Structure and Dynamics of the Sub-corotating Region of Saturn’s Magnetosphere: Cassini Magnetic Field Observations.

Abstract:

Measurements of the azimuthal magnetic field component, B_Φ, enable the study of the sub-corotating magnetosphere, the associated global current system, the ionospheric Pedersen current, I_P, and a comparison with the Cowley-Vasyliunas model. The study is strongly dependent on Cassini’s highly elliptical orbits that restrict useful observations to specific orbits and parts of orbits to avoid weak azimuthal fields associated with the magnetopause or other magnetospheric currents. An example is the selection of thirteen identical orbits near 22 +/- 2 hours Local Time and restriction of the data to 4 to 10 Saturn radii. Inferred I_P profiles between 10° and 25° ionospheric co-latitude, θ_i , when averaged, are similar in shape to model predictions with a peak value of 5 million amperes. However, the individual profiles vary significantly, the result of dynamics associated with large-scale motions of the magnetospheric field. The Cowley-Vasyliunas model also relates I_P(θ_i) to magnetospheric rotation rates, Ω_S, equivalent to the SKR period, and the sub-corotation rate, ω, and provides a means of estimating ω / Ω_S in the night-side magnetosphere as compared to the dayside rotation rates obtained from azimuthal plasma velocity measurements, V_Φ. Other carefully- selected orbits extend these results to different ranges in θ_i and Local Times and provide further values of important magnetospheric parameters and allow additional testing of the model.