If a corotating magentospheric plasma is in equilibrium under the action of gravitational, inertial, and electromagnetic forces alone (neglecting pressure), the corotation speed must be proportional to the local Keplerian orbital speed, as a result used by Alfv¿n as the basis for the ''cosmogonic shadow effect'' in the formation of the solar system (leading to Alfv¿n's well-known 2/3 law for asteroid-belt or planetary-ring structures). The electric field implied by this distribution of corotation speeds has components parallel to the magnetic field which require non-zero electron pressures and thus invalidate the initial neglect of pressure forces. Inclusion in the stress balance of the force on ions and electrons exerted by the parallel electric field leads to a state of isorotation along lines of the magnetic field modified by the vorticity of the plasma, with a corotation frequency presumably governed by interaction with the ionosphere and having no particular relation to the gravitational orbital period. ¿ American Geophysical Union 1987