Energetic charged particles near Europa

Abstract

Galileo energetic particles detector (EPD) data are presented from several close encounters of the spacecraft with Jupiter's moon Europa. These data reveal significant decreases in electron and ion count rates very close to the moon and a few Europa radii downstream. The relation between count rate decreases and satellite absorption is analyzed because these particles sputter and chemically modify Europa's surface. Decreases on the plasma wake side of the satellite have radial extents comparable to satellite dimensions. Furthermore, some energetic particle wakes are shifted toward Jupiter from predicted locations. At some wake locations, electron fluxes do not get above instrument background levels. Corresponding electrons are lost preferentially on Europa's trailing hemisphere. Upstream of Europa during the E12 encounter, deep decreases in energetic ion count rates are observed for some instrument look directions. These decreases are associated with satellite absorption because the E12 closest approach altitude is smaller than relevant ion gyroradii. Other E12 decreases, such as a “bite‐out” corresponding to a peak in the measured magnetic field intensity, are associated with “secondary” drifts in the radial direction caused by the field gradient. Pitch angle distributions measured during the E15 encounter (when Galileo was in Europa's wake) show much higher fluxes of locally mirroring particles than would be expected in a simple absorption signature. We suggest that a fraction of the upstream population escapes absorption because of the pileup of magnetic flux on Europa's trailing hemisphere.