Generation of Saturn's Periodicities Through A Global Mode of the Centrifugal Interchange Instability

The origin of the periodicities in the radio, plasma and magnetic fields of Saturn has long been debated. Given the high degree of alignment of Saturn's dipole with its rotation axis no strong rotational periodicities are expected. However, detailed analysis of the Cassini data demonstrated the existence of such periodicities not only in Saturn's kilometric radio emissions (SKR), but in the plasma and magnetic field signatures throughout the Kronian magnetosphere as well. Several mechanisms have been proposed to explain these periodicities, yet each have difficulties self-consistently explaining the features detected across all the observations. The results presented will show that many of the observed features can be explained by the development of the centrifugal interchange instability. The centrifugal interchange instability leads to the formation of outward moving cold dense plasma fingers interspersed with inward moving hot tenuous magnetospheric plasma. While each individual interchange finger is sub-corotational, the global nature of the instability leads density and magnetic perturbations at any fixed observing point that have the rotational period embedded in the perturbations. In other words, a stationary observer will see a plasma finger (but not necessarily the same finger) sweep past at the rotation period. We show that while the local perturbations can change in intensity from solar wind forcing, the periodicity remains approximately constant, although there are specific solar conditions that yield the strongest periodicity signal. Reconnection processes that generate plasmoids in the tail also generate density and magnetic fluctuations. However these processes occur over a much longer scale and as such are not the direct source of the observed periodicities. The interchange instability not only provides a means of describing both sub-corotational and rotational features but also provides a natural explanation for the difference in the periodicities seen between the northern and southern hemispheres.