Interpreting BepiColombo's observations of Mercury's solar wind interaction with global hybrid-particle simulations

We present global modelling of Mercury’s solar wind interaction with the open-source 3D hybrid-particle code framework RHybrid (paRallel Hybrid) in light of BepiColombo's flybys and forthcoming orbital science observations. In the hybrid-particle model, ions are treated kinetically as macroscopic particle clouds (macroparticles) moving under the Lorentz force, while electrons are described implicitly as a charge-neutralising, inertialess fluid governed by Ohm’s law. The kinetic ion dynamics are coupled to the evolution of the magnetic field through Faraday’s law, and the system is closed using Ampère’s law. This approach allows ion velocity distributions to evolve self-consistently, capturing wave-particle interactions, finite Larmor radius effects, and other ion-kinetic processes. The code is highly parallelised using message passing between compute nodes, enabling efficient use of large high-performance computing resources. This allows us to simulate spatial structures and ion velocity distributions in the Hermean plasma environment with high resolution, resolving the coupling between ion scales and global magnetospheric scales.

In the model, Mercury’s surface is represented as a particle-absorbing boundary, with the underlying crust–mantle region modelled as a resistive spherical shell overlying an ideally conducting core. The intrinsic planetary magnetic field is prescribed as a dipole offset northward from the planet’s centre, or alternatively by any other 3D planetary magnetic field model. The production of exospheric ion species is described through photoionisation of arbitrary 3D neutral density profiles.

Here we discuss the application of global hybrid modelling to study the formation, structure, and dynamics of different regions in the Hermean plasma environment, as well as ongoing development of the RHybrid code, including adaptive mesh refinement, temporal substepping, and improved electron physics. Mercury’s environment is characterised by strong couplings and feedbacks between the solar wind, magnetosphere, exosphere, surface, mantle, and core. Global hybrid modelling provides essential context for interpreting in situ observations, enables controlled numerical experiments under varying conditions, and supports systematic and flexible investigations of the scaling of solar wind interaction processes.