Kinetic plasma simulations of Mercury’s magnetosphere to prepare BepiColombo

Space plasmas permeate the Solar System, from the solar corona to the upper layers of planetary environments (e.g. magnetosphere and ionosphere). In the Solar System, only two telluric planets possess an intrinsic magnetic field, and therefore a magnetosphere, those are the Earth and Mercury. Differently from the Earth, Mercury has been rarely visited by exploratory space missions. Therefore, many of the properties of Mercury’s environment, and of its magnetosphere in particular, remain poorly investigated at present. This work contributes to the global understanding of Mercury’s plasma and planetary environment, in light of ongoing exploratory space missions. The ongoing ESA/JAXA BepiColombo mission provides in situ observations at Mercury with an advanced payload, able to observe the plasma dynamics –for the first time– down to electron kinetic scales. To interpret such observations, numerical models resolving electron kinetic scales are needed. In this work, I use two fully-kinetic models to study electron-scale processes in Mercury’s magnetosphere, both at local and global scales. I focus on the plasma processes at the origin of (i) electron acceleration by wave-particle interaction at the magnetopause, (ii) electron acceleration by magnetic reconnection in the magnetotail, and (iii) electron precipitation onto the surface of Mercury. The impact of these processes on Mercury’s magnetosphere-exosphere-surface coupling is also extensively studied. For this purpose, I develop and validate the first ab initio fully-kinetic global model of Mercury’s magnetosphere. In this PhD, I characterize the main processes that accelerate electrons in the magnetosphere of Mercury. First, electrons are accelerated by resonant wave-particle interaction with drift waves (generated by the lower-hybrid-drift instability) at the magnetopause. This process increases the parallel electron temperature up to a factor two, if the magnetopause width is of the order of the ion gyroradius. Second, electrons are accelerated by magnetic reconnection in the magnetotail. This process generates a flow of electrons with an energy of few keV directed towards the planet from the X-line in the tail. Such electrons populate the inner shells of the magnetosphere to form Mercury’s “partial ring current”. Third, a large fraction of the electrons in this “partial ring current” precipitates onto the surface of Mercury, thus driving plasma-exosphere and plasma-surface interactions. Magnetic reconnection in the tail is the main process accelerating electrons (up to few keV) in the magnetosphere of Mercury. These electrons, while being partially trapped in the nightside, precipitate onto the surface to drive (i) efficient ionization of exospheric H, He, O and Mn, (ii) a pattern of X-ray emissions more prominent at dawn consistent with MESSENGER/XRS observations, and (iii) differential space weathering of Mercury’s regolith. Finally, the findings of this work will be used to advance global models of Mercury’s coupled magnetosphere-exosphere-surface system and to interpret (ongoing) and to plan (future) observations by BepiColombo mission. The global model developed in this work for Mercury will also find applications to other bodies (such as the Moon, asteroids, Mars, and the Galilean Moons of Jupiter) in future works.