Analytical solutions are obtained for the equipotential (E X B) drift trajectories in a certain simplified (but non-dipolar) magnetospheric model. The required superposition of corotation and convection electric fields in the tail leads to drift paths that are concave relative to the earth's dawn meridian. The present model is equally applicable to other planetary magnetospheres having known parameters. For example, the larger magnetic moment and angular velocity of Jupiter lead to closed equipotential trajectories that lie entirely within the zenomagnetic tail, and thus preclude the adiabatic access of solar-wind plasma to the interior of Jupiter's magnetic tail. (Author)