Skip to main content Accessibility help

We use cookies to distinguish you from other users and to provide you with a better experience on our websites. Close this message to accept cookies or find out how to manage your cookie settings.

Close cookie message
×
Hostname: page-component-76fb5796d-vvkck Total loading time: 0 Render date: 2024-04-28T01:18:15.476Z Has data issue: false hasContentIssue false

4 - Saturn’s Magnetic Field and Dynamo

Published online by Cambridge University Press:  13 December 2018

By
Ulrich R. Christensen ,
Hao Cao ,
Michele K. Dougherty  and
Krishan Khurana
Edited by
Kevin H. Baines ,
F. Michael Flasar ,
Norbert Krupp  and
Tom Stallard
Kevin H. Baines
Affiliation:
University of Wisconsin, Madison
F. Michael Flasar
Affiliation:
NASA-Goddard Space Flight Center
Norbert Krupp
Affiliation:
Max-Planck-Institut für Sonnensystemforschung, Göttingen
Tom Stallard
Affiliation:
University of Leicester
Get access

Summary

The magnetometer measurements taken by Cassini have confirmed the unusual character of Saturn’s internal magnetic field known from previous flybys and have revealed additional properties that suggest a rather unique dynamo in this planet. Within measurement uncertainty, the internal magnetic field is completely symmetric with respect to Saturn’s spin axis. The upper limit on the tilt of the magnetic dipole could be reduced from 1 to 0.06 degree. Moreover, only axisymmetric quadrupole and octupole moments are needed to fit the data. The lack of non-axisymmetric field components prevents a reliable determination of the bulk rotation rate of Saturn’s deep interior. Using data from Cassini’s closest approach to Saturn during orbit insertion, the magnetic moments of degrees four and five have been determined. The spatial power spectrum shows a zig-zag pattern with high power in odd spherical harmonic degrees and low power in even degrees. Compared to a simple dipole field, this corresponds to a concentration of magnetic flux towards the rotation poles. The flux concentration becomes progressively more pronounced when the field is continued into the interior. Comparison of the Cassini field model with that based on the Pioneer 11 and Voyager 1 and 2 measurements taken roughly 30 years earlier suggests that the secular variation of Saturn’s field is at least one order of magnitude slower than that of the Earth. A viable explanation for most of the unusual field properties is that a stably stratified and electrically conducting layer, formed by a partial demixing of helium from metallic hydrogen, exists on top of a “standard” dynamo in Saturn’s deep interior. This dynamo, driven by thermal and compositional convection, generates a magnetic field that is moderately asymmetric and time dependent. Rapid time variations and non-axisymmetric field components are filtered out in the stable layer by a skin effect. This model also implies that the top of the active dynamo may be located rather deep in Saturn’s interior and the geometric drop-off of the dipole strength with the radius cubed could explain the unexpectedly low field strength at Saturn’s surface. The stable layer model does not provide an explanation for the magnetic flux concentration towards the poles. Strong differential rotation in the dynamo region can have this effect, but a physical mechanism for such a flow state remains to be explored. From magnetic measurements to be taken during the very close approaches in the Grand Finale of the Cassini mission, we can expect to characterize Saturn’s magnetic field up to at least spherical harmonic degree nine and possibly to detect weak non-axisymmetric field components, which would enable an accurate determination of Saturn’s rotation period.

Type
Chapter
Information
Saturn in the 21st Century , pp. 69 - 96
Publisher: Cambridge University Press
Print publication year: 2018

