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 I 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 (similar to 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 (similar to 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. (C) 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.