An explanation for the east–west asymmetry of the Io plasma torus

Abstract

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 prominently observed radial feature of the torus. The average radial position of the ribbon on the sky plane in both its ground‐based measured S+ optical emission (6716 Å, 6731 Å) and its Voyager measured S++ ultraviolet emission (685 A) is observed to be located closer 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 near 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 average position as a function of its ansa System III longitude. It is shown that the location of this asymmetrical radial structure for the S+ ribbon arises naturally in the presence of an east–west electric field from a space and time dependent plasma source that is highly concentrated at Io's instantaneous orbital location (and hence initially located at a constant distance from the planet) and a plasma transport rate that increases radially outward. Model calculations reproduce both the observed average east–west asymmetry and the System III longitude dependence 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 Jupiter just inside Io's orbit and does not exhibit the observed east–west asymmetry. Since the S+ ribbon during its slow outward transport will undergo electron impact ionization, the radial location of the S++ ribbon 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.