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Achilleos, N., Guio, P. and Arridge, C. S. (2010), A model of force balance in Saturn’s magnetodisc. Mon. Not. R. astr. Soc., 401, 2349–2371.CrossRefGoogle Scholar
Anderson, J. D. and Schubert, G. (2007), Saturn’s gravitational field, internal rotation, and interior structure. Science, 317, 1384–1387.CrossRefGoogle ScholarPubMed
Andrews, D. J., Bunce, E. J., Cowley, S. W. H. et al. (2008), Planetary period oscillations in Saturns magnetosphere: Phase relation of equatorial magnetic field oscillations and Saturn kilometric radiation modulation. J. Geophys. Res., 113, A09205.Google Scholar
Andrews, D. J., Coates, A. J., Cowley, S. W. H. et al. (2010), Magnetospheric period oscillations at Saturn: Comparison of equatorial and high-latitude magnetic field periods with north and south Saturn kilometric radiation periods. J. Geophys. Res., 115, A12252.Google Scholar
Arridge, C. S., André, N., Khurana, K. K. et al. (2011), Periodic motion of Saturns night-side plasma sheet. J. Geophys. Res., 116, A11205.Google Scholar
Aubert, J., Labrosse, S. and Poitou, C. (2009), Modelling the paleo-evolution of the geodynamo. Geophys. J. Int., 179, 1414–1429.CrossRefGoogle Scholar
Aurnou, J. M., Heimpel, M., Allen, L., King, E. and Wicht, J. (2008), Convective heat transfer and the pattern of thermal emission on the gas planets. Geophys. J. Int., 193, 793–801.Google Scholar
Bunce, E. J., Cowley, S. W. H., Alexeev, I. I. et al. (2007), Cassini observations of the variation of Saturn’s ring current parameters with system size. J. Geophys. Res., 112, A10202.CrossRefGoogle Scholar
Burton, M. E., Dougherty, M. K. and Russell, C. T. (2009), Model of Saturn’s internal planetary magnetic field based on Cassini observations. Planet. Space Sci., 57, 1706–1713.Google Scholar
Burton, M. E., Dougherty, M. K. and Russell, C. T. (2010), Saturn’s internal planetary magnetic field. Geophys. Res. Lett., 37, 24105.Google Scholar
Cao, H., Russell, C. T., Christensen, U. R., Dougherty, M. K. and Burton, M. E. (2011), Saturn’s very axisymmetric magnetic field: no secular variation or tilt. Earth Planet. Sci. Lett., 304, 22–28.Google Scholar
Cao, H., Russell, C. T., Wicht, J., Christensen, U. R. and Dougherty, M. K. (2012), Saturn’s high degree magnetic moments: Evidence for a unique planetary dynamo. Icarus, 221, 388–394.CrossRefGoogle Scholar
Carbary, J. F., Achilleos, N. and Arridge, C. S. (2012), Statistical ring current of Saturn. J. Geophys. Res., 117, 6223.Google Scholar
Christensen, U. R. (2010), Dynamo scaling laws: applications to the planets. Space Sci. Rev., 152, 565–590.CrossRefGoogle Scholar
Christensen, U. R. (2011), Geodynamo models: tools for understanding properties of Earth’s magnetic field. Phys. Earth Planet. Inter., 187, 157–169.Google Scholar
Christensen, U. R. and Aubert, J. (2006), Scaling properties of convection-driven dynamos in rotating spherical shells and applications to planetary magnetic fields. Geophys. J. Int., 166, 97–114.Google Scholar
Christensen, U. R., Holzwarth, V. and Reiners, A. (2009), Energy flux determines magnetic field strength of planets and stars. Nature, 457, 167–169.CrossRefGoogle ScholarPubMed
Christensen, U. R. and Tilgner, A. (2004), Power requirement of the geodynamo from ohmic losses in numerical and laboratory dynamos. Nature, 429, 169–171.Google Scholar
Christensen, U. R. and Wicht, J. (2008), Models of magnetic field generation in partly stable planetary cores: Applications to Mercury and Saturn. Icarus, 196, 16–34.Google Scholar
Christensen, U. R. and Wicht, J. (2015), Numerical dynamo simulations. Pages 245–277 of: Schubert, G. (ed.), Treatise on Geophysics, 2nd ed., Volume 8: Core Dynamics. Amsterdam: Elsevier.Google Scholar
Connerney, J. E. P., Acûna, M. H. and Ness, N. F. (1981), Modeling the Jovian current sheet and inner magnetosphere. J. Geophys. Res., 86, 8370–8384.CrossRefGoogle Scholar
