Axisymmetry Breaking of Saturn ’ s External Magnetic Field

Saturn’s magnetic field is remarkably axisymmetric. Its dipole axis is inclined by less than 0.2 with respect to its rotation axis. Early evidence for nonaxisymmetry came from the periodicity of Saturn’s kilometric radio bursts (SKR). Subsequently, percent variations of the SKR period were found to occur on timescales of years. A recent breakthrough has been the direct detection of a nonaxisymmetric component of the field that rotates with a period close to that of the SKR. Because this component’s magnitude varies only weakly with distance from Saturn, it must be supported by currents external to the planet. These currents flow along field lines that connect plasma in the equatorial region of the outer magnetosphere (the plasma disk) to the high latitude ionosphere. The plasma originates from mass lost by the planet’s rings and moons. It is tightly coupled to the magnetic field and its motion can be ascribed to the large scale interchange of flux tubes. Heavily loaded tubes drift outward and are replaced by lightly loaded ones which drift inward. This process of rotationally driven convection is responsible for breaking the axisymmetry of Saturn’s external magnetic field. The convection pattern rotates rigidly with the angular velocity of the plasma at its source. Its rotation provides the clock that controls the periods of both the SKR bursts and the nonaxisymmetric magnetic perturbations. We distinguish two types of currents. Those that flow in the azimuthal direction in both the ionosphere and the plasma disk provide a radial force that enables the plasma to corotate with the planet’s spin. They are responsible for stretching magnetic field lines and thus exposing the polar ionosphere to the incoming solar wind. Other currents flow in the latitudinal direction in the ionosphere and the radial direction in the plasma disk. These act to transfer angular momentum from the planet’s spin to the outflowing plasma. They slow down the rotation