How can Saturn impose its rotation period in a noncorotating magnetosphere?

A conceptual model is proposed, where Saturn can impose its rotation period in a noncorotating magnetosphere, as observed by Pioneer 11, Voyager 1 and 2. The fundamental hypothesis for this so‐called “Camshaft model” is that Saturn has an equatorial anomaly, likely to be magnetic. It is restricted in longitude, and the source is yet to be detected. This longitudinal asymmetry is equivalent to a variation of pressure for the magnetospheric subcorotating plasma, and therefore as the planet rotates, a compressional wave is generated. That is, we use the MHD fast mode, which can propagate across the magnetic field, rather than the transverse mode for momentum transfer from the planet to the magnetospheric plasma. The wave propagates radially outward across the background magnetic field, inducing a motion in the plasma that is decoupled from and superposed on its azimuthal motion. Consequently, as the planet rotates, magnetic field observations fixed in an inertial frame would present a periodic signature with the planetary rotation period. This is true at each local time, independently of the level of plasma subcorotation. We then show that the Camshaft model accounts very well for the previously reported observations of spin‐periodic perturbations in Saturn's magnetic field. Finally, we consider the perturbation magnetic field (obtained by subtracting only the model planetary field from the observations) measured by Pioneer 11 while outbound, and find its orientation consistent with the Camshaft model once the propagation delay of the compressional wave is included.