Electron dynamics in small magnetospheres: insights from global fully-kinetic plasma simulations of planet Mercury

TL;DR

This study uses fully kinetic 3D simulations to explore electron behavior, acceleration, and circulation in Mercury's small magnetosphere, revealing electron-driven currents and energetic particles linked to magnetic reconnection.

Contribution

It provides the first detailed kinetic simulation of Mercury's magnetosphere, highlighting electron acceleration and currents driven by small-scale physics and magnetic reconnection.

Findings

High electron-driven plasma currents at magnetospheric boundaries.

Strong electron acceleration up to tens of keV during magnetic reconnection.

Energetic electrons are partially trapped in Mercury's magnetic field, consistent with spacecraft observations.

Abstract

The planet Mercury possesses a small but highly dynamic magnetosphere in which the role and dynamics of electrons are still largely unknown. We aim at modeling 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. 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. Numerical simulations are carried out using two different solar wind conditions: purely northward or purely southward interplanetary magnetic field direction. We find a high plasma current (of the order of few $\mu$A/m2) 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.