Global hybrid modelling of Mercury's plasma environment in light of BepiColombo's first three flybys

We present a global hybrid-particle simulation study of Mercury's magnetosphere and plasma environment during BepiColombo's first three Mercury flybys, or swing-bys, (MFB1–3). We use the high-performance simulation code RHybrid (paRallel Hybrid) to model the interaction between the solar wind and the Hermean magnetosphere[1-3]. In the hybrid approach, ions are treated as macroscopic particle clouds (macroparticles), resolving the kinetic motion of ions via the Lorentz force. Electrons are described implicitly as a charge-neutralising, inertialess fluid governed by Ohm's law and a polytropic (adiabatic) closure. The kinetic ion dynamics are coupled to the evolution of the magnetic field through Ampère's and Faraday's laws. This approach allows ion velocity distributions to evolve self-consistently, capturing wave-particle interactions, finite Larmor radius effects, and other ion-kinetic processes. Mercury's surface is modelled as a particle-absorbing inner boundary, while the crust–mantle region is represented as a resistive spherical shell atop an ideally conducting core. The planetary magnetic field is modelled as a dipole offset northward from the planet's centre. The solar wind ion species include protons and alpha particles, and Na+ ions are produced by photoionisation of a neutral sodium exosphere profile.

For each flyby, we perform a dedicated simulation run using stationary upstream solar wind, interplanetary magnetic field (IMF), and photoionisation conditions representative of the observed environment [3-9]. Mercury was near perihelion during MFB3, while MFB1 and MFB2 occurred when the planet was about halfway between perihelion and aphelion. Under the adopted conditions, the solar wind was densest and the IMF strongest during MFB3, the solar wind speed was highest during MFB2, and the Alfvén Mach number was highest during MFB1. From these simulations, we generate virtual spacecraft observations along the MFB1-3 trajectories and compare boundary crossings, particle populations, electric and magnetic field properties, and wave activity in different regions of Mercury's plasma environment. The study aims to investigate how variations in upstream solar wind and IMF conditions, and exospheric ion loading influence Mercury's magnetospheric structure across the three encounters. We will discuss the comparative analysis of the model runs with BepiColombo and MESSENGER observations, as well as other modelling efforts.

[1] https://github.com/fmihpc/rhybrid, https://planets.fmi.fi

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