Spinning, breathing, and flapping: Periodicities in Saturn's middle magnetosphere

Abstract

AbstractIn Saturn's magnetosphere, ubiquitous fluctuations with a period of ~10.7 h have been observed in Saturn kilometric radiation (SKR), auroral emissions, the magnetic field, the electron density, and energetic particle fluxes. Here we characterize previously unstudied periodicities in plasma properties inside of 15 RS near the equatorial plane. Although periodically varying magnetic perturbations rotate relatively smoothly (spinning), plasma properties do not. The phase of the peak value of plasma density or pressure perturbations can change substantially across a few hours of local time or RS. As a means of interpreting observations, we use a magnetohydrodynamic simulation that generates field‐aligned currents centered at 70° invariant latitude in Saturn's southern ionosphere and rotating at the SKR period. The simulation reproduces many periodic features of the data including not only spinning perturbations but also global‐scale compression and expansion (breathing). Simulated plasma properties are also modulated by periodic large‐scale north‐south motion (flapping) in regions beyond ~15 Saturn radii (RS), which we do not analyze here. Inside of 15 RS, plasma responds to a superposition of spinning and breathing at the spin period, developing perturbations that peak at different phases depending on what is measured and where. Strong compressional effects act impulsively over a limited range of rotation phase. Superposition of local and global‐scale variations produces phase jumps across short distances and can introduce multiple peaks in the variation of plasma properties within one rotation period, accounting for anomalies in the phase dependence of periodic fluctuations identified in the sparse data available.