Nonaxisymmetrical component of Saturn's magnetic field derived from Cassini radio and magnetometer data
Résumé
A dipole model ("birotor dipole") was proposed for Saturn's inner magnetic field: this dipole presents the particularity to have North and South poles rotating around Saturn's axis at two different angular velocities; this dipole is tilted and not centered. The spin rates and the associated rotation phases are derived from a continuous wavelet transform analysis of the intensity of the Saturnian kilometric radiation (SKR) signal received at 290 kHz between July 2004 and June 2012 by the radio and plasma wave science (RPWS) experiment on board Cassini. 57 revolutions of the spacecraft, the periapsis of which is less than 5 Saturnian radii, have been selected for this study. For each of these chosen orbits, it is possible to fit with high precision the measurements of the MAG data experiment given by the magnetometer embarked on board Cassini. A nonrotating external magnetic field completes the model. This study suggests that Saturn's inner magnetic field is neither stationary nor fully axisymmetric. These results can be used as a boundary condition for modelling and constraining the planetary dynamo and they can be a starting point for the study of Saturn's inner structure and the comparison with the interior of Jupiter. An average of the birotor dipole magnetic potential is performed on the azimuthal angle revealing an average axisymmetrical component which can be compared with the published zonal models proposed for Saturn's magnetic field. The presence of a "birotor quadrupole" is required in the present model to improve the agreement for the third order Gauss coefficient.