Connerney, J. E. P., Acûna, M. H. and Ness, N. F. (1983), Currents in Saturn’s magnetosphere. J. Geophys. Res., 88, 8779–8789.Google Scholar
Connerney, J. E. P., Acûna, M. H. and Ness, N. F. (1984), The Z3 model of Saturn’s magnetic field and the Pioneer 11 vector helium magnetometer observations. J. Geophys. Res., 89, 7541–7544.Google Scholar
Connerney, J. E. P., Ness, N. F. and Acûna, M. H. (1982), Zonal harmonic models of Saturn’s magnetic field from Voyager 1 and 2 observations. Nature, 298, 44–46.Google Scholar
Conrath, B. and Gautier, D. (2000), Saturn helium abundance: a reanalysis of Voyager measurements. Icarus, 144, 124–134.Google Scholar
Davidson, P. A. (2013), Scaling laws for planetary dynamos. Geophys. J. Int., 195, 67–74.Google Scholar
Davis, L. and Smith, E. J. (1985), Comments on “The Z3 model of Saturn’s magnetic field and the Pioneer 11 vector helium magnetometer observations” by Connerney, J. E. P., Acuña, M. H. and Ness, N. F.. J. Geophys. Res., 90, 4461–4464.Google Scholar
Davis, L. and Smith, E. J. (1986), New models of Saturn’s magnetic field using Pioneer 11 Vector Helium Magnetometer data. J.Geophys. Res., 91, 1373–1380.Google Scholar
DavisJr., L. and Smith, E. J. (1990), A model of Saturn’s magnetic field based on all available data. J. Geophys. Res., 95, 15257–15261.Google Scholar
Dharmaraj, G. and Stanley, S. (2012), Effect of inner core conductivity on planetary dynamo models. Phys. Earth Planet.Inter., 212 –213, 1–9.Google Scholar
Dougherty, M. K., Achilleos, N., André, N. et al. (2005), Cassini magnetometer observations during Saturn orbit insertion. Science, 307, 1266–1270.Google Scholar
Dougherty, M. K., Kellock, S., Southwood, D. J. et al. (2004), The Cassini magnetic field investigation. Space Sci. Rev., 114, 331–383.Google Scholar
Dougherty, M. K., Khurana, K. K., Neubauer, F. M. et al. (2006), Identification of a dynamic atmosphere at Enceladus with the Cassini magnetometer. Science, 311, 1406–1409.Google Scholar
Duarte, L., Gastine, T. and Wicht, J. (2013), Anelastic dynamo models with variable electrical conductivity: an application to gas giants. Phys. Earth Planet. Inter., 222, 22–34.Google Scholar
Elphic, R. C. and Russell, C. T. (1978), On the apparent source depth of planetary magnetic fields. Geophys. Res. Lett., 5, 211–214.Google Scholar
Fischer, G., Gurnett, D. A., Kurth, W. S., Ye, S.-Y. and Groene, J. B. (2015), Saturn kilometric radiation periodicity after equinox. Icarus, 254, 72–91.Google Scholar
Fortney, J. J. and Hubbard, W. B. (2003), Phase separation in giant planets: inhomogeneous evolution of Saturn. Icarus, 164, 228–243.Google Scholar
French, M., Becker, A., Lorenzen, W. et al. (2012), Ab initio simulations for material properties along the Jupiter adiabat. Astrophys. J. Suppl., 202, 5.Google Scholar
Gastine, T., Duarte, L. and Wicht, J. (2012), Dipolar versus multipolar dynamos: the influence of the background density stratification. Astronom. Astrophys., 546, A19.CrossRefGoogle Scholar
Gastine, T. and Wicht, J. (2012), Effects of compressibility on driving zonal flow in gas giants. Icarus, 219, 428–442.Google Scholar
Gastine, T., Wicht, J., Duarte, L. D. V., Heimpel, M. and Becker, A. (2014), Explaining Jupiter’s magnetic field and equatorial jet dynamics. Geophys. Res. Lett., 41, 5410–5419.Google Scholar
Giampieri, G., Dougherty, M. K., Smith, E. J. and Russell, C. T. (2006), A regular period for Saturn’s magnetic field that may track its internal rotation. Nature, 441, 62–64.Google Scholar
Glatzmaier, G. A. (1984), Numerical Simulation of Stellar Convective Dynamos. 1. The Model and Methods. J. Comput. Phys., 55, 461–484.Google Scholar
Glatzmaier, G. A. (2008), A note on constraints on deep-seated zonal winds inside Jupiter and Saturn. Icarus, 196, 665–666.Google Scholar
Goldstein, J., Hill, T. W., Waite, H. and Burch, J. L. (2014), Analytical model of rotating two-cell convection at Saturn. J. Geophys. Res., 119, 1980–1993.Google Scholar
Gubbins, D. and Kelly, P. (1993), Persistent patterns in the geomagnetic field over the past 2.5 Ma. Nature, 365, 829–832.Google Scholar
