Energetic Proton Dynamics in Ganymede’s Magnetosphere under Different Upstream Conditions: The Role of Field-line Curvature Scattering

Abstract Ganymede’s mini-magnetosphere is embedded in the variable Jovian plasma environment, yet how upstream conditions regulate energetic proton dynamics within the magnetosphere remains an open question. Using global magnetohydrodynamic simulations coupled with a test-particle model under five distinct upstream regimes (including both Galileo-constrained and extreme cases), we investigate the probabilities of trapping, precipitation, and escape for 50–500 keV protons injected into the magnetosphere, and demonstrate that field-line curvature (FLC) scattering acts as a key control on proton behaviors. We find that for low-energy protons (with kinetic energies lower than 150 keV), strong FLC scattering leads to similar outcome probabilities across different initial pitch angles, whereas under weak scattering, drift-shell splitting becomes effective, causing enhanced escape of protons with pitch angles near 90° through the magnetopause. The total trapping probability decreases approximately exponentially with increasing effective upstream pressure, reflecting enhanced scattering efficiency. However, escape and precipitation display a threshold-like dependence: when the ratio of gyroradius to the FLC radius is above about 0.15, stronger scattering raises both probabilities; below the threshold, weaker scattering still enhances escape but reduces precipitation. These results unify diverse dynamical behaviors of protons under a single FLC-scattering framework and provide a basis for estimating Ganymede’s ring current intensity from upstream conditions.