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4 - Saturn’s Magnetic Field and Dynamo
- Edited by Kevin H. Baines, University of Wisconsin, Madison, F. Michael Flasar, NASA-Goddard Space Flight Center, Norbert Krupp, Tom Stallard, University of Leicester
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- Book:
- Saturn in the 21st Century
- Published online:
- 13 December 2018
- Print publication:
- 06 December 2018, pp 69-96
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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.
7 - Planetary fields and dynamos
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- By Ulrich R. Christensen, Max Planck Institute for Solar System Research
- Edited by Carolus J. Schrijver, George L. Siscoe, Boston University
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- Book:
- Heliophysics: Evolving Solar Activity and the Climates of Space and Earth
- Published online:
- 05 April 2013
- Print publication:
- 23 September 2010, pp 179-216
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Summary
Introduction
Over four centuries ago it was realized that the time-averaged direction of a compass needle is not affected by a force emanating from the sky, but by a magnetic field that is intrinsic to the Earth. The basic structure of the geomagnetic field and its slow variation with time was characterized long before magnetic fields were detected on other celestial bodies. By the middle of the twentieth century, the study of remanent magnetization of natural rocks had firmly established that the principal dipole component of the Earth's magnetic field had reversed its direction many times in the past.
Our understanding of the origin of the field by a dynamo process in the Earth's core has developed at a much slower pace, basically in parallel with that of astrophysical dynamos in general. Aside from understanding the intricate details of how a magnetic field is generated by a dynamo, we must ascertain that some fundamental requirements are fulfilled inside our planet. Geophysical observations have shown that one condition, namely the existence of an electrically conducting fluid region, is met inside the Earth, which has an outer core consisting of a liquid iron alloy. It is likely, but not completely certain, that all big planets have conducting fluid cores (see Fig. 7.5). However, some planets may not conform with another basic condition for a dynamo, namely sufficiently fast motion in the fluid layer.
Convection-driven planetary dynamos
- Ulrich R. Christensen, Julien Aubert, Peter Olson
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- Journal:
- Proceedings of the International Astronomical Union / Volume 2 / Issue S239 / August 2006
- Published online by Cambridge University Press:
- 01 August 2006, pp. 188-195
- Print publication:
- August 2006
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Numerical simulations of convection-driven dynamos in rotating spherical shells are employed to better understand the observed strength and geometry of planetary magnetic fields. The model computations cannot be performed for realistic values of several of the control parameters. By varying parameters within the accessible range, it is possible to derive scaling laws for the magnetic field strength and the flow velocity in the dynamo region and for the dipole moment. Our scaling laws suggest that, even though diffusivities are far too large in the models, diffusive processes do not play an important role, just as in planetary cores. Extrapolating the scaling laws to planetary values of the control parameters leads to reasonable predictions for the field strength in the dynamo region and fits the observed dipole moments decently, in particular in the cases of Earth and Jupiter. For Mercury, which does not fit well when applying the scaling laws in a straightforward way, a model is proposed in which the upper part of the fluid core is stably stratified and the dynamo operates only in its deep regions. The time-varying dynamo field must diffuse through the stable region and is attenuated by the skin effect. The model explains why Mercury has a very weak but probably dipole-dominated magnetic field.