Emission of Energetic Neutral Atoms From Ganymede's Magnetosphere-Atmosphere Interaction

This study analyzes the emission of energetic neutral atoms (ENAs), generated by charge exchange between energetic protons and Ganymede's atmosphere. We also constrain the observability of such ENAs by an imaging instrument aboard a spacecraft. Our approach employs tracing tools that calculate the trajectories of magnetospheric parent protons near Ganymede. We determine the ENA flux through a hypothetical spherical detector encompassing the moon's atmosphere. We additionally generate synthetic ENA images, as seen by a point-like detector with a finite field of view. The complexity of Ganymede's electromagnetic environment is successively increased; we consider (i) uniform Jovian fields, (ii) the superposition of the moon's internal dipole with Jupiter's field, and (iii) draped fields from a hybrid model of Ganymede's plasma interaction. Our major results are: (a) In uniform fields, the ENA flux is elevated within a circular band on the detector sphere. Synthetic ENA images record a cluster of high flux near the moon's limb, with the position of this enhancement determined by the viewing geometry. (b) When including Ganymede's internal dipole, the flux through the sphere displays a localized increase above the ramside apex, mainly generated by protons on open field lines at mid-latitudes. In the synthetic images, the reduced ENA emissions from the closed field line region produce local flux depletions along the equator. (c) Pile-up of Jupiter's field significantly reduces the ENA flux from Ganymede's ramside atmosphere. (d) At energies above several keV, the emissions from Ganymede's atmosphere clearly exceed the ENA flux released from the moon's surface.