Identifying Active Kelvin–Helmholtz Vortices on Saturn's Magnetopause Boundary

For ∼2,000 Cassini magnetopause encounters, we analyse plasma and magnetic fields. The boundary can be unstable to the Kelvin–Helmholtz instability (KHI), which can drive large‐scale flows identifiable in plasma measurements. Bulk flow reversed from the expected direction near the magnetopause can indicate vorticity associated with active KH, and events are found dominantly in the dawn–subsolar region. KHIs are also responsible for magnetic field fluctuations, and hybrid simulations indicate heating, and transport is significant in an actively growing vortex. Cassini observations are filtered for disturbed magnetic fields near the magnetopause, similar to the signatures from hybrid simulations, with significant fluctuation and current sheet crossings. We also find that these occur most frequently in the dawn–subsolar region. A turbulent heating rate density and mass diffusion coefficient are calculated for these disturbed events and compared with the hybrid simulation to test whether enhanced values for these quantities can identify active KH events. Plain Language Summary Ocean waves occur when even a light breeze blows over the surface of water. Without wind, the air–water interface would be flat, but strong gusts initiate circulating motion, producing the characteristic breaking wave shape. Similar waves occur anywhere two fluids flowing in different directions are in contact. In space plasmas, this situation is common where solar wind flows past a magnetized body. At Saturn, strong internal magnetic fields produce a cavity in the solar wind, which is filled with plasma by Enceladus. This plasma rotates with the planet, leading to asymmetry in the system, where solar wind flows opposite the magnetospheric flow on the dawn side of the planet, and solar wind flows in the same direction as magnetospheric flow on the dusk side. Previous studies have confoundingly concluded that these waves are most prevalent in the dusk sector, but this is due to the difficulty of observing waves on the dawn magnetopause. The difference being waves on the dawn magnetopause quickly roll‐up and “break” similarly to ocean waves, while on the dusk side they keep their structure and move across the magnetopause. We resolve the long‐standing conundrum from a comprehensive analysis of the plasma near Saturn's magnetopause boundary. Key Points Reversed azimuthal flows, likely driven by Kelvin‐Helmholtz instabilites, occur dominantly in the dawn‐subsolar region Many identified Kelvi