Saturn’s magnetospheric oscillations
Gabrielle Provan
University of Leicester
S. W. H. Cowley
Planetary period oscillations (PPOs) with periods close to Saturn’s rotational period are ubiquitous throughout Saturn’s magnetospheric system. Here we review the observational studies of PPOs determined from magnetospheric magnetic field data throughout the Cassini mission to date. As first shown using radio data, two oscillatory systems are present, one associated with the northern polar region and the other with the southern. The PPO periods are shown to lie in the range ~10.6 to 10.8 h, are persistently shorter north than south to date, and undergo a strong seasonal cycle together with the oscillation amplitudes. We show that within the northern (southern) open-field polar region only the northern (southern) PPO oscillations are detected. However, within the equatorial ‘core’ region of Saturn’s magnetosphere (dipole L ≤ 12), the two oscillations are superposed and interfere. Thus within the ‘core’ region the oscillations can be organized by the ‘beat frequency’ of the two oscillations, but outside the ‘core region’ organizing the oscillations by the beat-frequency will in general result in a phase drift. We discuss recent results showing good agreement between the PPO periods determined from the magnetic field with independently-determined SKR periods, clearly showing that the two periods are well separated and proving the robustness of determining the PPO periods from the magnetic and SKR data. Finally, we discuss how the reported abrupt changes in the periods and amplitudes of the two oscillations, with a cadence of 100-200 days, appears associated with solar wind activity.