Inference of Europa's conductance from the Galileo Energetic Particles Detector

Abstract

On Galileo's first approach to Europa on December 19, 1996, the Energetic Particles Detector (EPD) observed decreases in the count rates of low‐energy ions in Europa's plasma wake. We understand these decreases to be “microsignatures” of Europa, that is, the signatures of recent satellite absorptions of energetic ions. Using a previously developed simulation of count rates measured by a detector, we compare simulated data with measurements from Europa's immediate and distant wakes. We find that if Europa is considered to be an inert, solid body absorber of charged particles, we cannot account for the depth of observed microsignatures using our model. Instead, if Europa, its ionosphere, or its magnetosphere is sufficiently conducting that Jovian field lines become “hung up” on the moon with contact times of the order of the bounce time of energetic ions, then we can explain the decreases in the data using our model. For a conducting Europa, our simulation gives the best order‐of‐magnitude agreement with a global radial diffusion of the order of DLL ≈ 10−10L4. Simulation results put a lower limit of ΣE ≈ 8 mho on the global conductance of Europa and/or its environs, ∼50% the value for Io. Finally, higher‐energy microsignatures in the EPD data can be understood when we include the effects of a bend in magnetic field lines near Europa that causes a local curvature drift perpendicular to the direction of plasma motion.