Abstract :
Mercury’s highly dynamic magnetosphere provides a unique environment for studying charged particle acceleration and transport. While previous test-particle studies have often relied on static semi-empirical magnetic field models, we investigate proton dynamics using Winterfell, a test-particle code combining the guiding-center approximation and full-orbit integration, coupled to MPI-AMRVAC, a 3D global magnetohydrodynamic (MHD) model of Mercury’s magnetosphere. We compare the results with those obtained using the semi-empirical KT17 model.
We find significant differences between the two approaches : while KT17 allows 5 keV protons to form a ring current under typical aphelion conditions, a similarly favorable steady-state MHD configuration does not. Under more compressed, perihelion-like conditions, the mean proton precipitation rate in KT17, averaged across a broad range of initial energies and pitch angles, is approximately twice that obtained with MHD, highlighting the influence of electromagnetic convection and the associated ExB drift. Furthermore, introducing a transient magnetic hole into the MHD simulation enhances mean proton precipitation by a factor of 2.3 through further magnetospheric compression and changes in magnetic topology and convection.
These results demonstrate the importance of accounting for electric fields and transient solar-wind forcing when modeling particle dynamics at Mercury, with implications for surface sputtering, exospheric neutral release, and solar-wind–magnetosphere–exosphere coupling. They provide a framework for interpreting particle observations during the upcoming BepiColombo science phase.
