Electron dynamics in small magnetospheres Insights from global, fully kinetic plasma simulations of the planet Mercury

Context. The planet Mercury possesses a small but highly dynamic magnetosphere in which the role and dynamics of electrons are still largely unknown. Aims. We aim to model the global dynamics of solar-wind electrons impinging on Mercury’s magnetosphere. Particular relevance is given to local acceleration processes and the global circulation patterns. Methods. The goals of this work are pursued by means of three-dimensional, fully kinetic particle-in-cell simulations modeling the interaction of the solar wind with the Hermean magnetosphere. This method allows a self-consistent representation of the plasma dynamics from the large planetary scale down to the electron kinetic scale. We carried out numerical simulations using two different solar-wind conditions: purely northward or purely southward interplanetary magnetic field direction. Results. We find a high plasma current (of the order of few µA m^-2) flowing at the magnetospheric boundaries (bow shock and magnetopause) dominated by electrons. This current is driven by the small-scale electron physics resolved in our model. Furthermore, we observe strong electron acceleration up to tens of keV as a consequence of magnetic reconnection when the interplanetary magnetic field is directed southward. Such energetic electrons are partially trapped in the dipolar magnetic field of the planet mainly at nightside. Finally, by studying the distribution of electrons in our simulations along Mariner10 and BepiColombo first-Mercury-flyby trajectories, we propose that both spacecraft observed this energetic quasi-trapped electron population around closest approach.