Gurnett, D. A., Persoon, A. M., Kurth, W. S. et al. (2007), The variable rotation period of the inner region of Saturn’s plasma disk. Science, 316, 442–445.Google Scholar
Hanel, R. A., Conrath, B. J., Kunde, V. G., Pearl, J. C. and Pirraglia, J. A. (1983), Albedo, internal heat flux, and energy balance of Saturn. Icarus, 53, 262–285.CrossRefGoogle Scholar
Heimpel, M. and Gómez-Pérez, N. (2011), On the relationship between zonal jets and dynamo action in giant planets. Geophys. Res. Lett., 38, L1401.Google Scholar
Heimpel, M. H., Aurnou, J. M. and Wicht, J. (2005), Simulation of equatorial and high-latitude jets on Jupiter in a deep convection model. Nature, 438(10), 193–196.CrossRefGoogle Scholar
Holland, P. W. and Welsch, R. E. (1977), Robust regression using iteratively reweighted least squares. Commun. Stat. Theor. M., 6, 813–827.Google Scholar
Hollerbach, R. and Jones, C. A. (1993), Influence of the Earth’s inner core on the geomagnetic fluctuations and reversals. Nature, 365, 541–543.CrossRefGoogle Scholar
Holme, R. (2007), Large-scale flow in the core. Pages 107–130 of: Schubert, G. (ed.), Treatise on Geophysics, Volume 8: Core Dynamics. Amsterdam: Elsevier.CrossRefGoogle Scholar
Ingersoll, A. P., Orton, G. S., Münch, G., Neugebauer, G. and Chase, S. C. (1980), Pioneer Saturn infrared radiometer experiment: preliminary results. Science, 207, 439–443.Google Scholar
Jia, X. and Kivelson, M. G. (2012), Driving Saturns magnetospheric periodicities from the upper atmosphere/ionosphere magnetotail response to dual sources. J. Geophys. Res., 117, A11219.Google Scholar
Jia, X., Kivelson, M. G. and Gombosi, T. I. (2012), Driving Saturns magnetospheric periodicities from the upper atmosphere/ionosphere. J. Geophys. Res., 117, A05215.Google Scholar
Jones, C. A. (2011), Planetary magnetic fields and fluid dynamos. Ann. Rev. Fluid Mech., 43, 583–614.Google Scholar
Jones, C. A. (2014), A dynamo model for Jupiter’s magnetic field. Icarus, 241, 148–159.CrossRefGoogle Scholar
Jones, C. A., Boronski, P., Bruns, A. S. et al. (2011), Anelastic convection-driven dynamo benchmarks. Icarus, 216, 120–135.Google Scholar
Khurana, K. K., Mitchell, D. G., Arridge, C. S. et al. (2009), Sources of rotational signals in Saturn’s magnetosphere. J. Geophys. Res., 114, A02211.Google Scholar
King, E. M. and Buffett, B. A. (2013), Flow speeds and length scales in geodynamo models: the role of viscosity. Earth Planet. Sci. Lett., 371–372, 156–162.CrossRefGoogle Scholar
Kippenhahn, R. and Weigert, A. (1990), Stellar structure and evolution. Berlin: Springer.Google Scholar
Leconte, J. and Chabrier, G. (2013), Layered convection as the origin of Saturn’s luminosity anomaly. Nature Geosci., 6, 347–350.Google Scholar
Lhuillier, F., Fournier, A., Hulot, G. and Aubert, J. (2011), The geomagnetic secular variation timescale in observations and numerical dynamo models. Geophys. Res. Lett., 38, L09306.Google Scholar
Liu, J., Goldreich, P. M. and Stevenson, D. J. (2008), Constraints on deep-seated zonal winds inside Jupiter and Saturn. Icarus, 196, 653–664.Google Scholar
Lorenzen, W., Holst, B. and Redmer, R. (2011), Metallization in hydrogen-helium mixtures. Phys. Rev. B, 84, 23519.Google Scholar
Lowes, F. J. (1974), Spatial power spectrum of the main geomagnetic field, and extrapolation to the core. Geophys. J. Int., 36, 717–730.Google Scholar
Morales, M. A., Schwelger, E., Ceperley, D. et al. (2009), Phase separation in hydrogen-helium mixtures at Mbar pressures.Proc. Natl. Acad. Sci., 106, 1324–1329.Google Scholar
Nellis, W. J., Weir, S. T. and Mitchell, A. C. (1999), Minimum metallic conductivity of fluid hydrogen at 140 MPa (1.4 Mbar). Phys. Rev. B, 59, 3434–3449.Google Scholar
Nettelmann, N., Püstow, R. and Redmer, R. (2013), Saturn layered structure and homogeneous evolution models with different EOSs. Icarus, 225, 548–557.Google Scholar
Olson, P. and Aurnou, J. (1999), A polar vortex in the Earth’s core. Nature, 402, 170–173.Google Scholar
