AN EXPLANATION FOR THE EAST-WEST ASYMMETRY OF THE IO PLASMA TORUS

One of the most interesting and unexplained spatial structures in the Io plasma torus located near Io's orbit about Jupiter is the east-west (or alternatively dawn-dusk) asymmetry in the planetocentric distance of the so called plasma ''ribbon,'' the brightest and most prominentl y observed radial feature of the torus. The average radial position of the ribbon on the sky plane in both its ground-based measured S+ opti cal emission (6716 Angstrom, 6731 Angstrom) and its Voyager measured S ++ ultraviolet emission (685 Angstrom) is observed to be located close r to the planet and well within Io's orbit when it is west of Jupiter at the dusk (or receding) ansa and farther from the planet and very ne ar Io's orbit when it is east of Jupiter at the dawn (or approaching) ansa. In addition, the ribbon is also observed to move about this aver age position as a function of its ansa System III longitude. It is sho wn that the location of this asymmetrical radial structure for the Sribbon arises naturally in the presence of an east-west electric field from a space and time dependent plasma source that is highly concentr ated at Io's instantaneous orbital location (and hence initially locat ed at a constant distance from the planet) and a plasma transport rate that increases radially outward. Model calculations reproduce both th e observed average east-west asymmetry and the System III longitude de pendence of the S+ ribbon location in the plasma torus. In the absence of an east-west electric field, however, the model-calculated density peak for the S+ ribbon is located essentially symmetrically about Jup iter just inside Io's orbit and does not exhibit the observed east-wes t asymmetry. Since the S+ ribbon during its slow outward transport wil l undergo electron impact ionization, the radial location of the S++ r ibbon can be expected to be created naturally with the same System III longitude dependence as for the S+ ribbon and at a position radially just beyond that of the S+ ribbon, as has been observed.