Low-energy electrons in Saturn’s inner magnetosphere from the Cassini Langmuir Probe

It is known that Enceladus is one of the main mass sources in Saturn’s inner magnetosphere, with other bodies – e.g. Dione, Tethys, Saturn’s rings and atmosphere – thought to play a smaller role. In this study we analysed electron density and temperature as estimated from Cassini’s Langmuir Probe (LP) for the entire duration of the Cassini mission (August 2004 – September 2017).  The main motivation for this work is to understand the impact of Enceladus as a main mass source and search for evidence of additional sources.  We also have been investigating the impact of the spacecraft-plasma interaction, and illumination conditions on the LP current-voltage curves and henceforth on the bulk electron parameters.

One particular facet of this work is the discrimination of spacecraft-generated photoelectrons in the current-voltage curves so that they can be excluded from further analysis of the ambient magnetospheric electrons.  To develop a robust algorithm to identify photoelectrons we have been working with data during transitions in and out of shadow caused by two main events: i) when the LP is shadowed by the spacecraft, or ii) during eclipses by Saturn or its moons.

With a cleaned data set we focused on analysing data within 0.5R_S of the Kronian in the inner magnetosphere (r<7R_S). We compared our data with the hybrid power-law model, as described in Persoon et al. (2013, JGR), that used the electron densities estimated from the upper hybrid resonance frequency. We also looked for local time asymmetries in electrons, as reported in ion plasma in the same region (e.g. Holmberg et al., 2007, GRL).