly the electron total energy, as the thermal energy component dominates over the kinetic component in the total energy of the electrons.