Proton Trapping in Ganymede’s Magnetosphere: A Quantitative Test Particle Study

Abstract Ganymede, the largest moon in the solar system, possesses an intrinsic dipole magnetic field, creating a mini-magnetosphere within Jupiter’s giant magnetosphere. Energetic protons from the upstream Jovian plasma can become temporarily confined within this small system, giving rise to a partial ring current whose asymmetric configuration has been suggested by previous models. However, the specific conditions that allow proton trapping and the physical origin of the partial ring current remain unclear. In this study, we perform test particle simulations based on three-dimensional magnetohydrodynamics electromagnetic fields corresponding to the Galileo G8 flyby to quantitatively investigate proton trapping in Ganymede’s magnetosphere. Our results indicate that less than 1% of protons injected from the magnetotail remain trapped within the closed field line region, whereas most precipitate to the surface or escape from the magnetosphere, leading to a partial ring current structure. In addition, the trapping region is restricted to a narrow phase-space domain that corresponds to kinetic energies below ∼75 keV, pitch angles between 60° and 120°, and gyrophases near 45°. These findings demonstrate the fundamental constraint from the small size of Ganymede’s magnetosphere on proton trapping, a feature that can be further verified by future in situ observations.