Magnetospheric Physics
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Spontaneous axisymmetry breaking of the external magnetic field at Saturn
Abstract
[1] 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 cos
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 104 ≲ 
≲ 105g 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.
Citing Literature
Number of times cited: 53
- C.G.A. Smith, Interaction of Saturn’s dual rotation periods, Icarus, 10.1016/j.icarus.2017.11.016, 302, (330-342), (2018).
- M. F. Thomsen, C. M. Jackman, S. W. H. Cowley, X. Jia, M. G. Kivelson and G. Provan, Evidence for periodic variations in the thickness of Saturn's nightside plasma sheet, Journal of Geophysical Research: Space Physics, 122, 1, (280-292), (2017).
- J. F. Carbary and G. Provan, Saturn's magnetic field periodicities at high latitudes and the effects of spacecraft motion and position, Journal of Geophysical Research: Space Physics, 122, 2, (1979-1989), (2017).
- James F. Carbary, Update on Saturn's energetic electron periodicities, Journal of Geophysical Research: Space Physics, 122, 1, (156-165), (2017).
- Emil Khalisi, Dust clouds and plasmoids in Saturn’s magnetosphere as seen with four Cassini instruments, Advances in Space Research, 59, 6, (1614), (2017).
- J. F. Carbary, A new spiral model for Saturn's magnetosphere, Geophysical Research Letters, 43, 2, (501-507), (2016).
- J. Goldstein, J. H. Waite, J. L. Burch and R. Livi, Evidence of m = 1 density mode (plasma cam) in Saturn's rotating magnetosphere, Journal of Geophysical Research: Space Physics, 121, 3, (2335-2348), (2016).
- Ulyana A. Dyudina, Andrew P. Ingersoll, Shawn P. Ewald and Danika Wellington, Saturn’s aurora observed by the Cassini camera at visible wavelengths, Icarus, 10.1016/j.icarus.2015.05.022, 263, (32-43), (2016).
- Henrik Melin, S.V. Badman, T.S. Stallard, S.W.H. Cowley, U. Dyudina, J.D. Nichols, G. Provan, J. O’Donoghue, W.R. Pryor, K.H. Baines, S. Miller, J. Gustin, A. Radioti, C. Tao, C.J. Meredith, J.S.D. Blake and R.E. Johnson, Simultaneous multi-scale and multi-instrument observations of Saturn’s aurorae during the 2013 observing campaign, Icarus, 263, (56), (2016).
- Xianzhe Jia, Margaret G. Kivelson and Tamas I. Gombosi, Global MHD Modeling of the Coupled Magnetosphere‐Ionosphere System at Saturn, Magnetosphere‐Ionosphere Coupling in the Solar System, (319-334), (2016).
- Nicholas Achilleos, Nicolas André, Xochitl Blanco-Cano, Pontus C. Brandt, Peter A. Delamere and Robert Winglee, Transport of Mass, Momentum and Energy in Planetary Magnetodisc Regions, The Magnetodiscs and Aurorae of Giant Planets, 10.1007/978-1-4939-3395-2_7, (229-299), (2015).
- James L. Burch, Magnetosphere‐Ionosphere Coupling, Past to Future, Magnetosphere‐Ionosphere Coupling in the Solar System, (1-17), (2016).
- Sven Simon, Elias Roussos and Carol S. Paty, The interaction between Saturn’s moons and their plasma environments, Physics Reports, 10.1016/j.physrep.2015.09.005, 602, (1-65), (2015).
- M. Blanc, D. J. Andrews, A. J. Coates, D. C. Hamilton, C. M. Jackman, X. Jia, A. Kotova, M. Morooka, H. T. Smith and J. H. Westlake, Saturn Plasma Sources and Associated Transport Processes, Space Science Reviews, 10.1007/s11214-015-0172-9, 192, 1-4, (237-283), (2015).
- D. J. Southwood, Formation of Magnetotails, Magnetotails in the Solar System, (197-215), (2015).
- Nicholas Achilleos, Nicolas André, Xochitl Blanco-Cano, Pontus C. Brandt, Peter A. Delamere and Robert Winglee, 1. Transport of Mass, Momentum and Energy in Planetary Magnetodisc Regions, Space Science Reviews, 10.1007/s11214-014-0086-y, 187, 1-4, (229-299), (2014).
- G. Fischer, D.A. Gurnett, W.S. Kurth, S.-Y. Ye and J.B. Groene, Saturn kilometric radiation periodicity after equinox, Icarus, 10.1016/j.icarus.2015.03.014, 254, (72-91), (2015).
- J. Goldstein, T. W. Hill, J. H. Waite and J. L. Burch, Analytical model of rotating two‐cell convection at Saturn, Journal of Geophysical Research: Space Physics, 119, 3, (1980-1993), (2014).
