Periodic modulation of gas giant magnetospheres by the neutral upper atmosphere

Periodic signatures present in the magnetospheres of both Jupiter and Saturn have yet to be fully explained. At Jupiter the unexplained signatures are related to emissions from the Io torus (System IV); at Saturn they are observed in emissions of kilometric radiation (SKR) and in magnetometer data. These signatures are often interpreted in terms of magnetic field anomalies. This paper describes an alternative mechanism by which the neutral atmosphere may impose such periodic signatures on the magnetosphere. The mechanism invokes a persistent zonal asymmetry in the neutral wind field that rotates with the planet. This asymmetry must be coupled to substantial ionospheric conductivity. It is then able to drive divergent currents in the upper atmosphere that close in and perturb the magnetosphere. We estimate the conductivities and wind speeds required for these perturbations to be significant, and argue that they are most likely to be important at auroral latitudes where the conductivity may be enhanced by particle precipitation.