Field-aligned currents in Saturn’s magnetosphere: Relationship between Sub-corotation and Planetary Period Oscillation Currents
Magnetic fields of planets and cool stars
Gregory Hunt
University of Leicester
S. W. H. Cowley (University of Leicester), G. Provan (University of Leicester), E. J. Bunce (University of Leicester), I. I. Alexeev (MSU), E. S. Belenkaya (MSU), V. V. Kalegaev (MSU), M. K. Dougherty (Imperial College London), A. J. Coates (UCL)
We present analyses of magnetic field data from the Cassini spacecraft during 2008 showing the presence of field-aligned currents. In the southern hemisphere these currents are found to be strongly modulated in form, magnitude and position by the phase of the southern planetary period oscillations (PPOs), these oscillations are observed as magnetic perturbations. While in the northern hemisphere we show that these currents are modulated in form and magnitude by both the northern and southern PPO phases, thus giving the first direct evidence of inter-hemispheric currents. We separate currents independent of PPO phase from the PPO-related currents, by exploiting the expected anti-symmetry of the latter with respect to PPO phase. We find that in both hemispheres the PPO-independent and PPO-related currents are typically co-located and comparable in magnitude. We show that in the southern hemisphere the latitude of current layers are modulated with amplitude ~1°, such that the maximum equatorward and poleward excursions are adjacent to the maximum upward and downward PPO currents, respectively. This phasing requires the current system to be driven upward from the planetary atmosphere rather than downward from the magnetosphere. Latitudinal variations in the dual-modulated northern hemisphere are more difficult to discern. In common with previous studies we find that in the polar regions the oscillations are hemispherically pure to within ~10%. Both north and south oscillations are present on closed field lines interior to the current region, where we newly examine how the amplitude of the oscillations vary with latitude along the field lines.
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