Saturn's Very Axisymmetric Magnetic Field: Implication for the Interior of the Planet

Details Saturn's Very Axisymmetric Magnetic Field: Implication for the Interior of the Planet Saturn's Very Axisymmetric Magnetic Field: Implication for the Interior of the Planet

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.4105c&link_type=abstract

American Astronomical Society, DPS meeting #42, #41.05; Bulletin of the American Astronomical Society, Vol. 42, p.1039

Astronomy and Astrophysics

Astronomy

Scientific paper

Saturn's intrinsic magnetic field is very spin-axisymmetric as found by Pioneer 11 and confirmed by the Voyager 1 and 2 flybys. Since 2004, the Cassini spacecraft has been providing continuous measurements of the magnetic field from Saturn orbit. Orbits with high inclination and low altitude provide a good dataset for the study of the intrinsic magnetic field of the planet. Our investigation shows no evidence for any detectable non-axisymmetric intrinsic magnetic field. The biggest possible dipole tilt we can find is 0.06 degree, which is the noise level of the analysis. Our axisymmetric model for Saturn's intrinsic magnetic field is characterized by Schmidt-normalized spherical harmonic coefficients g11 = 21191nT, g20 = 1586nT, g30 = 2374nT. No significant degree 4 or 5 terms can be determined from the current data. Compared with the SPV model, which fits the measurements from Pioneer 11, Voyager 1 and 2 equally well, the secular variation is found to be an order of magnitude smaller than for similar terms of the Earth's field for degree 1 and 2. This implies no fast decay of the Saturn's dipole moment, which is fundamentally different from the current secular variation of the Earth's field.
The extremely high degree of axisymmetry of Saturn's magnetic field is challenging for dynamo theory. Our analysis of the magnetic measurement from Cassini requires a damping factor of 0.006 if the dipole tilt in the dynamo region is 10 degrees. To meet this damping factor, the stable layer must has significant thickness, L, is greater than or equal to 4,000km. This thicker stable layer is also favored by the prediction of Saturn's observed dipole moment from scaling theory. Numerical dynamo models with predicted damping factors are needed to determine possible limits to the degree of axisymmetrization by a conducting layer.

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