Olson, P., Christensen, U. R. and Driscoll, P. E. (2012), From superchrons to secular variation: a broadband dynamo frequency spectrum for the geomagnetic dipole. Earth Planet. Sci. Lett., 319 –320, 75–82.Google Scholar
Provan, G., Cowley, S. W. H. and Nichols, J. D. (2009), Phase relation of oscillations near the planetary period of Saturn’s auroral oval and the equatorial magnetospheric magnetic field. J. Geophys. Res., 114, A04205.Google Scholar
Pryor, W. R., Rymer, A. M., Mitchell, D. G. et al. (2011), The auroral footprint of Enceladus on Saturn. Nature, 472, 331–333.Google Scholar
Read, P. L., Dowling, T. E. and Schubert, G. (2009), Saturn’s rotation period from its atmospheric planetary-wave configuration. Nature, 460, 608–610.Google Scholar
Roberts, P. H. and Gubbins, D. (1987), Origin of the main field: Kinematics. Pages 185–249 of: Jacobs, J. A. (ed.), Geomagnetism, Vol. 2. London: Academic Press.Google Scholar
Schubert, G. and Soderlund, K. M. (2011), Planetary magnetic fields: observations and models. Phys. Earth Planet. Inter., 187, 92–108.Google Scholar
Smith, C. G. A. and Achilleos, N. (2012), Axial symmetry breaking of Saturn’s thermosphere. Mon. Not. R. astr. Soc., 422, 1460–1488.CrossRefGoogle Scholar
Smith, E. J., Davis, L., Jones, D. E. et al. (1980), Saturn’s magnetic field and magnetosphere. Science, 207, 407–410.Google Scholar
Southwood, D. J. and Kivelson, M. G. (2007), Saturnian magnetospheric dynamics: Elucidation of a camshaft model. J. Geophys. Res., 112, A12222.Google Scholar
Stanley, S. (2010), A dynamo model for axisymmetrizing Saturn’s magnetic field. Geophys. Res. Lett., 37, L05201.Google Scholar
Stanley, S. and Mohammadi, A. (2008), Effects of an outer thin stably stratified layer on planetary dynamos. Phys. Earth Planet. Inter., 168, 179–190.Google Scholar
Starchenko, S. V. and Jones, C. A. (2002), Typical velocities and magnetic field strengths in planetary interiors. Icarus, 157, 426–435.Google Scholar
Stelzer, Z. and Jackson, A. (2013), Extracting scaling laws from numerical dynamo models. Geophys. J. Int., 193, 1265–1276.Google Scholar
Sterenborg, M. G. and Bloxham, J. (2010), Can Cassini magnetic field measurements be used to find the rotation period of Saturn’s interior? Geophys. Res. Lett., 37, L11201.Google Scholar
Stevenson, D. J. (1980), Saturn’s luminosity and magnetism. Science, 208, 746–748.Google Scholar
Stevenson, D. J. (1982), Reducing the non-axisymmetry of a planetary dynamo and an application to Saturn. Geophys. Astrophys. Fluid Dyn., 21, 113–127.Google Scholar
Stevenson, D. J. (2003), Planetary magnetic fields. Earth Planet. Sci. Lett., 208, 1–11.Google Scholar
Stevenson, D. J. and Salpeter, E. E. (1977), The dynamics and helium distribution in helium-hydrogen fluid planets. Astrophys. J. Suppl., 35, 239–261.Google Scholar
Talboys, D. L., Bunce, E. J., Cowley, S. W. H. et al. (2011), Statistical characteristics of field-aligned currents in Saturn’s nightside magnetosphere. J. Geophys. Res., 116, A04213.Google Scholar
von Zahn, U., Hunten, D. M. and Lehmacher, G. (1998), Helium in Jupiter’s atmosphere: results from the Galileo probe helium interferometer experiment. J. Geophys. Res., 103, 22,815–22,829.Google Scholar
Wicht, J. and Tilgner, A. (2010), Theory and modeling of planetary dynamos. Space Sci. Rev., 152, 501–542.Google Scholar
Wilson, H. F. and Militzer, B. (2010), Sequestration of noble gases in giant planet interiors. Phys. Rev. Lett., 104, 121101.Google Scholar

Save book to Kindle

To save this book to your Kindle, first ensure coreplatform@cambridge.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.

Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

Find out more about the Kindle Personal Document Service.

Available formats
×

Save book to Dropbox

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.

Available formats
×

Save book to Google Drive

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.

Available formats
×