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Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction

Sreenivasan, B. and Gubbins, D. (2008) Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction. Geophysical & Astrophysical Fluid Dynamics, 102 (4). pp. 395-407. ISSN 1029-0419

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Convection in the Earth's core is driven much harder at the bottom than the top. This is partly because the adiabatic gradient steepens towards the top, partly because the spherical geometry means the area involved increases towards the top, and partly because compositional convection is driven by light material released at the lower boundary and remixed uniformly throughout the outer core, providing a volumetric sink of buoyancy. We have therefore investigated dynamo action of thermal convection in a Boussinesq fluid contained within a rotating spherical shell driven by a combination of bottom and internal heating or cooling. We first apply a homogeneous temperature on the outer boundary in order to explore the effects of heat sinks on dynamo action; we then impose an inhomogeneous temperature proportional to a single spherical harmonic Y2² in order to explore core-mantle interactions. With homogeneous boundary conditions and moderate Rayleigh numbers, a heat sink reduces the generated magnetic field appreciably; the magnetic Reynolds number remains high because the dominant toroidal component of flow is not reduced significantly. The dipolar structure of the field becomes more pronounced as found by other authors. Increasing the Rayleigh number yields a regime in which convection inside the tangent cylinder is strongly affected by the magnetic field. With inhomogeneous boundary conditions, a heat sink promotes boundary effects and locking of the magnetic field to boundary anomalies. We show that boundary locking is inhibited by advection of heat in the outer regions. With uniform heating, the boundary effects are only significant at low Rayleigh numbers, when dynamo action is only possible for artificially low magnetic diffusivity. With heat sinks, the boundary effects remain significant at higher Rayleigh numbers provided the convection remains weak or the fluid is stably stratified at the top. Dynamo action is driven by vigorous convection at depth while boundary thermal anomalies dominate in the upper regions. This is a likely regime for the Earth's core.

Item Type: Article
Copyright, Publisher and Additional Information: © 2008 Taylor and Francis. This is an author produced version of the published paper. Uploaded in accordance with the publisher's self-archiving policy.
Institution: The University of Leeds
Academic Units: The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds)
Depositing User: Sherpa Assistant
Date Deposited: 03 Oct 2008 13:43
Last Modified: 08 Feb 2013 17:05
Published Version: http://dx.doi.org/10.1080/03091920801900047
Status: Published
Publisher: Taylor & Francis
Refereed: Yes
Identification Number: 10.1080/03091920801900047
URI: http://eprints.whiterose.ac.uk/id/eprint/4729

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