Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43e..04s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P43E-04
Physics
[1507] Geomagnetism And Paleomagnetism / Core Processes, [1510] Geomagnetism And Paleomagnetism / Dynamo: Theories And Simulations
Scientific paper
Magnetic field measurements by the Cassini mission have confirmed the earlier Pioneer 11 and Voyager missions' results that Saturn's internally generated magnetic field has an extremely small dipole tilt. Whereas Earth, Jupiter and Mercury have axial-dipole dominated magnetic fields with dipole tilts of approximately 10 degrees, Saturn's dipole tilt is less than 1 degree. The nearly-perfect axisymmetry of Saturn's dipole is troubling because of Cowling's Theorem which states that an axisymmetric magnetic field cannot be maintained by a dynamo. However, Cowling's Theorem applies to the magnetic field generated inside the dynamo source region and we can avert any contradiction with Cowling's Theorem if we can find reason for a non-axisymmetric field generated inside the dynamo region to have an axisymmetrized potential field observed at satellite altitude. Stevenson (1980) proposed a mechanism for this axisymmetrization. He suggested that differential rotation in a stably-stratified electrically conducting layer surrounding the dynamo could act to shear out the non-axisymmetry and hence produce an axisymmetric observed magnetic field. Here we use three-dimensional self-consistent numerical dynamo models to study the axisymmetrizing effects of stably-stratified layers surrounding the dynamo. We find that a thin stably-stratified layer which undergoes differential rotation due to thermal winds as a result of pole to equator temperature differences can produce a more axisymmetrized field. Surprisingly, we find that the direction of the zonal flows is a crucial factor for magnetic field axisymmetry since some zonal flows act to destabilize the dynamo producing non-axisymmetric fields.
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