The dusk side of the Io plasma torus is brighter than the dawn side (the ''east-west effect''), which has been explained in terms of a uniform dawn-to-dusk cross-tail magnetospheric electric field. Such a field would cause the torus to be offset so the dusk ansa is closer to Jupiter and the dawn ansa is farther, the two displacements being equal. We find the displacement of the two ansae to be unexpectedly complex. The displacements are unequal and both ansae are in motion with the motion of the approaching ansa being the lesser of the two. The radial motions, as measured from either the center of Jupiter or the offset-tilted dipole, are of unequal magnitude and have the System III periodicity. We conclude that the cross-tail electric field that causes these torus motions is concentrated on the dusk ansa, varies with the System III period, and shows magnetic-anomaly phase control. Finally, we find that the dawn-dusk asymmetry in brightness (the east-west effect) is not explained simply by the cross-tail electric field. We conclude that there is a heating mechanism, yet to be fully identified, that causes the dusk side of the Io plasma torus to be brighter than the dawn side. ¿ American Geophysical Union 1992