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Field-Aligned Currents in Saturn's Nightside Magnetosphere: Subcorotation and Planetary Period Oscillation Components During Northern Spring

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journal contribution
posted on 10.08.2018, 11:17 by T. J. Bradley, S. W. H. Cowley, G. Provan, G. J. Hunt, E. J. Bunce, S. J. Wharton, I. I. Alexeev, E. S. Belenkaya, V. V. Kalegaev, M. K. Dougherty
We newly analyze Cassini magnetic field data from the 2012/2013 Saturn northern spring interval of highly inclined orbits and compare them with similar data from late southern summer in 2008, thus providing unique information on the seasonality of the currents that couple momentum between Saturn’s ionosphere and magnetosphere. Inferred meridional ionospheric currents in both cases consist of a steady component related to plasma subcorotation, together with the rotating current systems of the northern and southern planetary period oscillations (PPOs). Subcorotation currents during the two intervals show opposite north-south polar region asymmetries, with strong equatorward currents flowing in the summer hemispheres but only weak currents flowing to within a few degrees of the open-closed boundary (OCB) in the winter hemispheres, inferred due to weak polar ionospheric conductivities. Currents peak at ~1 MA rad^-1 in both hemispheres just equatorward of the open-closed boundary, associated with total downward polar currents ~6 MA, then fall across the narrow auroral upward current region to small values at subauroral latitudes. PPO-related currents have a similar form in both summer and winter with principal upward and downward field-aligned currents peaking at ~1.25 MA rad^-1 being essentially collocated with the auroral upward current and approximately equal in strength. Though northern and southern PPO currents were approximately equal during both intervals, the currents in both hemispheres were dual modulated by both systems during 2012/2013, with approximately half the main current closing in the opposite ionosphere and half cross field in the magnetosphere, while only the northern hemisphere currents were similarly dual modulated in 2008.

Funding

Work at Leicester was supported by STFC Consolidated grant ST/N000749/1, while work at Imperial College London was supported by STFC Consolidated grant ST/N000692/1. T. J. B. was supported by STFC Quota Studentship ST/N504117/1. Model calculations by the Lomonosov MSU team were partially supported by the Ministry of Education and Science of the Russian Federation grant 14.616.21.0084. We thank S Kellock and the Cassini Mag team at Imperial College for access to processed magnetometer data and J. F. Carbary for access to the UV auroral boundary data shown in Figures 4d and 4e.

History

Citation

Journal of Geophysical Research: Space Physics, 2018, 123 (5), pp. 3602-3636 (35)

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Journal of Geophysical Research: Space Physics

Publisher

American Geophysical Union (AGU), Wiley

issn

2169-9380

eissn

2169-9402

Acceptance date

04/04/2018

Copyright date

2018

Available date

10/08/2018

Publisher version

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2017JA024885

Notes

Calibrated data from the Cassini mission are available from the NASA Planetary Data System at the Jet Propulsion Laboratory (https://pds.jpl.nasa.gov/)

Language

en