Saturn's planetary period oscillations: Interhemispheric coupling and enforcement of the 'plasma cam' by vertically propagating atmospheric waves

We develop a three-dimensional, linear model of the atmospheric waves postulated to explain Saturn's planetary period oscillations (PPOs). The polar atmospheres in both hemispheres are represented using a beta-plane model; the equatorial magnetosphere is also represented using plane Cartesian geometry. We describe the interaction of quasi-geostrophic Rossby waves with a layer of Hall conductance whose magnitude varies with latitude. We investigate a situation in which upward-propagating Rossby waves in one hemisphere drive currents which flow into the magnetosphere and the opposite hemisphere. We find that the model describes well the observed current systems when the opposite hemispheres have distinct PPO periods. We present a qualitative model of how driving by atmospheric waves could lead to the two PPO current systems becoming 'locked' together, as observed in 2013-2014. However, our model is not able to explain why the PPO current systems became locked together with a specific phase relationship. We also discuss the observed presence of a 'plasma cam' density distribution in the magnetosphere, and show that our model can explain how a cam-shaped density distribution is maintained close to corotation in a magnetosphere that is almost universally subcorotating.

Automated summary

A three-dimensional linear model was developed to explain Saturn's planetary period oscillations, focusing on the interaction of Rossby waves and current systems. The model successfully describes observed current systems when opposite hemispheres have distinct PPO periods. However, the model falls short in explaining why the PPO current systems become 'locked' together with a specific phase relationship.