the Mercury-centered Solar Orbital (MSO) coordinate system) for our two simulations. The left (right) panel corresponds to a simulation with a northward (southward) IMF. Blue lines correspond to magnetic field lines not connected to the planet (i.e., solar wind field lines). Red lines correspond to magnetic field lines with one end connected to the planet (i.e., magnetospheric open field lines). Green lines correspond to magnetic field lines with both ends connected to the planet (i.e., magnetospheric closed field lines). The horizontal gray lines indicate the geographic equator of the planet.]

4 electron gyroradius.

In Lavorenti et al. (2022), the authors validated these compressed length scales by comparison of the model results with in situ MESSENGER observations averaged over the mission time period.

The solution of our numerical model depends upon the upstream solar wind parameters.

We use solar wind (SW) parameters corresponding to typical mean values at Mercury’s aphelion (Sarantos et al. 2007; James et al. 2017) with a plasma density nSW = 30 cm−3, speed VSW = 400 km s−1, magnetic field amplitude BSW = 20 nT, and proton and electron temperatures Ti,SW = Te,SW = 21.5 eV.

These parameters are kept fixed in our two numerical simulations, while we vary the direction of the interplanetary magnetic field (IMF) in our two runs from purely northward to purely southward.

Different directions of the IMF corresponds to different magnetic configurations at the surface of Mercury as shown in Fig. 2.

These configurations in turn affects plasma precipitation at the surface.

In this work, we address two extreme IMF configurations (with Bx = By = 0) rarely found in the real system, but which are useful to grasp the role of the magnetic field in shaping the plasma precipitation at the surface.

Our IMF configurations show the range of variability of the system, by providing conditions for minimal (maximal) magnetic coupling between the solar wind and the planet when the IMF is northward (southward), i.e., when the IMF is anti-parallel (parallel) to the planetary magnetic dipole moment.

Plasma precipitation maps as a function of local time were integrated along two full Mercury orbits, from 2022 January 23 at 18:44:32 UTC to 2022 July 18 at 05:48:44 UTC, this corresponds to the time interval shown in Fig. 1.

We do not consider variations in the solar wind parameters along the orbit, which will be included in a future work including also a more realistic IMF configurations.

We subdivide the orbit into 515 steps of equal time dt = 8.21 hours.

At each timestep, we rotate the planet and map local time to surface longitude using Jet Propulsion Laboratory’s NAIF (Navigation and Ancillary Information Facility) SPICE files and routines.

The NAIF/SPICE files enable determining precisely the location of the Sun with respect to Mercury at each time of the orbit, and consist of ephemeris files for the orbit of Mercury and planetary body information for its rotation.

The geographical registration of local time to longitude through this time period is provided with the ancillary files to this publication (Lavorenti et al. 2023).
3.

RESULTS 3.1.

Spatial distribution of particle fluxes at the surface The spatial distribution of particle fluxes at the surface is organized into latitudinal bands with enhancements in longitude driven by the 3:2 resonance.

For protons, this pattern is shown by the maps in Fig. 3, along with the latitudinal and longitudinal averages in Fig. 4.

The proton data are shown for energy bins of 0 −500, 500 −1500, and 1500 −∞eV, which we refer to as low, moderate, and high energies.

For electrons, Fig. 5 and Fig. 6 show the corresponding data.

The electron data are shown for energy bins of 0 −100, 100 −4000, and 4000 −∞eV, which we refer to as low, moderate, and high energies.

The results from these maps are summarized in Tab. 1, using a coarse spatial grid and averaging between the two IMF conditions under study.

In these maps, longitudinal variations are controlled by Mercury’s rotation, while latitudinal variations are controlled by the IMF.

In the following, we discuss these two effects separately.

Mercury’s rotation is responsible for the differential accumulation of particles versus longitude.

Due to the 3:2 spin-orbit resonance of Mercury, subsolar (local time 12 h) high-latitude proton precipitation is enhanced at the hot poles (longitude 0◦and 180◦), as shown in Fig. 3.