Spin‐period effects in magnetospheres with no axial tilt

A magnetic axis that is tilted with respect to the spin axis causes waves that generate spin‐periodic effects in a planet's outer magnetosphere. Saturn's magnetic axis is aligned with its spin axis, and yet its outer magnetosphere exhibits spin‐periodicities in all species of charged particles, just as expected from a wavy magnetodisk. In a spin‐aligned geometry, however, a rotating anomaly (or “cam”) in the inner magnetosphere can also generate spin‐periodic waves by shaking the plasma sheet of the outer magnetosphere. Like a wobbling magnetodisk, this model predicts oscillations of the plasma sheet at distances beyond ∼20 RS (1 RS = 60268 km). When viewed from a moving spacecraft, these oscillations cause charged particle periodicities near the spin period of the planet. The phase and polarity of these periodicities should change systematically with local time, and spin‐periodic effects should appear along the magnetopause.