An Explanation for the East-West Asymmetry of the Io Plasma Torus
W. H. Smyth, M. L. Marconi (AER)
One of the most interesting and unexplained of the structures in the plasma torus, organized near Io's orbit, is the radial peak in the brightness distribution of the plasma clearly observed in both the optical S (6716Å, 6731Å) and ultraviolet S (685Å) emission lines. For both S and S , the planetocentric location of the brightest observed feature in its radial structure, the so called plasma ``ribbon'', is asymmetrically positioned about the planet, exhibiting an average distance farther from Jupiter when near eastern elongation (dawn or approaching ansa) and also exhibiting a System III longitude variation about this average position (Schneider and Trauger, Ap. J. 450, 459, 1995; Dessler and Sandel, GRL 19, 2099, 1992). The key question raised by these observations is: how is this east-west asymmetrical radial structure of the plasma torus established within Io's orbit for a plasma source that is concentrated at Io's instantaneous orbital location and hence is initially at a constant distance from the planet? To address this question, we have developed a time-dependent, two-dimensional plasma transport model (L-shell and System III longitude angle) containing an Io plasma source that moves about Jupiter in the plasma torus described by an offset tilted dipole magnetic field in the presence of an east-west electric field. We have first undertaken studies for the S ribbon created by atomic sulfur loss from Io, where the spacetime dependent plasma source for S produced by ionization and charge exchange in the plasma torus has been calculated using our neutral sulfur cloud model for Voyager epoch plasma properties. Transport calculations show that the Iogenic S plasma density evolves in time and produces, as it approaches steady state, a maximum which is asymmetrically located within Io's orbit just as observed. The east-west asymmetry and the System III longitude dependence of the position of the S ribbon are thus shown to be produced naturally by the combined influence of three factors: a plasma source that is highly peaked at Io's position, a nominal east-west electric field in the Jupiter system, and a plasma transport rate that increases radially outward with decreasing magnetic field.