The Saturnian plasma sheet as revealed by energetic particle measurements

Since July 2004 Cassini is in orbit around Saturn providing in-situ measurements of the Saturnian magnetosphere. One of the three sensors of the Magnetospheric Imaging Instrument (MIMI) is the Low Energy Magnetospheric Measurement System (LEMMS) that responds to energetic particles which can serve as indicators of key regions (Krimigis et al., 2005) and ongoing plasma processes in the magnetosphere. In this paper we identify and characterize, based on energetic particle and magnetic field measurements, the radiation belts, the plasma sheet, and the lobe region. The transition between plasma sheet and lobe region sometimes occurs very rapidly, and sometimes occurs with the period of Saturn's rotation. We explain the highly variable nature of the Saturnian plasma sheet as a combination of the geometry of the Cassini trajectory, together with the variable location of the boundary between open and closed field lines caused by a strong localized magnetic anomaly in the Saturnian field.