- C. G. A. Smith, On the nature and location of the proposed twin vortex systems in Saturn's polar upper atmosphere, Journal of Geophysical Research: Space Physics, 119, 7, (5964-5977), (2014).
- D. J. Southwood and S. W. H. Cowley, The origin of Saturn's magnetic periodicities: Northern and southern current systems, Journal of Geophysical Research: Space Physics, 119, 3, (1563-1571), (2014).
- M. Andriopoulou, E. Roussos, N. Krupp, C. Paranicas, M. Thomsen, S. Krimigis, M.K. Dougherty and K.-H. Glassmeier, Spatial and temporal dependence of the convective electric field in Saturn’s inner magnetosphere, Icarus, 229, (57), (2014).
- Junjun Liu, Tapio Schneider and Leigh N. Fletcher, Constraining the depth of Saturn’s zonal winds by measuring thermal and gravitational signals, Icarus, 239, (260), (2014).
- M. F. Thomsen, Saturn's magnetospheric dynamics, Geophysical Research Letters, 40, 20, (5337-5344), (2013).
- G. Provan, S. W. H. Cowley, J. Sandhu, D. J. Andrews and M. K. Dougherty, Planetary period magnetic field oscillations in Saturn's magnetosphere: Postequinox abrupt nonmonotonic transitions to northern system dominance, Journal of Geophysical Research: Space Physics, 118, 6, (3243-3264), (2013).
- J. F. Carbary and D. G. Mitchell, Periodicities in Saturn's magnetosphere, Reviews of Geophysics, 51, 1, (1-30), (2013).
- J. Olson and N. Brenning, The magnetospheric clock of Saturn—A self-organized plasma dynamo, Physics of Plasmas, 20, 8, (082901), (2013).
- Xianzhe Jia, Margaret G. Kivelson and Tamas I. Gombosi, Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere, Journal of Geophysical Research: Space Physics, 117, A4, (2012).
- M. F. Thomsen, E. Roussos, M. Andriopoulou, P. Kollmann, C. S. Arridge, C. P. Paranicas, D. A. Gurnett, R. L. Powell, R. L. Tokar and D. T. Young, Saturn's inner magnetospheric convection pattern: Further evidence, Journal of Geophysical Research: Space Physics, 117, A9, (2012).
- Xianzhe Jia and Margaret G. Kivelson, Driving Saturn's magnetospheric periodicities from the upper atmosphere/ionosphere: Magnetotail response to dual sources, Journal of Geophysical Research: Space Physics, 117, A11, (2012).
- J. Olson and N. Brenning, Dust-driven and plasma-driven currents in the inner magnetosphere of Saturn, Physics of Plasmas, 19, 4, (042903), (2012).
- C. G. A. Smith and N. Achilleos, Axial symmetry breaking of Saturn’s thermosphere, Monthly Notices of the Royal Astronomical Society, 422, 2, (1460-1488), (2012).
- J. D. Nichols, S. W. H. Cowley and L. Lamy, Dawn‐dusk oscillation of Saturn's conjugate auroral ovals, Geophysical Research Letters, 37, 24, (2010).
- D. A. Gurnett, J. B. Groene, A. M. Persoon, J. D. Menietti, S.‐Y. Ye, W. S. Kurth, R. J. MacDowall and A. Lecacheux, The reversal of the rotational modulation rates of the north and south components of Saturn kilometric radiation near equinox, Geophysical Research Letters, 37, 24, (2010).
- D. J. Andrews, S. W. H. Cowley, M. K. Dougherty and G. Provan, Magnetic field oscillations near the planetary period in Saturn's equatorial magnetosphere: Variation of amplitude and phase with radial distance and local time, Journal of Geophysical Research: Space Physics, 115, A4, (2010).
- S.‐Y. Ye, D. A. Gurnett, J. B. Groene, Z. Wang and W. S. Kurth, Dual periodicities in the rotational modulation of Saturn narrowband emissions, Journal of Geophysical Research: Space Physics, 115, A12, (2010).
- D. J. Andrews, A. J. Coates, S. W. H. Cowley, M. K. Dougherty, L. Lamy, G. Provan and P. Zarka, Magnetospheric period oscillations at Saturn: Comparison of equatorial and high‐latitude magnetic field periods with north and south Saturn kilometric radiation periods, Journal of Geophysical Research: Space Physics, 115, A12, (2010).
- A. D. DeJong, J. L. Burch, J. Goldstein, A. J. Coates and D. T. Young, Low‐energy electrons in Saturn's inner magnetosphere and their role in interchange injections, Journal of Geophysical Research: Space Physics, 115, A10, (2010).
