Europa's Plasma Environment: A Reanalysis of Galileo PLS and PWS Observations

Abstract:

We present a new analysis of Galileo plasma observations in the vicinity of Europa’s orbit. We estimated plasma densities and temperatures from the Plasma Wave Spectrometer and Plasma Spectrometer instruments. Both density and temperature show extreme fluctuations. As expected some of this results from Europa's varying distance from the centrifugal equator, but most of the variability is random. An anti-correlation in ion density and temperature suggests that these fluctuations are due to flux tube interchange—cold and dense plasma traveling outward from the Io plasma torus interchanging with hot and tenuous plasma traveling inward from the outer magnetosphere. The unusually large plasma density observed during Galileo’s E12 Europa encounter falls onto this temperature/density regression, suggesting that Galileo observed a cold flux tube traveling outward from the Io plasma torus. We also find that the plasma is significantly denser than generally appreciated, with a median electron density of ~160 cm-3. These new results have important implications for models of Europa’s interaction with the jovian magnetosphere. Many models assume a much lower plasma density, and telescopic observations have been interpreted using these low values. These much higher densities suggest, e.g., much larger charge exchange rates, a process that is often ignored at Europa. Assumptions built into such models will need to be revisited.