Photoelectrons in the Enceladus plume

摘要

The plume of Enceladus is a remarkable plasma environment containing several charged particle species. These include cold magnetospheric electrons, negative and positive water clusters, charged nanograins, and magnetospheric photoelectrons produced from ionization of neutrals throughout the magnetosphere near Enceladus. Here we discuss observations of a population newly identified by the Cassini Plasma Spectrometer (CAPS) electron spectrometer instrumentphotoelectrons produced in the plume ionosphere itself. These were found during the E19 encounter, in the energetic particle shadow where penetrating particles are absent. Throughout E19, CAPS was oriented away from the ram direction where the clusters and nanograins are observed during other encounters. Plume photoelectrons are also clearly observed during the E9 encounter and are also seen at all other Enceladus encounters where electron spectra are available. This new population, warmer than the ambient plasma population, is distinct from, but adds to, the magnetospheric photoelectrons. Here we discuss the observations and examine the implications, including the ionization source these electrons provide.