Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43e..01c&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P43E-01
Physics
[1510] Geomagnetism And Paleomagnetism / Dynamo: Theories And Simulations, [1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths
Scientific paper
For many published dynamo models an Earth-like magnetic field has been claimed, even though properties such as the viscosity are unrealistic. However, it has also been noted that as the Ekman number (viscosity) is lowered to less unrealistic values, the magnetic field tends to become less Earth-like. Here we define quantitative criteria for the degree of similarity between the field of a dynamo model and the geomagnetic field, concentrating on the agreement in field morphology. Main criterion is (1) the ratio between the axial dipole component and non axial dipole parts of the field. In addition, we include (2) the ratio between equatorially symmetric and antisymmetric components and (3) between zonal and non-zonal components in the non-dipole field and (4) a measure for the degree of concentration of magnetic flux at the core surface into small patches. We test for a large number of dynamo models that cover the accessible parameter space the degree of compliance with these criteria. We tentatively order models according to the magnetic Reynolds number Rm and magnetic Ekman number Em (ratio between rotation period and magnetic diffusion time). We find that requirements for an Earth-like field morphology are that Em <= 10-4 and that Rm falls into a limited range which depends on Em and ranges between 140 and 450 at Em=10-4 and increases to 1500-5000 when extrapolated to the Earth value Em=5x10-8. Dynamo models with zero buoyancy flux on the outer boundary (compositional convection) are generally somewhat more Earth-like than models driven by a fixed temperature contrast between the boundaries.
Aubert Julien
Christensen Ulrich R.
Hulot Gauthier
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