Energetic ion events upstream from the Saturnian bow shock: A multi-instrument study with Cassini measurements

SERGIS, N.; KRIMIGIS, S. M.; MASTERS, A.; ARRIDGE, C. S.; CESAR BERTUCCI; NAHUEL ANDRÉS; ANDRÉ, N.; MITCHELL, D. G.

San Francisco, USA

Congreso; American Geophysical Union; 2010

AGU

Energetic particle events associated with IMF fluctuations upstream of Saturn were first seen by Voyager. The extensive spatial coverage of both dawn and dusk upstream regions provided by Cassini offered the possibility to detect, identify, list and statistically study ~30 particle events upstream from the Kronian bow shock for distances ranging between ~10 and 100 Rs. Using the Magnetospheric Imaging Instrument (MIMI) and the Cassini Plasma Spectrometer (CAPS), measurements of H+, O+ and electrons are made in a broad energy range (few eV to MeV), together with plasma wave measurements obtained by the Radio and Plasma Wave Science experiment (RPWS). Magnetic field data (Cassini/MAG) is also utilised to reveal low frequency waves associated with the upstream flux enhancements. A 3D model of the Saturnian bow shock is used to determine the points of magnetic connection. The statistical analysis shows that particle events are strongly correlated with magnetic connection of the spacecraft to the planetary shock, with the onset of the events associated with a change in the IMF direction. The energy extent is >700 keV for O+ ions with particle fluxes usually anisotropic. Upstream plasma waves (electron plasma oscillations) are driven by the free energy in the particle distribution function. It is possible that the O+ events appear in a periodic fashion, as a weak signal near the planetary period is observed in the dawn sector. The unambiguous presence of upstream water-product ions (MIMI/CHEMS compositional data) and the energy spectrum of the observed population strongly supports the magnetospheric origin of these upstream particles (magnetospheric leakage). Typical example of a O+ particle event upstream from the Kronian bow shock, as seen with various Cassini instruments.