Cassini observations of a Kelvin‐Helmholtz vortex in Saturn's outer magnetosphere

Abstract

We present Cassini observations of a plasma vortex in Saturn's dayside outer magnetosphere. The vortex encounter took place on 13 December 2004 as Cassini was travelling toward the planet. The spacecraft crossed the magnetopause 3 times, before being immersed in the low‐latitude boundary layer. During the transition between the boundary layer and the magnetosphere proper, the spacecraft observed deflected boundary layer plasma, a twisted magnetic field topology, and high‐energy (>20 keV) directional electron fluxes. These observations are consistent with an encounter with a vortex on the inner edge of the boundary layer, an interface that is expected to be susceptible to the growth of the Kelvin‐Helmholtz (K‐H) instability due to its low magnetic shear. The size of the vortex is determined to be at least 0.55 RS, and a simple model of the current system resulting from the formation of the vortex is proposed. The possible acceleration mechanisms responsible for the high‐energy electrons are discussed. The identification of the structure provides compelling evidence of the operation of the nonlinear K‐H instability at Saturn's morning magnetospheric boundaries and has implications for our understanding of the transfer of energy and momentum between the solar wind and Saturn's magnetosphere.