Ionosphere of Ganymede: Galileo observations versus test particle simulation

Abstract

In this paper, we model the plasma environment of Ganymede by means of a collisionless test particle simulation. By coupling the outputs from a DSMC simulation of Ganymede's exosphere (i.e. number density profiles of neutral species such as $\mathrm{H}$, $\mathrm{H_2}$, $\mathrm{O}$, $\mathrm{HO}$, $\mathrm{H_2O}$, $\mathrm{O_2}$ for which we provide parametrisation) 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 6 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 $\mathrm{H_2^+}$, $\mathrm{O_2^+}$, and occasionally $\mathrm{H_2O^+}$. 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 magnetised environment and that additional mechanisms may be at play to energise/accelerate newborn ions from the neutral exosphere.