Rotation rate of Saturn's interior from magnetic field observations

Analysis of magnetic field data from the Pioneer 11, Voyager 1 and Voyager 2 spacecraft produced the first models of Saturn's internal field. One striking feature of these models is the lack of non axisymmetric terms, which poses strong constraints on the dynamo mechanism. Resolution of the internal field using magnetic field data from different epochs is complicated by the uncertainty in our knowledge of the planetary rotation rate. By reanalyzing the flyby data, using modern inversion techniques, we derive the first tentative direct measurement of the rotation rate of the magnetic field. The measured rotation period agrees within 0.6 s with the value obtained from remote radio emission measurements, and its uncertainty is reduced to +/- 2.4 s. From an inversion of all available magnetic field data, we conclude that it is premature to exclude the presence of non-axisymmetric terms when describing the internal planetary field, and in particular we find a significant dipole tilt of 0.17degrees.