Magnetic Field Observations on Cassini's Proximal Periapsis Passes: Planetary Period Oscillations and Mean Residual Fields

We analyze periapsis pass magnetic field data from the final 23 orbits of the Cassini spacecraft at Saturn, uniquely encompassing auroral, subauroral, ring region, and intra-ring field lines, to determine the planetary period oscillations (PPOs) and mean residual fields in these regions. Dual modulation by northern and southern PPO systems is found almost continuously, demonstrating for the first time the presence of PPOs on and inside ring region field lines. The azimuthal component displays the largest ~10–15nT PPO amplitudes on auroral field lines, falling across the subauroral region to ~3–5 nT on main ring field lines in the northern hemisphere, less in the southern hemisphere, while increasing to ~5–8 nT on D ring and intra-D ring field lines. Auroral and subauroral amplitudes mapped along field lines are in good agreement with previous analyses in regions of overlap. Colatitudinal and radial field oscillations generally have a half and a quarter the amplitude of the azimuthal component, respectively. Inner region oscillation phases are typically several tens of degrees “earlier” than those of outer subauroral and auroral regions. Mean residual poloidal fields (internal and ring current fields subtracted) show quasi-sinusoidal latitude variations of ~2.5nT amplitude, with radial and colatitudinal fields approximately in quadrature. Mean azimuthal fields peaking at ~15 nT are approximately symmetrical about the equator on and inside D ring field lines as previously reported, but are unexpectedly superposed on ~3–5nT “lagging” fields which extend continuously through the ring region onto subauroral field lines north and south.