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The landscape of Saturn’s internal magnetic field from the Cassini Grand Finale

Cao, Hao and Dougherty, Michele K. and Hunt, Gregory J. and Provan, Gabrielle and Cowley, Stanley W. H. and Bunce, Emma J. and Kellock, Stephen and Stevenson, David J. (2020) The landscape of Saturn’s internal magnetic field from the Cassini Grand Finale. Icarus, 344 . Art. No. 113541. ISSN 0019-1035. https://resolver.caltech.edu/CaltechAUTHORS:20191111-071031304

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Abstract

The Cassini mission entered the Grand Finale phase in April 2017 and executed 22.5 highly inclined, close-in orbits around Saturn before diving into the planet on September 15th 2017. Here we present our analysis of the Cassini Grand Finale magnetometer (MAG) dataset, focusing on Saturn’s internal magnetic field. These measurements demonstrate that Saturn’s internal magnetic field is exceptionally axisymmetric, with a dipole tilt less than 0.007 degrees (25.2 arcsecs). Saturn’s magnetic equator was directly measured to be shifted northward by ∼0.0468 ± 0.00043 (1σ) R_S, 2820 ± 26 km, at cylindrical radial distances between 1.034 and 1.069 R_S from the spin-axis. Although almost perfectly axisymmetric, Saturn’s internal magnetic field exhibits features on many characteristic length scales in the latitudinal direction. Examining B_r at the a = 0.75 R_S, c = 0.6993 R_S isobaric surface, the degree 4 to 11 contributions correspond to latitudinally banded magnetic perturbations with characteristic width ∼15∘, similar to that of the off-equatorial zonal jets observed in the atmosphere of Saturn. Saturn’s internal magnetic field beyond 60∘, in particular the small-scale features, are less well constrained by the available measurements, mainly due to incomplete spatial coverage in the polar region. Magnetic fields associated with the ionospheric Hall currents were estimated and found to contribute less than 2.5 nT to Gauss coefficients beyond degree 3. The magneto-disk field features orbit-to-orbit variations between 12 nT and 15.4 nT along the close-in part of Grand Finale orbits, offering an opportunity to measure the electromagnetic induction response from the interior of Saturn. A stably stratified layer thicker than 2500 km likely exists above Saturn’s deep dynamo to filter out the non-axisymmetric internal magnetic field. A heat transport mechanism other than pure conduction, e.g. double diffusive convection, must be operating within this layer to be compatible with Saturn’s observed luminosity. The latitudinally banded magnetic perturbations likely arise from a shallow secondary dynamo action with latitudinally banded differential rotation in the semi-conducting layer.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1016/j.icarus.2019.113541DOIArticle
ORCID:
AuthorORCID
Cao, Hao0000-0002-6917-8363
Dougherty, Michele K.0000-0002-9658-8085
Hunt, Gregory J.0000-0002-9154-7081
Provan, Gabrielle0000-0001-7442-4154
Cowley, Stanley W. H.0000-0002-4041-0034
Bunce, Emma J.0000-0002-9456-0345
Kellock, Stephen0000-0002-1369-7172
Stevenson, David J.0000-0001-9432-7159
Additional Information:© 2019 Published by Elsevier Inc. Received 19 July 2019, Revised 31 October 2019, Accepted 7 November 2019, Available online 11 November 2019. We acknowledge support from the Cassini Project. Work at Imperial College London was funded by Science and Technology Facilities Council (STFC), UK consolidated grant ST/N000692/1. Work at the University of Leicester was funded by STFC, UK consolidated grant ST/N000749/1. M.K.D. is funded by Royal Society, UK Research Professorship RP140004. H.C. is funded by NASA Jet Propulsion Laboratory (JPL), USA contract 1579625. H.C.’s visit to Imperial College London was funded by the Royal Society, UK grant RP 180014. E.J.B. was supported by a Royal Society Wolfson Research Merit Award. The derived model parameters are given in Table 3, Table 4, Table 5, Table 6 and Supplementary Table 1. We thank Burkhard Militzer for providing the interior shape of Saturn and helpful discussions. Fully calibrated Cassini magnetometer data are available at the NASA Planetary Data System at https://pds.nasa.gov.
Group:Astronomy Department
Funders:
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)ST/N000692/1
Science and Technology Facilities Council (STFC)ST/N000749/1
Royal SocietyRP140004
JPL1579625
Royal SocietyRP180014
Subject Keywords:Saturn; Magnetic fields; Geophysics; Saturn; Interior
Record Number:CaltechAUTHORS:20191111-071031304
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20191111-071031304
Official Citation:Hao Cao, Michele K. Dougherty, Gregory J. Hunt, Gabrielle Provan, Stanley W.H. Cowley, Emma J. Bunce, Stephen Kellock, David J. Stevenson, The landscape of Saturn’s internal magnetic field from the Cassini Grand Finale, Icarus, Volume 344, 2020, 113541, ISSN 0019-1035, https://doi.org/10.1016/j.icarus.2019.113541. (http://www.sciencedirect.com/science/article/pii/S0019103519305299)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99759
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 Nov 2019 22:19
Last Modified:26 Jun 2020 16:52

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