Strong depletions of energetic protons (115-244 keV) were observed during Galileo flyby E26of Europa. We simulate the flux of energetic protons using a Monte Carlo particle backtracing code and show that energetic proton depletions during E26 are reproduced by taking into account the perturbations of the electromagnetic fields calculated by magnetohydrodynamic (MHD) simulations and charge exchange with a global atmosphere and plume. A depletion feature occurring shortly after closest approach is driven by plume associated charge exchange, or a combination with plume associated field perturbations. We therefore conclude, with a new method and independent data set, that Galileo could have encountered a plume during E26. Plain Language SummaryWe investigate why (normally abundant) fast protons were disappearing during Europa flyby E26 by Galileo. We do this by simulating the proton motion. In some cases we detect few protons because Europa is blocking the field of view. What is new here is that part of the decrease can be explained by charge exchange, a process whereby the protons are removed after they lose their electrical charge in Europa's thin atmosphere. Furthermore, we see that there is a special decrease, which can be explained by an erupting plume of water vapor, thereby providing additional evidence for an active plume during Galileo flyby E26.

We present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009),

HIGHLIGHTS-The in-situ detection of H2O and H2O + from Europa's plumes is possible with JUICE/PEP -This work focusses on plumes with a low mass flux (1 kg/s) -The geometry of the plume source is not a major factor for the detectability -Knowledge of Europa's exosphere is required to separate exospheric and plume H2O/H2O + ABSTRACT We investigate the feasibility of detecting water molecules (H2O) and water ions (H2O + ) from the Europa plumes from a flyby mission. A Monte Carlo particle tracing method is used to simulate the trajectories of neutral particles under the influence of Europa's gravity field and ionized particles under the influence of Jupiter's magnetic field and the convectional electric field. As an example mission case we investigate the detection of neutral and ionized molecules using the Particle Environment Package (PEP), which is part of the scientific payload of the future JUpiter ICy moon Explorer mission (JUICE). We consider plumes that have a mass flux that is three orders of magnitude lower than what has been inferred from recent Hubble observations (Roth et al., 2014a). We demonstrate that the in-situ detection of H2O and H2O + from these low mass flux plumes is possible by the instruments with large margins with respect to background and instrument noise. The signal to noise ratio for neutrals is up to~5700 and ~ 33 for ions. We also show that the geometry of the plume source, either a point source or 1000 km-long crack, does not influence the density distributions, and thus, their detectability. Furthermore, we discuss how to separate the plume-originating H2O and H2O + from exospheric H2O and H2O + . The separation depends strongly on knowledge of the density distribution of Europa's exosphere.