Characterization of Saturn's bow shock: Magnetic field observations of quasi-perpendicular shocks

TL;DR

This study analyzes Saturn's bow shock using in-situ Cassini data, revealing its hybrid characteristics and the influence of magnetic Mach number and shock angle on collisionless shock behaviors across regimes.

Contribution

First comprehensive in-situ analysis of Saturn's bow shock across a wide parameter space, linking shock properties to upstream magnetic conditions and Mach number.

Findings

Saturn's bow shock exhibits both terrestrial and astrophysical shock characteristics.

The shock is predominantly quasi-perpendicular due to the Parker spiral at 10 AU.

Physical mechanisms like non-stationarity depend strongly on the Mach number.

Abstract

Collisionless shocks vary drastically from terrestrial to astrophysical regimes resulting in radically different characteristics. This poses two complexities. Firstly, separating the influences of these parameters on physical mechanisms such as energy dissipation. Secondly, correlating observations of shock waves over a wide range of each parameter, enough to span across different regimes. Investigating the latter has been restricted since the majority of studies on shocks at exotic regimes (such as supernova remnants) have been achieved either remotely or via simulations, but rarely by means of in-situ observations. Here we present the parameter space of MA bow shock crossings from 2004-2014 as observed by the Cassini spacecraft. We find that Saturn's bow shock exhibits characteristics akin to both terrestrial and astrophysical regimes (MA of order 100), which is principally controlled by the upstream magnetic field strength. Moreover, we determined the {\theta}Bn of each crossing to show that Saturn's (dayside) bow shock is predominantly quasi-perpendicular by virtue of the Parker spiral at 10 AU. Our results suggest a strong dependence on MA in controlling the onset of physical mechanisms in collisionless shocks, particularly non-time stationarity and variability. We anticipate our comprehensive assessment will yield deeper insight into high MA collisionless shocks and provide a broader scope for understanding the structures and mechanisms of collisionless shocks.