Applying Ion Energy Spectrograms to Search for Plumes at Europa

We constrain the diagnostic potential of ion energy spectrograms to identify signatures of water vapor plumes in the thermal plasma environment of Jupiter's moon Europa. For this purpose, we apply a hybrid model of Europa's Alfvenic plasma interaction to calculate the perturbations of the flow and the electromagnetic fields near the moon for various plume locations on its surface, combined with different sets of magnetospheric upstream conditions (corresponding to different distances between Europa and the center of Jupiter's plasma sheet). The model output is used to generate synthetic time series for the count rates of the observable thermal ion population as a function of energy along several hypothetical spacecraft trajectories as well as for the Galileo E26 flyby. We demonstrate that the observability of characteristic plume signatures depends strongly on the viewing direction of the detector. Most surprisingly, for certain plume locations a particle detector facing away from Europa captures more clearly discernible signatures of a plume passage than a detector looking into the direction of the moon. This puzzling result is caused by the deflection of magnetospheric and plume ions near Europa's Alfven wings as well as a ``contamination'' of the spectrograms by cold plasma from the moon's global exosphere. The signature of the plume crossed during E26 is most clearly visible for a detector orientation that simultaneously captures the cold plume ions and a portion of the incident magnetospheric ion population. The results of this study will facilitate the planning of synergistic measurements during upcoming missions to Europa.