On the nature and location of the proposed twin vortex systems in Saturn's polar upper atmosphere

Twin vortices in Saturn's northern and southern polar ionospheres have been proposed as the drivers of the similar to 10.7 h periodicities in the magnetosphere. We critically analyze this model from an atmospheric perspective, identifying two broad problems: first, the difficulty in reconciling a similar to 10.7 h periodicity with the strongly subcorotational flows that are observed in the polar ionosphere; second, the very large energy requirement for sustaining the vortices against Joule dissipation. We propose a model in which the twin vortex originates neither in the plasma nor in the thermosphere but deeper in the atmosphere, around 750 km altitude. At this altitude the gas is expected to be close to corotation, and Joule dissipation is low. A twin vortex system at this altitude blowing laterally across the Hall conductance gradient at the main auroral oval generates field-aligned currents which are qualitatively similar to those driven by a plasma vortex or thermospheric vortex at higher altitudes.