the northern hemisphere and latitudes from −30◦to −70◦in the southern hemisphere, as shown in Fig. 4(b) for protons and Fig. 6(b) for electrons. The north-south asymmetry in the size of the cusp precipitation region is a consequence of the northward shift of Mercury’s magnetic dipole. The position of the northern cusp is in agreement with recent estimates by Raines et al.

[Figure: Figure 5. Same as Fig. 3 but for electron precipitation.]

[Figure: Figure 6. Same as Fig. 4 but for electron precipitation.]

(2022) using MESSENGER/FIPS data. Under a northward IMF, particle precipitation is negligible in the equatorial region at latitudes spanning from +60◦to −30◦. Conversely, under a southward IMF, the topology of the magnetosphere results in stronger precipitation at low latitudes, as shown in Fig. 4(d) for protons and 6(d) for electrons. This low-latitude precipitating plasma is composed of high-energy protons and moderate-energy electrons ejected planetward from the reconnection site in the tail. Respectively, comparing Fig. 4(b) and Fig. 4(d) with Fig. 4(a) and Fig. 4(c) for the protons, or equivalently Fig. 6(b) and Fig. 6(d) with Fig. 6(a) and Fig. 6(c) for electrons, we observe that latitudinal variations driven by the IMF are about one order of magnitude more important than longitudinal variations driven by Mercury’s rotation. A coarse energy sampling of the precipitation maps in three energy bins, that are shown in Figs. 3-6, enables one to identify the importance of various particle distributions in driving different processes at the surface. The fluence of the particles in each of the three energy bins are summarized in Tab. 1, using a coarse spatial grid and averaging over the two IMF configurations under study. In this table, we show the mean fluence of protons and electrons in each energy bin in three regions at the surface of Mercury, namely the north pole (above 60◦latitude), the equatorial region (from 60◦to −30◦latitude) and the south pole (below −30◦latitude). These fluence values can be used for a first order estimate of space-weathering due to plasma-surface interaction processes at Mercury. This point is further discussed in Sec. 4.
3.2. Energy distribution of particles at the surface For protons, the net effect of Mercury’s magnetic field is to broaden their energy distribution. In our simulations, solar wind protons are initialized with a narrow Maxwellian energy distribution centered around miV 2 SW/2 = 826 eV, corresponding to a solar wind speed of VSW = 400 km s−1 (shown by the red vertical solid line in Fig. 7(a)-(b)). The distribution width is of the order of Ti,SW = 21.5 eV, as shown by the orange curves in Fig. 7(a)-(b). At the surface,

| Energy [eV] | North Pole | Equatorial region | South Pole |
| --- | --- | --- | --- |
| Proton fluence [1014 cm−2] 0-500 500-1500 1500-∞ | 0.9 0.8 0.5 | small small 0.7 | 1.8 1.2 0.9 |
| Electron fluence [1014 cm−2] 0-100 100-4000 4000-∞ | 1.8 5.8 0.7 | small 2.2 small | 2.8 4.0 small |

[Figure: Figure 7. Normalized energy distribution of the particles in the solar wind (orange) and at the surface (blue). Panels (a)-(b) show the proton distribution functions. Panels (c)-(d) show the electron distribution functions. The red vertical solid lines show the mean energy of the solar wind plasma, which is equal to KSW = miV 2 SW/2 = 826 eV for protons, and to Te,SW = 21.5 eV for electrons. The green vertical dashed lines in panels (a)-(b) show an estimate of the proton energy downstream of the bow shock equal to KSW/M 2 f,SW, where Mf,SW = 3.3 is the fast magnetosonic Mach number in the solar wind upstream of the bow shock. The green vertical solid lines show an estimate of the maximum magnetic energy available for magnetic reconnection equal to]