Wave activity in Europa's wake: Implications for ion pickup

Abstract

Intense wave power at low frequencies (near and below the cyclotron frequencies of heavy ions) was present in Europa's wake during the E11 and E15 flybys. The fluctuations are mainly transverse to the background magnetic field, and wave characteristics indicate that they are ion cyclotron waves driven by positively charged pickup ions. In both flybys there is evidence, from the wave polarization, for pickup of negatively charged chlorine ions. The pickup rate inferred for the E15 flyby, when the moon is near the center of the Jovian current sheet, is larger than that for the E11 flyby, when the moon is outside the Jovian current sheet. The wave power does not provide absolute pickup density values, because the waves are observed in regions where their growth has not yet saturated. Low-frequency (