In this paper, we model the plasma environment of Ganymede by means of a collisionless test particle simulation. By coupling
the outputs from a Direct Simulation Monte Carlo (DSMC) simulation of Ganymede’s exosphere (i.e. number density profiles of
neutral species such as H, H2, O, HO, H2O, O2 for which we provide parametrization) with those of a MagnetoHydroDynamic
simulation of the interaction between Ganymede and the Jovian plasma (i.e. electric and magnetic fields), we perform a
comparison between simulated ion plasma densities and ion energy spectra with those observed in situ during six close flybys of
Ganymede by the Galileo spacecraft. We find that not only our test particle simulation sometimes can well reproduce the in situ
ion number density measurement, but also the dominant ion species during these flybys are H+
2 , O+
2 , and occasionally H2O+.
Although the observed ion energy spectra cannot be reproduced exactly, the simulated ion energy spectra exhibit similar trends
to those observed near the closest approach and near the magnetopause crossings but at lower energies. We show that the neutral
exosphere plays an important role in supplying plasma to Ganymede’s magnetized environment and that additional mechanisms
may be at play to energize/accelerate newborn ions from the neutral exosphere.