Hunt_et_al-2018-Journal_of_Geophysical_Research%3A_Space_Physics.pdf (5.48 MB)
Field-Aligned Currents in Saturn's Magnetosphere: Observations From the F-Ring Orbits
journal contributionposted on 2018-08-09, 13:34 authored by G. J. Hunt, G. Provan, E. J. Bunce, S. W. H. Cowley, M. K. Dougherty, D. J. Southwood
We investigate the azimuthal magnetic field signatures associated with high‐latitude field‐aligned currents observed during Cassini's F‐ring orbits (October 2016–April 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the field‐aligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the F‐ring data cover the post‐dawn and dusk sectors in the northern and southern hemispheres, respectively. The F‐ring field‐aligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the field‐aligned currents in a hemisphere are modulated by that hemisphere's “planetary period oscillation” (PPO) systems. We separate the PPO‐independent and PPO‐related currents in both hemispheres using their opposite symmetry. The average PPO‐independent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPO‐related currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPO‐related currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete field‐aligned currents that act to “turn off” the PPOs.
Work at Imperial College London is funded by the Science and Technology Facilities Council (STFC) consolidated grant ST/N000692/1. M. K. D. is funded by Royal Society Research Professorship RP140004. Work at the University of Leicester is funded by STFC consolidated grant ST/N000749/1. We thank J. F. Carbary for access to the UV auroral boundary data. We thank Steve Kellock and the Cassini Mag team at Imperial College for the processed magnetometer data. Magnetometer data will be available on the NASA Planetary Data System (https://pds.jpl.nasa.gov). The PPO phases are from Provan et al. (2018). G. J. H., G. P., and D. J. S. acknowledge the support of the International Space Science Institute, as this study was discussed at the international team on “Rotational phenomena in Saturn's magnetosphere.”
CitationJournal of Geophysical Research: Space Physics, 2018, 123 (5), pp. 3806-3821 (16)
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
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