The magnetosphere of Neptune changes its magnetic configuration continuously as the planet rotates, leading to a strong modulation of the convection electric field. Even though the corotation speed is considerably larger, the modulation causes the small convection speed to have a cumulative effect, much like the acceleration of particles in a cyclotron. A model calculation shows that plasma on one side off the planet convects out of the magnetosphere in a few planetary rotations, while on the other side it convects slowly palanetward. The oservation of nitrogen ions from a Triton plasma torus may provide a critical test of the model. ¿American Geophysical Union 1990