Plasma Transition Across the Bow Shock at Saturn: Initial Results From the Cassini Plasma Spectrometer

The Cassini spacecraft entered the Saturn system in June 2004. The first observation of Saturn's bow shock occurred at 09:48 UT on 27 June 2004 at an absolute distance of 49.2 Rs from the planet. At this time Cassini was approaching Saturn towards dawn, ~30 degrees below the orbital plane with a velocity of ~7.3 kms-1. During this, and six subsequent crossings on the inbound leg, the Cassini plasma spectrometer (CAPS) measured the flux of 0.5 eV to 28.0 keV electrons as they were slowed and heated at the shock front and made some measurements of ions in the downstream region. After the successful insertion burn Cassini encountered the bow shock a further ten times from 82.5 to 87.0 Rs. At this time the orbiter was exiting the Saturn system with a velocity of ~2.6 kms-1 within ~15 degrees of Saturn's equator. During these crossings CAPS was oriented such that the instrument was able to measure the fluxes of 1 eV to 50 keV ions in addition to the electron measurements (which are always available irrespective of spacecraft pointing). The CAPS observations show that the solar wind plasma is strongly modified at the bow shock with electron density enhancements up to a factor of 4 and electron temperature increasing by as much as a factor of ~20. The ion bulk flow in the sheath was typically ~250 kms-1 with a thermal speed of ~100 kms-1. In this presentation we will characterise the plasma transitions observed and compare them with previous studies made by the same instrument at Jupiter. Supporting data from the Cassini magnetometer and the radio and plasma wave instruments will also be presented where appropriate.