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Saturn's Nightside Ring Current During Cassini's Grand Finale

Provan, G. and Bradley, T. J. and Bunce, E. J. and Cowley, S. W. H. and Cao, H. and Dougherty, M. and Hunt, G. J. and Roussos, E. and Staniland, N. R. and Tao, C. (2021) Saturn's Nightside Ring Current During Cassini's Grand Finale. Journal of Geophysical Research. Space Physics, 126 (3). Art. No. e2020JA028605. ISSN 2169-9380.

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During Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.

Item Type:Article
Related URLs:
URLURL TypeDescription
https://pds.jpl.nasa.govRelated ItemNASA Planetary Data System
Provan, G.0000-0001-7442-4154
Bradley, T. J.0000-0001-9277-6751
Bunce, E. J.0000-0002-9456-0345
Cowley, S. W. H.0000-0002-4041-0034
Cao, H.0000-0002-6917-8363
Dougherty, M.0000-0002-9658-8085
Hunt, G. J.0000-0002-9154-7081
Roussos, E.0000-0002-5699-0678
Staniland, N. R.0000-0003-2510-8846
Tao, C.0000-0001-8817-0589
Additional Information:© 2021. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Issue Online: 08 March 2021; Version of Record online: 08 March 2021; Accepted manuscript online: 02 February 2021; Manuscript accepted: 05 January 2021; Manuscript revised: 30 December 2020; Manuscript received: 19 August 2020. Work at the University of 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. EJB was supported by a Royal Society Wolfson Research Merit Award. M.K.D. was supported by a Royal Society Research Professorship. T.J.B. was supported by STFC Quota Studentship ST/N504117/1. The authors thank S. Kellock and the Cassini magnetometer team at Imperial College for access to processed magnetic field data. Data Availability Statement: Calibrated magnetic field data from the Cassini mission are available from the NASA Planetary Data System at the Jet Propulsion Laboratory (
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)ST/N000749/1
Science and Technology Facilities Council (STFC)ST/N000692/1
Science and Technology Facilities Council (STFC)ST/N504117/1
Subject Keywords:Cassini; ring current; Saturn
Issue or Number:3
Record Number:CaltechAUTHORS:20210203-090928596
Persistent URL:
Official Citation:Provan, G., Bradley, T. J., Bunce, E. J., Cowley, S. W. H., Cao, H., Dougherty, M., et al. (2021). Saturn's nightside ring current during Cassini's Grand Finale. Journal of Geophysical Research: Space Physics, 126, e2020JA028605.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107888
Deposited By: Tony Diaz
Deposited On:03 Feb 2021 18:04
Last Modified:10 Mar 2021 19:13

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