Convective motions in the Earth's fluid core

Details Convective motions in the Earth's fluid core Convective motions in the Earth's fluid core

Sep 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994georl..21.1939z&link_type=abstract

Geophysical Research Letters (ISSN 0094-8276), vol. 21, no. 18, p. 1939-1942

Other

15

Convective Flow, Coriolis Effect, Earth Core, Lorentz Force, Magnetic Fields, Boundary Conditions, Dynamo Theory, Mathematical Models, Rayleigh Number, Two Dimensional Flow

Scientific paper

Convection in the Earth's core is affected by Lorentz and Coriolis forces. The relative importance of these is measured by the Elsasser number Lambda. Previous work suggested that the optimum condition for convection occurs when the Elsasser number Lambda is O(1); in particular, the critical modified Rayleigh number R* had a minimum value at some O(1) value of Lambda. This gave rise to the view that the size of the magnetic field generated by the dynamo would adjust to this Lambda-value because it optimised the convection. We have investigated convection in a rotating spherical shell with magnetic field distributions satisfying appropriate boundary conditions in the form of the toroidal decay modes which we believe are more realistic for the Earth's core. Our results are rather different from the classical picture. We find no optimum Elasser number that minimises R*, but instead a monotonic decay of R* as Lambda increases. At Lambda approximately 10, R* goes negative, so that rolls are driven by magnetic instability rather than convection. Naturally, as this regime is entered the rolls must be destroying magnetic field rather than creating it by dynamo action. This suggests that in the Earth's fluid outer core, the field strength is such that 1 is less than Lambda is less than 10, so that the magnetic field is stable and the corresponding convection is dominated by large scales is efficient. Another important feature of these solutions is that the azimuthal flow is primarily two-dimensional. This is surprising, as it has generally been believed that a magnetic field of this strength would break the Taylor-Proudman constraint. However, it appears that the magnetic field adjusts to preserve two-dimensionality even in the range 1 less than Lambda less than 20.

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