Cassini observations of a vortex structure in Saturn’s dayside magnetosphere driven by the Kelvin-Helmholtz instability

We present the first observations of a vortex structure in Saturn’s dayside, outer magnetosphere. The identification of the structure provides conclusive evidence of the operation of the Kelvin-Helmholtz (K-H) instability at Saturn’s magnetospheric boundaries. Cassini observations taken during the inbound pass of the spacecraft’s Revolution B orbit in Decmber 2004 are analysed. Magnetic field conditions during the magnetopause crossings that occurred on this orbital pass suggest that the boundary was highly K-H unstable. Following multiple magnetopause crossings the spacecraft encountered the low-latitude boundary layer. Magnetic field, thermal plasma, and superthermal plasma observations made by Cassini during the spacecraft transition between the boundary layer and magnetosphere proper are consistent with an encounter with a vortex structure on the edge of the boundary layer - this interface is also anticipated to be K-H unstable. High-energy (>20 keV) electrons observed while the spacecraft was within the vortex suggest that the structure was associated with auroral emissions. A model of the coupling between an outer magnetospheric vortex and Saturn’s ionsphere via field-aligned currents is proposed. Estimates based on Knight’s theory imply that field-aligned potentials of a few kV were associated with the region of upward-directed field-aligned current in the northern ionosphere, and that the resulting precipitation of accelerated electrons produced UV auroral emissions with an intensity of a few kR. We propose that K-H vortices in Saturn’s outer magnetosphere produce bright spots of UV aurora. This discovery has implications for our understanding of the interaction between the solar wind and Saturn’s magnetosphere.