Rotating Planetary Period Oscillations in Saturn’s Magnetosphere

Summary

The rotating phenomena termed planetary period oscillations (PPOs), observed persistently and ubiquitously in Saturn’s coupled magnetosphere–ionosphere system, have formed a central topic of investigation using data from the Cassini Saturn orbiter space mission (2004–2017). It has been found that two superposed systems are generally present, driven by rotating twin-vortex flows in Saturn’s northern and southern polar upper atmospheres, whose periods are generally slightly different from each other and slightly longer than the likely rotation period of Saturn’s deep interior ~10.56 h. The periods are also found to change somewhat irregularly with Saturn’s seasons over Saturn yearly intervals of ~29.4 Earth years, those driven from the summer polar atmosphere-ionosphere generally being longer ~10.8 h than those driven from the winter polar atmosphere-ionosphere ~10.6–10.7 h. The polar atmospheric-ionospheric flows drive rotating modulations throughout Saturn’s magnetosphere in magnetic fields, plasma flows, and charged particle populations that propagate in the sense of planetary rotation through the generally subcorotating magnetospheric plasma. Rotating systems of field-aligned electric currents associated with the magnetosphere–ionosphere coupling also give rise to rotating modulations of Saturn’s current-driven polar auroras and related modulated radio emissions at kilometer wavelengths, termed Saturn kilometric radiation (SKR).

The PPOs play a significant role in Saturn’s magnetospheric dynamics by modulating the stability of the nightside equatorial plasma sheet to magnetic reconnection. Magnetospheric plasma originates primarily from water gas emitted from the moon Enceladus, orbiting at ~4 Saturn radii deep within the inner magnetosphere, which is transported outward by flux tube interchange motions. The plasma is lost downtail to the solar wind following PPO-modulated plasma sheet reconnection, which pinches off closed-loop plasmoids from distended outflowing plasma sheet flux tubes, while the remaining mass-reduced flux tubes collapse back toward the planet, injecting heated tenuous plasma into Saturn’s midnight-dawn middle magnetosphere. Magnetospheric dynamical events in this PPO-modulated “Vasyliūnas flow cycle” thus tend to recur on planetary rotation (PPO) timescales. Related PPO effects also appear to modulate the magnetospheric response to strong solar wind compressions, which result in the closure of significant quantities of open flux in Saturn’s magnetic tail, these dynamic events forming a major feature of the “Dungey flow cycle” at Saturn.