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Spontaneous axisymmetry breaking of the external magnetic field at Saturn

Goldreich, Peter and Farmer, Alison J. (2007) Spontaneous axisymmetry breaking of the external magnetic field at Saturn. Journal of Geophysical Research A, 112 (A5). Art. No. A05225. ISSN 0148-0227.

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Saturn’s magnetic field is remarkably axisymmetric. Early evidence for nonaxisymmetry came from the periodicity of Saturn’s kilometric radio bursts (SKR). Subsequently, percent-level variations of the SKR period were found to occur on timescales of years. A recent breakthrough has been the direct detection of a nonaxisymmetric component of the field that rotates with a period close to that of the SKR and whose magnitude varies only weakly with distance from Saturn. The latter implies that it must be supported by currents external to the planet. We explore the hypothesis that centrifugally driven convection spontaneously breaks the axisymmetry of the external magnetic field at Saturn. The density of the outflowing plasma close to its source is assumed to contain a substantial part that varies as cosø and rotates uniformly. We demonstrate that the plasma stream must narrow with distance from the planet, while the field-aligned currents joining stream to ionosphere increase rapidly. These currents produce a nonaxisymmetric component of magnetic field whose magnitude varies inversely with radial distance in the planet’s equatorial plane. For a rate of plasma outflow 10^4 ≾ Ṁ ≾ 10^5g s^(-1), this component’s strength is compatible with that observed. Additionally, we postulate that the SKR is associated with the narrow range of longitudes over which large currents flow along magnetic field lines connecting the tip of the outflow to the auroral ionosphere.

Item Type:Article
Related URLs:
URLURL TypeDescription 10.1029/2006JA012163DOIUNSPECIFIED
Additional Information:© 2007 by the American Geophysical Union. Received 6 November 2006; revised 20 February 2007; accepted 23 February 2007; published 26 May 2007. Wolfgang Baumjohann thanks the reviewers for their assistance in evaluating this paper.
Subject Keywords:Magnetic fields and magnetism; Magnetosphere/ionosphere interactions; Planetary magnetospheres; Plasma convection
Issue or Number:A5
Record Number:CaltechAUTHORS:20130228-102458528
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Official Citation:Goldreich, P., and A. J. Farmer (2007), Spontaneous axisymmetry breaking of the external magnetic field at Saturn, J. Geophys. Res., 112, A05225, doi:10.1029/2006JA012163
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37200
Deposited By: Tony Diaz
Deposited On:28 Feb 2013 20:27
Last Modified:03 Oct 2019 04:45

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