A Consistent Ribbon Structure for the Io Plasma Torus at the Voyager 1 and Galileo J0 Epochs

Author: William H. Smyth and Charles A. Peterson
June 2, 2008
Journal Article
Bulletin of the American Astronomical Society

Smyth, W.H., C.A. Peterson, and M. L. Marconi (2008) A Consistent Ribbon Structure for the Io Plasma Torus at the Voyager 1 and Galileo J0 Epochs, BAAS 40, 460.

The peak density structure for the plasma torus electrons and heavy ions occurs radially near and inside of Io's orbit. This structure has been documented remotely by ground-based observations at the dawn and dusk ansae in S+ (6716 Å, 6731 Å) and S++ (9531 Å) emissions and from Voyager 2 UVS observations at the 35 degree pre-dawn and pre-dusk ansae for S++ (685Å) emission. It has also been documented by in situ observations by the Voyager 1 PLS instrument for the O+, S+, S++ ions and electrons at pre-dusk local-times and by the Galileo PWS and PLS instruments for electrons at noon local-time. The focus of this presentation is to compare the radial location of the peak density structure for the S+ ions (so-called ribbon feature) acquired at different local-times from ground-based (Schneider and Trauger, Ap. J. 450, 450, 1995) and Voyager (Bagenal, JGR 99, 11043, 1994) measurements and inferred from Galileo measurements (Gurnett et al., Science, 274, 391, 1996; Frank and Paterson, JGR 99, 11043, 2000). This is accomplished by the application of a newly developed more accurate, four-dimensional (three spatial dimensions and local-time) empirical model for the plasma torus. This model includes System III longitudinal asymmetries and also local-time asymmetries created by a dawn-dusk electric field that is equipotential along the magnetic field lines and has an adjustable magnitude and direction. It is found that all of the measurements can be fit by a set of different magnitudes and directions for the dawn-dusk electric field and that a limited range of such solutions is possible for the three measurements that allow a consistent radial location for the S+ ribbon feature, resolving a long-standing issue for the plasma torus (Bagenal et al., GRL 24, 2119, 1997). Information will be presented to illustrate the nature of these solutions.