Cassini Observations of Saturn's High-Mach Number Bow Shock

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernova remnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks. However, the Cassini spacecraft has shown that the shock standing in the solar wind sunward of Saturn (Saturn's bow shock) can occasionally reach this high-Mach number astrophysical regime. In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we review the most recent published work based on Cassini data taken at Saturn's bow shock. We then present an interpretation and discussion of the sum of the Cassini findings to date, with emphasis on the implications for shock-acceleration of charged particles. Future directions for work in this area are outlined.