We utilize an earlier model of the Jovian magnetosphere in which the centrifugal stress of corotating plasma distends the outer magnetosphere and opens the fail field. Because of a longitudinal asymmetry in the ionospheric plasma source strength, caused principally by the nonaxisymmetric surface field, the closed field region in the tail expands and contracts with the rotation period, resulting in a 10-hour modulation of the flux of energetic particles escaping from the magnetosphere into interplanetary space.