Saturn’s magnetic field revealed by the Cassini Grand Finale

INTRODUCTION Starting on 26 April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15 September 2017. This phase offered an opportunity to investigate Saturn’s internal magnetic field and the electromagnetic environment between the planet and its rings. The internal magnetic field is a diagnostic of interior structure, dynamics, and evolution of the host planet. Rotating convective motion in the highly electrically conducting layer of the planet is thought to maintain the magnetic field through the magnetohydrodynamic (MHD) dynamo process. Saturn’s internal magnetic field is puzzling because of its high symmetry relative to the spin axis, known since the Pioneer 11 flyby. This symmetry prevents an accurate determination of the rotation rate of Saturn’s deep interior and challenges our understanding of the MHD dynamo process because Cowling’s theorem precludes a perfectly axisymmetric magnetic field being maintained through an active dynamo. RATIONALE The Cassini fluxgate magnetometer was capable of measuring the magnetic field with a time resolution of 32 vectors per s and up to 44,000 nT, which is about twice the peak field strength encountered during the Grand Finale orbits. The combination of star cameras and gyroscopes onboard Cassini provided the attitude determination required to infer the vector components of the magnetic field. External fields from currents in the magnetosphere were modeled explicitly, orbit by orbit. RESULTS Saturn’s magnetic equator, where the magnetic field becomes parallel to the spin axis, is shifted northward from the planetary equator by 2808.5 ± 12 km, confirming the north-south asymmetric nature of Saturn’s magnetic field. After removing the systematic variation with distance from the spin axis, the peak-to-peak “longitudinal” variation in Saturn’s magnetic equator position is CONCLUSION Cassini Grand Finale observations confirm an extreme level of axisymmetry of Saturn’s internal magnetic field. This implies the presence of strong zonal flows (differential rotation) and stable stratification surrounding Saturn’s deep dynamo. The rapid latitudinal variations in the field suggest a second shallow dynamo maintained by the background field from the deep dynamo, small-scale helical motion, and deep zonal flows in the semiconducting region closer to the surface. Some of the high-degree magnetic moments could result from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. The periapse azimuthal field originates from a strong interhemispherical electric current system flowing along magnetic field lines between Saturn and the inner edge of the D-ring, with strength comparable to that of the high-latitude field-aligned currents (FACs) associated with Saturn’s aurorae.