Varying the core radius ratio in dynamo models: Effect of the initial seed field.

Details Varying the core radius ratio in dynamo models: Effect of the initial seed field. Varying the core radius ratio in dynamo models: Effect of the initial seed field.

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusmgp33a..07g&link_type=abstract

American Geophysical Union, Spring Meeting 2004, abstract #GP33A-07

Physics

1510 Dynamo Theories, 1513 Geomagnetic Excursions, 8115 Core Processes (1507)

Scientific paper

The onset of dynamo action is studied as a function of the radius ratio and the initial seed magnetic field in self-sustaining 3D spherical numerical dynamo models. Two different initial seed magnetic fields are used; a weak and a relatively strong field. The radius ratio χ = ro/r_i, where ri and ro are the inner and outer core radii, respectively, covers the range 0.1 < χ < 0.75. Dynamo onset is characterized using the ratio of the Rayleigh number at the onset of dynamo action and the critical Rayleigh number for convection Rad = Rad'/Ra_c. We find that Rad is a decreasing function of χ and also depends on whether the seed magnetic field is weak or strong. In addition the seed field dependence of Rad is a function of χ. In some cases the specification of the seed field places a strong constraint on the onset of dynamo action. The seed field dependence of Rad seems to be caused by the existence of two different types of dynamos. For strong magnetic field initial conditions at relatively low Ra, we obtain, in many cases, strongly dipolar dynamos that are not accessible with a weak initial magnetic field. Some dynamos initiated with weak seed fields have stable but very weak dipole fields. At higher Ra dynamos obtained using strong or weak seed field merge for long time integrations, in the sense that their features are similar. Thus, moderately supercritical dynamo action can vary strongly with the structure and strength of the magnetic field. We explore the implications of these results for planets with weak observed magnetic fields, such as Mercury, and for polarity reversals of the Geodynamo.

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