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Saturn’s magnetic field revealed by the Cassini Grand Finale

Dougherty, Michele K. and Cao, Hao and Khurana, Krishan K. and Hunt, Gregory J. and Provan, Gabrielle and Kellock, Stephen and Burton, Marcia E. and Burk, Thomas A. and Bunce, Emma J. and Cowley, Stanley W. H. and Kivelson, Margaret G. and Russell, Christopher T. and Southwood, David J. (2018) Saturn’s magnetic field revealed by the Cassini Grand Finale. Science, 362 (6410). Art. No. eaat5434. ISSN 0036-8075. http://resolver.caltech.edu/CaltechAUTHORS:20181005-093938794

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Abstract

INTRODUCTION: Starting on 26 April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15 September 2017. This phase offered an opportunity to investigate Saturn’s internal magnetic field and the electromagnetic environment between the planet and its rings. The internal magnetic field is a diagnostic of interior structure, dynamics, and evolution of the host planet. Rotating convective motion in the highly electrically conducting layer of the planet is thought to maintain the magnetic field through the magnetohydrodynamic (MHD) dynamo process. Saturn’s internal magnetic field is puzzling because of its high symmetry relative to the spin axis, known since the Pioneer 11 flyby. This symmetry prevents an accurate determination of the rotation rate of Saturn’s deep interior and challenges our understanding of the MHD dynamo process because Cowling’s theorem precludes a perfectly axisymmetric magnetic field being maintained through an active dynamo. RATIONALE: The Cassini fluxgate magnetometer was capable of measuring the magnetic field with a time resolution of 32 vectors per s and up to 44,000 nT, which is about twice the peak field strength encountered during the Grand Finale orbits. The combination of star cameras and gyroscopes onboard Cassini provided the attitude determination required to infer the vector components of the magnetic field. External fields from currents in the magnetosphere were modeled explicitly, orbit by orbit. RESULTS: Saturn’s magnetic equator, where the magnetic field becomes parallel to the spin axis, is shifted northward from the planetary equator by 2808.5 ± 12 km, confirming the north-south asymmetric nature of Saturn’s magnetic field. After removing the systematic variation with distance from the spin axis, the peak-to-peak “longitudinal” variation in Saturn’s magnetic equator position is <18 km, indicating that the magnetic axis is aligned with the spin axis to within 0.01°. Although structureless in the longitudinal direction, Saturn’s internal magnetic field features variations in the latitudinal direction across many different characteristic length-scales. When expressed in spherical harmonic space, internal axisymmetric magnetic moments of at least degree 9 are needed to describe the latitudinal structures. Because there was incomplete latitudinal coverage during the Grand Finale orbits, which can lead to nonuniqueness in the solution, regularized inversion techniques were used to construct an internal Saturn magnetic field model up to spherical harmonic degree 11. This model matches Cassini measurements and retains minimal internal magnetic energy. An azimuthal field component two orders of magnitude smaller than the radial and meridional components is measured on all periapses (closest approaches to Saturn). The steep slope in this component and magnetic mapping to the inner edge of the D-ring suggests an external origin of this component. CONCLUSION: Cassini Grand Finale observations confirm an extreme level of axisymmetry of Saturn’s internal magnetic field. This implies the presence of strong zonal flows (differential rotation) and stable stratification surrounding Saturn’s deep dynamo. The rapid latitudinal variations in the field suggest a second shallow dynamo maintained by the background field from the deep dynamo, small-scale helical motion, and deep zonal flows in the semiconducting region closer to the surface. Some of the high-degree magnetic moments could result from strong high-latitude concentrations of magnetic flux within the planet’s deep dynamo. The periapse azimuthal field originates from a strong interhemispherical electric current system flowing along magnetic field lines between Saturn and the inner edge of the D-ring, with strength comparable to that of the high-latitude field-aligned currents (FACs) associated with Saturn’s aurorae.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1126/science.aat5434DOIArticle
