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.

Downstream of the magnetosheat, at the magnetopause, a part of the magnetic field carried by the solar wind connects with Mercury’s magnetic field through magnetic reconnection. This process allows a fraction of shocked solar wind plasma to precipitate onto the surface, spiraling along newly-opened magnetic field lines. On the nightside, particles are accelerated and ejected planetward as magnetic field lines reconnect in the tail of the magnetosphere. This high-energy plasma is accelerated through the plasma sheet horns (i.e., regions connecting the plasma sheet to the surface at mid latitudes, analogous to auroral ovals at Earth) and precipitates onto the surface spiraling along magnetic field lines. The magnetic topology of the planet determine, to a large extent, the geographical distribution of plasma precipitation on both sides of the planet. Plasma precipitation onto the surface of Mercury is further affected by the 3:2 spin-orbit resonance of the planet, as shown in Fig. 1. As a consequence of this resonance, an observer standing at longitude 0◦(or 180◦) faces the Sun for a longer time and at a closer distance compared to longitude 90◦(or −90◦). Therefore, Mercury’s surface at longitudes 0◦and 180◦(called the hot poles) experience a higher mean photon flux than at longitudes 90◦and −90◦(called the warm poles). This was confirmed by the NASA MESSENGER mission (Solomon et al. 2007), which found a bimodal longitudinal pattern characterized by hot (warm) poles with maximal temperatures of 700◦K (570◦K) (Bauch et al. 2021). A similar longitudinal pattern is expected to arise in the plasma fluxes at the surface when integrating over two full Mercury orbits. Past numerical works have addressed this problem in terms of “time of day” (local time) on the surface (Benna et al. 2010; Schriver et al. 2011; Fatemi et al. 2020; Lavorenti et al. 2023), but plasma precipitation has yet to be examined with regard to geographic location. Looking at plasma precipitation from a geographic perspective will enable to correlate plasma fluxes with spectral and compositional properties of the surface and, thus, to explore potential causal relationships with space weathering driven by the solar wind. Precipitation of solar wind particles onto the surface of Mercury drives space-weathering processes such as ion sputtering, ion implantation, electron stimulated desorption (ESD), and X-ray fluorescence (XRF) (Domingue et al. 2014; Wurz et al. 2022). Ion irradiation affects the surface at an atomic level and the

[Figure: Figure 1. Sketch of plasma precipitation onto Mercury’s surface over orbital time. The 3:2 spin-orbit resonance is responsible for increasing the cusp precipitation at the hot poles (red dashed lines) compared to the warm poles (blue dashed lines).]