Constraints on the Observability of Energetic Neutral Atoms From the Magnetosphere-Atmosphere Interactions at Callisto and Europa

We investigate the emission of energetic neutral atoms (ENAs) from the magnetosphere-atmosphere interactions at Europa and Callisto that may be observed by an ENA imaging instrument. To do so, we combine the draped electromagnetic fields from the AIKEF hybrid model with a tracing tool for the energetic magnetospheric parent ions. Using a realistic viewing geometry, synthetic ENA images are generated for multiple detector positions and orientations with respect to the magnetospheric field. At both moons, we investigate the ENA emission morphology at different distances to the center of Jupiter's plasma sheet. By generating ENA images for uniform electromagnetic fields as a baseline, we also constrain the influence of field line draping and induction signals on the observable ENA emission signatures. Our major findings are: (a) In uniform fields, regions of elevated ENA flux in the images populate segments of a ring around each moon's disc. (b) The radial and angular extensions of these segments decrease with growing magnetospheric field strength. (c) The angle between the detector's boresight vector and the ambient field determines the regions of the images populated with elevated ENA flux. (d) Non-uniformities in the fields are discernible in the ENA images only when Europa and Callisto are located near the center of Jupiter's plasma sheet. Even in this case, the visibility of such features is restricted to certain viewing geometries. (e) Anisotropies in the moons' atmospheric density profiles produce gradients in the observable ENA emissions. This facilitates a global characterization of these neutral envelopes through ENA imaging.