On the Hermean near-planet boundaries response under different orbital interplanetary conditions

This work aimed to study the dynamical response of the near-Mercury environment to different interplanetary conditions experienced along its orbit by means of 3D multi-species hybrid simulations.Mercury features an eccentric and rapid orbit around the Sun, with its extreme aphelion (0.47 AU) and perihelion (0.31 AU) positions being embedded in significantly different interplanetary conditions. Given the different environments and its weak magnetic field strength, the position, size and behavior of the bow-shock, magnetosheath and magnetopause can vary significantly as highly coupled with the interplanetary medium. In this work, we consider an interplanetary magnetic field aligned along the Parker spiral direction at the Mercury's distance from the Sun, i.e., quasi-radial, and the case of an interaction with slow and fast solar winds. We show the response of the bow-shock due to such quasi-radial interplanetary field and the compression of the bow-shock / magnetosheath /  magnetopause system as the planet passes from the aphelion to perihelion orbital points, as well as the solar wind velocity increases. In particular, certain portions of the planet no longer present a significant protection from the interplanetary environment, so that the surface is exposed to the precipitation of interplanetary ions. An analysis of their fluxes revealed that the high latitude polar cusps are still the main regions for the interplanetary particles to reach out the surface. However, when the interplanetary conditions are sharp enough to cause a strong bow-shock and magnetosheath compression, the interplanetary particles are able to directly penetrate these boundaries and can precipitate at much lower latitudes. These results are particularly timely for the BepiColombo mission, and are compared with its first fly-bys observations.