Europa's near-surface radiation environment

摘要

As flux tubes are carried over Europa, hundreds of keV to tens of MeV electrons are absorbed by the moon due to their short bounce times. This results in an asymmetry in energetic charged particles in the near surface environment and in the bombardment of Europa's surface. For example, electrons between about 0.1 and 50 MeV do not have direct access to more than 50% of the sphere at 100 km Europa altitude. Moreover, a hypothetical orbiting spacecraft at this altitude would see a reduction of about 33% of the sky, based on Europa's obstructing its field-of-view. We find that the predicted radiation reduction due to the moon's presence is consistent with data from Galileo's Energetic Particles Detector (EPD). In this paper, we present data that illustrate this consistency and use standard models of the radiation environment to evaluate the energetic electron fluence at 100 km.