The variable extent of Saturn's electron radiation belt

The structure of Saturn's radiation belts is significantly different for electrons and protons. The permanent MeV proton belts are relatively stable in intensity over both short and long time scales, they have a outer boundary that continuously coincides with the L-shell of Saturn's moon Tethys (L=4.89) and comprise different sectors, each separated from the other by a proton depleted region that is centered on the L-shells of the planet's inner icy moons. On the other hand, the electron radiation belt (>500 keV) is a continuous structure that extends between the outer edge of the main rings and has its outer boundary at an average distance of about 8 Rs (Saturn radii) from the planet. The latter distance, however, appears to scatter considerably from orbit to orbit, while flux levels within the belts may vary by several orders of magnitude. Using 8 years of MIMI/LEMMS and CAPS observations, we study the variable extent of the Saturnian electron belt. Preliminary results show a series of interesting features, such as recurrent sudden belt expansions with periods in the order of one to several weeks and considerably variable responses following periods of CME interactions with Saturn's magnetosphere. Of particular interest is a period in the second half of 2011, when, following a CME, the outer boundary of the electron radiation belt drops to a distance of about 4.5-5.0 Rs, taking about 2-3 months to recover to its typical values.During that period, energetic electron and ion fluxes measured by the LEMMS detector are up to two or even three orders of magnitude below the typical levels. We will discuss how these observations relate to the global dynamics of Saturn's magnetosphere and whether the outer boundary of the electron belt is a useful index for organising other magnetospheric datasets.