Spinning, Breathing, and Flapping: The Changing Size and Shape of Saturn's Middle Magnetosphere

Abstract:

In Saturn's magnetosphere, periodic fluctuations are observed in Saturn Kilometric Radiation (SKR), auroral emissions, magnetic field, electron density, and energetic particle fluxes. We have extended previous Cassini investigations at Saturn by characterizing periodicities in additional plasma and magnetic field properties in Saturn's middle magnetosphere near the equatorial plane. It is customary to model perturbations in the middle magnetosphere as if they rotate rigidly, but we find that this assumption does not work well for all properties of interest and that the phase dependence of the perturbations may vary with radius and local time. We use a magnetohydrodynamic (MHD) simulation [Jia et al., 2012], which generates a rotating pattern of field aligned currents centered at 70° invariant latitude in Saturn's southern ionosphere that impose periodic variations on the entire magnetosphere, to understand how the changing size and shape affects the observed properties. In particular, we find that the dayside magnetopause roughly follows the 80 invariant latitude field lines as they move in and out. We identify three different modes of magnetospheric periodicity linked to rotation (spinning), compression (breathing), and north-south motion (flapping). All have the same ~10.7 hour period, but impose significant changes at phases that depend on the plasma property considered and the location of the measurement. Multiple modes acting concurrently can produce distinctly non-sinusoidal waveforms of the variations of plasma parameters through a rotation cycle. Within limitations of data coverage, we find good agreement between the simulation and the data in the rotation phase modulation of magnetic pressure, plasma pressure, and density perturbations.