Galileo plasma observations at Europa: Ion energy spectra and moments

Observations from the Galileo plasma analyzer (PLS) recorded during two near encounters with Europa are reported. The measured ion energy spectra show that the ions near Europa are a mix of thermalized torus plasmas with approximately Maxwellian ion velocity distributions and partially thermalized pickup ions with ring distributions. The measurements are used to determine plasma moments including ion number densities, bulk flow velocities, and ion temperatures. These parameters provide information concerning the interaction that occurs as corotating torus plasmas sweep past this moon. The first encounter on December 19, 1996, took the spacecraft through the wake of the moon with the altitude at closest approach approximately 700 km. The trajectory for the second encounter on February 20, 1997, was on the upstream side of the moon with closest approach at an altitude approximately 600 km. Features of the interaction are found to include (1) deflection of plasma flow away from the moon on the upstream side and into the wake on the downstream side, (2) evidence of boundaries in the near wake that indicate a structured wake, and (3) maximum heavy‐ion densities near closest approach ∼40/cm 3 that place a limit on the density of a high‐altitude ionosphere at Europa.