Galileo PLS Plasma Observations During the E12 Europa Flyby Refuting an Encounter With a Cryovolcanic Plume

Abstract During Galileo's closest (“E12”) flyby of Europa, a brief burst of wave activity was recorded by the plasma wave instrumentation, PWS. This was speculatively interpreted by Jia et al. (2018, https://doi.org/10.1038/s41550‐018‐0450‐z ) as a 2,100 cm −3 spike in plasma densities from a water plume encounter. While the plasma instrument, PLS, could have provided an independent density measurement and resolved ambiguity in the PWS analysis, calibrated PLS data were never published from E12. Here we present high‐resolution (∼18 s) PLS moments and velocity distributions from E12. Plasma densities near closest approach were between (assuming ) and (assuming ), in agreement with the original interpretation of PWS densities by Kurth et al. (2001, https://doi.org/10.1016/s0032‐0633(00)00156‐2 ). We thus discount a plume encounter. PLS measurements near Europa imply a highly asymmetrical and inhomogeneous ionosphere (as expected). Europa's Alfvén Wings were more compact than expected, implying stronger electrical currents than predicted by existing models.