Field-aligned currents in Saturn’s magnetosphere: Relationship between Subcorotation and Planetary Period Oscillation Currents
Tuesday, 24 May 2016: 9:20 AM
Gregory James Hunt1, Stanley W H Cowley2, Gabrielle Provan2, Emma J Bunce3, Igor I Alexeev4, Elena Semenovna Belenkaya4, Vladimir V Kalegaev4, Michele Karen Dougherty5 and Andrew J Coates6, (1)University of Leicester, Department of Physics and Astronomy, Leicester, United Kingdom, (2)University of Leicester, Leicester, United Kingdom, (3)Univ Leicester, Leicester, United Kingdom, (4)Lomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics, Moscow, Russia, (5)Imperial College London, Blackett Laboratory, London, SW7, United Kingdom, (6)University College London, Centre for Planetary Sciences (at UCL/Birkbeck), London, United Kingdom
Abstract:
We present analyses of magnetic field data from the Cassini spacecraft during 2008 showing the presence of field-aligned currents in the midnight local time (LT) sector. 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). In the northern hemisphere, however, we show that the currents are modulated by both the northern and southern PPO phases, thus giving the first direct evidence of inter-hemispheric PPO 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 (subcorotation) and PPO-related currents are typically co-located and comparable in magnitude. In common with previous studies we find that in the polar regions the oscillations are hemispherically pure to within ~10% by amplitude. Both northern and southern oscillations are present on closed field lines interior to the current region, where we examine how the amplitude of the oscillations varies with latitude along the field lines. Comparing the results for the southern hemisphere to an earlier interval of data from 2006/07 in the dawn-noon LT sector also in the southern hemisphere, we find that the PPO-independent currents are essentially identical within uncertainties in the dawn-noon and midnight sectors, thus providing no explanation for the LT dependence of the Saturn kilometric radiation (SKR) emissions, which peak at ~08 h LT. The main PPO-related currents are, however, found to be slightly stronger and narrower in latitudinal width at dawn-noon than at midnight, leading to estimated precipitating electron powers, and hence emissions, that are on average a factor of ~1.3 larger at dawn-noon than at midnight, inadequate to account for the observed LT asymmetry in SKR power. Some other factor must also be involved, such as a LT asymmetry in the hot magnetospheric auroral source electron population.
