Saturn's Very Axisymmetric Magnetic Field: New Upper Limit on the Dipole Tilt and Implications for the Interior of the Planet

Details Saturn's Very Axisymmetric Magnetic Field: New Upper Limit on the Dipole Tilt and Implications for the Interior of the Planet Saturn's Very Axisymmetric Magnetic Field: New Upper Limit on the Dipole Tilt and Implications for the Interior of the Planet

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmgp23b1008c&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #GP23B-1008

Physics

[1510] Geomagnetism And Paleomagnetism / Dynamo: Theories And Simulations, [1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [5734] Planetary Sciences: Fluid Planets / Magnetic Fields And Magnetism, [6275] Planetary Sciences: Solar System Objects / Saturn

Scientific paper

Saturn is the only planet in the solar system whose magnetic field is highly axisymmetric. At least a small deviation from perfect symmetry is required for a dynamo-generated magnetic field. Here we show that the tilt of Saturn's dipole field relative to the rotation axis is more than a factor ten smaller than previously reported. Since 2004 Cassini has provided continuous measurements of Saturn's field. We invert those observations that were obtained in the “current-free” inner magnetosphere for an internal model, making different assumptions on the unknown rotation rate of Saturn's deep interior. We find that the largest possible dipole ‘tilt’ is 0.06 degree, the detection threshold. If we model the contribution of the external field to the azimuthal component and remove that contribution, the maximum dipole tilt is 0.03 degree. An axisymmetric internal model with Schmidt-normalized spherical harmonic coefficients g10=21191nT, g20=1586nT, g30=2374nT is derived from these measurements, the upper bounds on the axial degree 4 and 5 terms are 720nT and 3200nT respectively. The secular variation from Pioneer 11 to the present epoch is an order of magnitude smaller than the variation in similar terrestrial terms for degrees 1 and 2, and it is also within the probable error of each term. Differentially rotating conducting stable layers above Saturn's dynamo region have been proposed to symmetrize the magnetic field [Stevenson 1982]. Our result implies that this stable layer must have a thickness L>=4,000km. This thickness is also consistent with our weak secular variation finding.

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