Unexpected periodic perturbations in Saturn's magnetic field data from Pioneer 11 and Voyager 2

A direct comparison between the three magnetic field data sets from Saturn's magnetosphere is carried out. These data arise from the three spacecraft encounters with Saturn (Pioneer 11, Voyager 1 and 2). A standard planetocentric spherical polar coordinate system is used for the analysis. The inbound observations of the magnetic field from Pioneer 11 and Voyager 2 show a periodic perturbation of a few nanoTeslas (with a period close to the planetary rotation period) in the azimuthal component of the field, from the inbound magnetopause crossing to closest approach (∼ 17 hours). The outbound observations present an equivalent feature only in the case of Pioneer 11, where the radial and azimuthal components of the field are perturbed throughout the whole outbound pass (∼ 40 hours). A dipole tilt and/or current sheet crossings are unlikely causes for such periodic perturbations. A large scale ballooning of the magnetic field and plasma is a possible generating mechanism, and is used for the time-dependent scenario proposed here to explain the observed feature. The Pioneer 11 outbound magnetopause crossings may provide some evidence for such ballooning.