- Tomoki Kimura, Fuminori Tsuchiya, Hiroaki Misawa, Akira Morioka and Hiromasa Nozawa, Occurrence statistics and ray tracing study of Jovian quasiperiodic radio bursts observed from low latitudes, Journal of Geophysical Research: Space Physics, 115, A5, (2010).
- W.-L. Tseng, W.-H. Ip, R.E. Johnson, T.A. Cassidy and M.K. Elrod, The structure and time variability of the ring atmosphere and ionosphere, Icarus, 206, 2, (382), (2010).
- J. L. Burch, A. D. DeJong, J. Goldstein and D. T. Young, Periodicity in Saturn's magnetosphere: Plasma cam, Geophysical Research Letters, 36, 14, (2009).
- G. Provan, D. J. Andrews, C. S. Arridge, A. J. Coates, S. W. H. Cowley, S. E. Milan, M. K. Dougherty and D. M. Wright, Polarization and phase of planetary‐period magnetic field oscillations on high‐latitude field lines in Saturn's magnetosphere, Journal of Geophysical Research: Space Physics, 114, A2, (2009).
- Krishan K. Khurana, Donald G. Mitchell, Christopher S. Arridge, Michele K. Dougherty, Christopher T. Russell, Chris Paranicas, Norbert Krupp and Andrew J. Coates, Sources of rotational signals in Saturn's magnetosphere, Journal of Geophysical Research: Space Physics, 114, A2, (2009).
- G. Provan, S. W. H. Cowley and J. D. Nichols, Phase relation of oscillations near the planetary period of Saturn's auroral oval and the equatorial magnetospheric magnetic field, Journal of Geophysical Research: Space Physics, 114, A4, (2009).
- David J. Southwood and Margaret G. Kivelson, The source of Saturn's periodic radio emission, Journal of Geophysical Research: Space Physics, 114, A9, (2009).
- P. L. Read, T. E. Dowling and G. Schubert, Saturn’s rotation period from its atmospheric planetary-wave configuration, Nature, 460, 7255, (608), (2009).
- D.G. Mitchell, S.M. Krimigis, C. Paranicas, P.C. Brandt, J.F. Carbary, E.C. Roelof, W.S. Kurth, D.A. Gurnett, J.T. Clarke, J.D. Nichols, J.-C. Gérard, D.C. Grodent, M.K. Dougherty and W.R. Pryor, Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions, Planetary and Space Science, 57, 14-15, (1732), (2009).
- J. F. Carbary, D. G. Mitchell, P. Brandt, E. C. Roelof and S. M. Krimigis, Periodic tilting of Saturn's plasma sheet, Geophysical Research Letters, 35, 24, (2008).
- D. J. Andrews, E. J. Bunce, S. W. H. Cowley, M. K. Dougherty, G. Provan and D. J. Southwood, Planetary period oscillations in Saturn's magnetosphere: Phase relation of equatorial magnetic field oscillations and Saturn kilometric radiation modulation, Journal of Geophysical Research: Space Physics, 113, A9, (2008).
- J. D. Nichols, J. T. Clarke, S. W. H. Cowley, J. Duval, A. J. Farmer, J.‐C. Gérard, D. Grodent and S. Wannawichian, Oscillation of Saturn's southern auroral oval, Journal of Geophysical Research: Space Physics, 113, A11, (2008).
- N. André, M. Blanc, S. Maurice, P. Schippers, E. Pallier, T. I. Gombosi, K. C. Hansen, D. T. Young, F. J. Crary, S. Bolton, E. C. Sittler, H. T. Smith, R. E. Johnson, R. A. Baragiola, A. J. Coates, A. M. Rymer, M. K. Dougherty, N. Achilleos, C. S. Arridge, S. M. Krimigis, D. G. Mitchell, N. Krupp, D. C. Hamilton, I. Dandouras, D. A. Gurnett, W. S. Kurth, P. Louarn, R. Srama, S. Kempf, H. J. Waite, L. W. Esposito and J. T. Clarke, Identification of Saturn's magnetospheric regions and associated plasma processes: Synopsis of Cassini observations during orbit insertion, Reviews of Geophysics, 46, 4, (2008).
- Vytenis M. Vasyliūnas and Duane H. Pontius, Rotationally driven interchange instability: Reply to André and Ferrière, Journal of Geophysical Research: Space Physics, 112, A10, (2007).
- D. J. Southwood and M. G. Kivelson, Saturnian magnetospheric dynamics: Elucidation of a camshaft model, Journal of Geophysical Research: Space Physics, 112, A12, (2007).
- J. D. Anderson and G. Schubert, Saturn's Gravitational Field, Internal Rotation, and Interior Structure, Science, 317, 5843, (1384), (2007).