https://www.sciencemag.org/content/362/6410/eaat5434/suppl/DC1PublisherSupporting Information
https://doi.org/10.1126/science.aav6732DOIErratum
ORCID:
AuthorORCID
Dougherty, Michele K.0000-0002-9658-8085
Cao, Hao0000-0002-6917-8363
Khurana, Krishan K.0000-0002-2856-1171
Hunt, Gregory J.0000-0002-9154-7081
Provan, Gabrielle0000-0001-7442-4154
Kellock, Stephen0000-0002-1369-7172
Bunce, Emma J.0000-0002-9456-0345
Cowley, Stanley W. H.0000-0002-4041-0034
Kivelson, Margaret G.0000-0003-3859-8581
Russell, Christopher T.0000-0003-1639-8298
Southwood, David J.0000-0001-6483-9241
Additional Information:© 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works http://www.sciencemag.org/about/science-licenses-journal-article-reuse. This is an article distributed under the terms of the Science Journals Default License. Received for publication March 9, 2018. Accepted for publication September 5, 2018. All authors acknowledge support from the Cassini Project. H.C. acknowledges Royal Society Grant RP\EA\180014 to enable an academic visit to Imperial College London, during which some of the work has been carried out. Work at Imperial College London was funded by Science and Technology Facilities Council (STFC) consolidated grant ST/N000692/1. M.K.D. is funded by Royal Society Research Professorship RP140004. H.C. is funded by NASA’s CDAPS program NNX15AL11G and NASA Jet Propulsion Laboratory (JPL) contract 1579625. Work at the University of Leicester was supported by STFC grant ST/N000749/1. E.J.B. is supported by a Royal Society Wolfson Research Merit Award. Work at the University of California, Los Angeles is funded by NASA JPL contract 1409809. K.K.K. is funded by NASA JPL contract 1409806:033. M.G.K. is funded by JPL under contract 1416974 at the University of Michigan. M.E.B. and T.A.B. are supported by the Cassini Project. Author contributions: M.K.D. led the instrument team and supervised the data analysis. H.C., K.K.K., and S.K. carried out the magnetometer calibration analysis. T.A.B. carried out the spacecraft attitude reconstruction. H.C., G.J.H., and G.P. carried out the magnetic field data analysis. M.E.B., E.J.B., S.W.H.C., M.G.K., C.T.R., and D.J.S. provided theoretical support and advised on the data analysis. All authors contributed to the writing of the manuscript. Data and materials availability: The derived model parameters are given in Table 1 and tables S1 and S2. Fully calibrated Cassini magnetometer data are released on a schedule agreed with NASA via the Planetary Data System at https://pds-ppi.igpp.ucla.edu/mission/Cassini-Huygens/CO/MAG; we used data from rev 271 through to rev 280, excluding rev 277. The authors declare no competing interests.
Errata:In the Research Article “Saturn’s magnetic field revealed by the Cassini Grand Finale,” a copyediting error led to the inadvertent omission of the sign of the value for Gauss coefficient g_7^0 in Table 1 online. The correct value is –59.6. The online version has been corrected. In addition, the authors corrected three supplementary figure callouts on p. 6 and an in-text citation on p. 7.
Funders:
Funding AgencyGrant Number
Royal SocietyRP\EA\180014
Science and Technology Facilities Council (STFC)ST/N000692/1
Royal SocietyRP140004
NASANNX15AL11G
JPL1579625
Science and Technology Facilities Council (STFC)ST/N000749/1
JPL1409809
JPL1409806:033
JPL1416974
Record Number:CaltechAUTHORS:20181005-093938794
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20181005-093938794
Official Citation:Saturn’s magnetic field revealed by the Cassini Grand Finale BY MICHELE K. DOUGHERTY, HAO CAO, KRISHAN K. KHURANA, GREGORY J. HUNT, GABRIELLE PROVAN, STEPHEN KELLOCK, MARCIA E. BURTON, THOMAS A. BURK, EMMA J. BUNCE, STANLEY W. H. COWLEY, MARGARET G. KIVELSON, CHRISTOPHER T. RUSSELL, DAVID J. SOUTHWOOD, Science 362, eaat5434 (2018). DOI: 10.1126/science.aat5434
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90135
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:05 Oct 2018 21:37
Last Modified:23 Oct 2018 22:20

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