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.

This effect is more prominent in the simulation with a northward IMF, as shown in Fig. 3(a)-(d) and Fig. 4(a).

Under a northward IMF, the topology of the magnetosphere (i.e., the “closed” topology shown in Fig. 2(a)) channels plasma precipitation to the high-latitude cusps at local time 12 h.

This hot pole enhancement drifts dawnward (by roughly -20◦longitude, as shown in Fig. 4(a)) with increasing proton energy; we speculate that this drift is due to finite-Larmor-radius effects as the proton gyroradius increases with energy.

Under southward IMF and the resulting “open” magnetic field topology of the magnetosphere shown in Fig. 2(b), plasma precipitation occurs both at the cusps and at low latitudes.

Cusp precipitation is enhanced at the hot poles due to the same mechanism at play with the northward IMF, as shown in Fig. 3(e)-(g).

This contrasts with the low-latitude proton precipitation, as shown in Fig. 3(g)-(h).

These low-latitude protons are ejected from the reconnection site in the tail and precipitate between roughly midnight and dawn in local time due to gradient-curvature drifts in the magnetosphere (Lavorenti et al. 2022; Glass et al. 2022).

Such proton precipitation around dawn in local time translates to precipitation around the warm poles in geographical coordinates.

Electrons precipitate onto the surface of Mercury with a pattern somewhat similar to that of the protons, as both are driven by the magnetic field topology.

Longitudinal enhancements for electrons are weaker than for protons due to their higher thermal energy component (that accounts for random motion) as compared to their kinetic energy component (that accounts for ordered motion).

[Figure: Figure 3. Proton precipitation maps integrated over two full Mercury orbits. Panels (a)-(d) are obtained from a simulation with a purely northward IMF and panels (e)-(h) with a purely southward IMF, respectively shown by the green vectors on the left. The different columns correspond to different energy bins, given at the top of each column. The grey horizontal arrows in the left sketches indicate the solar wind velocity. At times when longitude 0◦is subsolar, longitudes -90◦and +90◦correspond to local dawn and dusk, respectively.]

[Figure: Figure 4. Proton precipitation maps averaged over latitude and longitude. The curves in panels (a) and (c) are obtained from the maps in Fig. 3(a)-(d) and Fig. 3(e)-(h), respectively, by averaging in latitude. The curves in panels (b) and (d) are obtained, respectively, from the same maps by averaging in longitude. The black dots and line show the averages summed over all energies and the colored areas the contributions for the labeled energy bins. The horizontal dashed black lines show the mean value of the fluence (averaged both in latitude and longitude).]