Energetic ions trapped in Saturnʹs inner magnetosphere

The low-energy magnetospheric measurement system (LEMMS) of the magnetosphere imaging instrument (MIMI) aboard the Cassini orbiter observed energetic ions and electrons during Saturn orbit insertion (SOI) of July 1, 2004. Salient features of the trapped ion fluxes observed in the L=2–4RS region include the occurrence of two distinct components of the energy spectrum of energetic protons. We shall refer to protons below 10 MeV as the low-energy component and above 10 MeV as the high-energy component. The low-energy component has a power law energy spectrum that falls at approximately E−2.5. At about 1 MeV/nucleon, the ion pitch angle distributions tend to peak along and opposite to the magnetic field. The high-energy component has a separate peak in energy at about 20 MeV/nucleon and a pitch angle distribution that peaks at 90° to the magnetic field direction. The pitch angle distributions intermediate in energy evolve systematically from peaking along the field at low energies through isotropy to peaking perpendicular to the field at high energies. Ion species heavier than protons are present at energies from several MeV/nucleon up to 25 MeV/nucleon. Oxygen is separately observed to be present. Molecular hydrogen, H2 and H3 and helium are also present although the LEMMS instrumentation is not capable of unambiguously separating these species at multi-MeV energies. These species are measured separately in the outer magnetosphere (L=6.3–11RS) with the MIMI CHEMS instrument at energies from 1 to 100 keV/nucleon. This paper will report details of the observations and the results of modeling the abundances of the inner magnetosphere ions to determine constraints on source material and acceleration processes.