Energetic Particle Losses During Galileo's E12 Flyby of Europa: Potential Signature of Water Plume And Role of Smaller Scale Magnetic Field Perturbations.

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Abstract
We investigate losses of energetic electrons, protons, sulfur ions and oxygen ions, measured by Galileo's Energetic Particle Detector (EPD) during the E12 Europa flyby. We use a particle tracing code and previously published magnetohydrodynamic simulations of the interaction of plasma with Europa's atmosphere and plume. We find that localized dropouts in energetic protons (~100 keV) and electrons (~42 keV), whose gyroradii are smaller than Galileo’ s altitude, are not reproducible under nominal (plume-less) conditions. These losses are located near the central axis of the plume proposed by Jia et al., 2018. Particle tracing simulations show that these localized dropouts may be created by perturbed electromagnetic fields associated with a plume (protons and electrons) and charge exchange with the plume neutrals (protons). Interpretation of higher energy protons (>250 keV) and oxygen (>500 keV) dropouts is more complicated, as dropouts can be caused by surface impact, and, therefore, are harder to isolate from other loss processes. However, our simulations show that dropouts of large-gyroradius ions near the central axis of the plume should also be modified by the plume-perturbed fields. A single model setup fully reproducing all species and energies is lacking. Accounting for the magnitude of smaller scale magnetic field perturbations, presumably related to the plume, improves the fit to the measurements. We conclude that a plume may have played a role in producing the measured dropouts of energetic particles, but that magnetic field models which reproduce perturbations at smaller scales are required.