Particle and magnetic field properties of the Saturnian magnetosheath: Presence and upstream escape of hot magnetospheric plasma

Abstract

We analyze plasma, energetic particle, and magnetic field data from all available Cassini passes through the Saturnian magnetosheath between July 2004 and July 2011 and provide a statistical overview of particle and field properties. The results show that magnetosheath plasma has an average number density of ~0.1 cm−3 and a temperature of ~300 eV. The measured magnetic field strength is ~1 nT, and the plasma beta is in the range of 10 to 100. The prevailing flow and magnetic field configuration is close to that theoretically expected, with flow speed values of ~200 km/s. Compositional data reveal that although at low energies (<few keV) the magnetosheath is comprised by shocked solar wind plasma, at high energies (>few keV) there is a strong presence of water group ions (W+) forming localized structures we refer to as W+ “islands” that travel downstream convected in the plasma flow. Under average magnetic field conditions in the Saturnian magnetosheath, the kinetic properties of these hot W+ ions can enable escape upstream from the bow shock. Based on the measured particle and field distributions and the modeled bow shock and magnetopause positions, we describe the energetic ion escape as a function of energy and pitch angle and successfully predict the energy distribution of the escaping W+ ions. Comparison with the ion spectra measured in the nearby solar wind confirms that the suggested escape mechanism due to large ion gyroradii is sufficient to explain the observed leakage of heavy energetic ions upstream from the Saturnian bow shock.