tons and Fig. 6 for electrons, we derive the effective shielding parameters for solar wind protons and electrons:
αp ≈20, (3) αe ≈10. (4) Our results indicate that the magnetic field effectively shields about 90% of the incoming solar wind particles, reducing the fluence at the surface by one order of magnitude as compared to the unmagnetized case. The shielding values do not vary appreciably between our two runs using a purely northward or a purely southward IMF, suggesting that the 9 IMF direction weakly affects the total number of particles precipitating onto the surface.

A future study using a more realistic IMF direction for Mercury (including Bx and By components) will address this point specifically.

The IMF direction controls where solar wind particles precipitate onto the surface.

Although the total number of precipitating particles weakly depends on the IMF direction, at least for the configurations under study, the regions of strongest precipitation significantly differs between the simulations with a purely northward and a purely southward IMF, as shown in Sec. 3.1.

Therefore, time variations in the IMF direction – not included in our computation – would modify the location of the regions of strongest particle precipitation.

Since the IMF direction changes on time scales of the order of tens of minutes due to the turbulence in the solar wind (James et al. 2017), we expect a continuously changing pattern of precipitation.

Therefore, the use of the effective shielding parameter α in Eq. 2 represents a first order measure of a non-homogeneous and time-varying process.

As recently highlighted by MESSENGER/FIPS observations of proton precipitation at the northern cusp (Raines et al. 2022), one pervasive characteristic of Mercury’s interaction with the solar wind is high variability.

This study, however, focuses on representative cases with fixed solar wind conditions; we then computed the effect of these fixed cases over two full Mercury orbits.

In doing so, several types of events which may provide substantial contributions to space weathering (such as coronal mass ejections (CMEs), interplanetary shocks and solar energetic particle (SEP) events) are omitted.

These extreme events profoundly alter the ability of the planetary magnetic field to shield the surface.

From MESSENGER observations, Winslow et al. (2015) identified a total of 61 CMEs from March 2011 to September 2014, corresponding to an occurring frequency of roughly one CME every 20 days.

The increase in ram pressure associated to these events compresses the dayside magnetopause, increases the surface exposed to solar wind flux and, in few abnormally strong and rare cases, rips off completely the dayside magnetosphere (Slavin et al. 2014; Winslow et al. 2017; Exner et al. 2018; Slavin et al. 2019; Jia et al. 2019).

Concerning SEP events, Gershman et al. (2015) inferred the high-latitude precipitation of solar energetic electrons onto the surface of Mercury for 11 SEP events.

As SEP events include protons over 15 MeV and electrons up to 3 MeV in energy at substantial fluxes, they can make substantial contributions to space weathering over periods of hours to days (Lario et al. 2013).

More details concerning the probability of all of these events occurring as well as their full impact on space weathering processes, however, remains poorly known at present.

Ion precipitation onto the surface of Mercury is one of the drivers of space weathering via ion sputtering and ion implantation.

Space weathering alters the spectral properties of the regolith by darkening the surface, decreasing absorption band depths, and changing the spectral slope (reddening from the visible to near-infrared, bluing from the ultraviolet to the visible) (Noble et al. 2007; Blewett et al. 2021).

The infrared spectrometer MERTIS onboard BepiColombo will be able to measure those spectral variations at Mercury (Hiesinger et al. 2020; Maturilli et al. 2014).

The geographical distributions of proton fluxes at the surface of Mercury shown in Fig. 3 when compared to spectral surface properties, can provide key information to infer the role of solar wind ion irradiation on the spectral processing of the surface.

However, given the strong dependence of these maps on the time-variable IMF direction, we introduced the proton effective shielding parameter αp ≈20 in Eq. 2.

The use of this parameter will enable researchers to compare the partially shielded surface of Mercury with those unshielded of the Moon and of unmagnetized asteroids such as asteroids Ryugu (Sawada et al. 2017) and Bennu (Lauretta et al. 2017), paving the way to comparative space weathering studies.