Auroral spirals at Saturn

We report Cassini/UVIS observations of auroral vortices at Saturn propagating from midnight to noon via dawn. The emission in the dawn auroral sector is observed to consist of several detached features that swirl with time. They have a diameter of 6000 km in the ionosphere, which would correspond to plasma vortices in the magnetosphere of 12 to 15 Rs. ENA enhancements are observed simultaneously. However, they do not show any clear vortices. We estimate the velocity of the UV auroral feature to decrease from 85% of rigid corotation (28o/h) in the most equatorward part of the aurora to 68% of rigid corotation (22o/h) in the poleward part and we demonstrate that such velocity gradient could result in swirling auroral features. Particle velocities derived from magnetospheric data in previous studies, confirm large variations of the corotation fraction as a function of radial distance. We suggest that the auroral vortices could be the ionospheric footprints of hot dynamic populations containing strong velocity gradients. Alternatively, we consider another scenario that could generate auroral vortices based on field line deformation from the magnetosphere to the ionosphere, like it is proposed for the Earth. In that case the auroral spiral is the result of some processes that occurred in the transition region between the centers of vortices where strong shear flows existed. Finally, a third possibility is considered, according to which the auroral vortices reported here are the direct optical signatures of the plasma vortical flows in the magnetopause related to Kelvin-Helmholtz instabilities. However, this might be less possible due to the very different spatial scales of the auroral features (12-15 Rs) and the observed plasma vortices in the magnetopause (1 Rs).