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Cassini observations of the ring current in Saturn’s magnetosphere

By Stephanie Kellett

Abstract

In this thesis we have employed plasma and magnetic field data from the Cassini spacecraft in order to better understand Saturn’s ring current region. Three data studies are presented along with a derivation of the general expression for the field-perpendicular current density in terms of the plasma bulk parameters. \ud In the first data study an essentially direct determination of the equatorial current sheet thickness was made using data from six north-south Cassini orbits. The dayside data indicated the presence of an equatorial current disk with a near constant half-thickness of ~1.5 RS. More variable conditions were found on the nightside. The data examined also provided evidence of a northward displacement of the current layer from the equatorial plane. \ud Next, the nature of the ring current in Saturn’s dayside magnetosphere was investigated. The total azimuthal current density was found to rise from small values near ~6 RS, peak at ~100 pA m-2 near ~8 RS, and then reduce to values below ~25 pA m-2 at distances beyond ~15 RS, up to the 20 RS limit of our study. The overall total current density profile was found to be similar to that produced by the pressure gradient current, but augmented in strength by factors of ~1.5-2.0 by the difference between the inertia and pressure anisotropy currents. Comparison of the current density profiles deduced from plasma data with those obtained from current disk modelling of the magnetic field perturbations showed good agreement with the gross features. \ud Finally, both the local time dependency and temporal variability of Saturn’s ring current was explored using data obtained from eleven near-equatorial Cassini orbits. In general, the plasma parameters, azimuthal current, and related magnetic perturbation fields were found to exhibit only modest variations with local time and from pass-to-pass over the interval of this study

Publisher: University of Leicester
Year: 2011
OAI identifier: oai:lra.le.ac.uk:2381/9633

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