Experimental simulation of magnetoconvection in the Earth's tangent cylinder

The Earth has a substantial dipolar magnetic field, a fact of historical importance because of the role of the magnetic compass in the exploration of our planet. The Earth’s magnetic field is generated by a dynamo in the liquid iron core, which convects in response to cooling of the overlying mantle. Observations of the Earth’s magnetic field on the surface reveal interesting aspects about the Earth’s deep interior: first, the magnetic field is characterised by four high-latitude flux lobes symmetrically placed North and South of the equator. They are centered just outside the Earth’s tangent cylinder, an imaginary cylinder touching the inner core boundary and parallel to the Earth’s rotation axis. Second, time variations of the magnetic field indicate that there are anticyclonic vortices in the polar regions of the core. Convection in the Earth’s tangent cylinder most likely plays a role in both observations. Numerical simulations of the Earth’s dynamo tend to support this idea. Experiments of rotating convection in a magnetic field in regimes that mimic the Earth’s core have never been attempted. Our proposed study is a decisive step in this direction.

An experimental investigation of rotating magnetoconvection is justified by previous analytical and numerical studies. In particular, it has been previously shown that, in the presence of a magnetic field, the flow often takes the form of a single coherent plume that extends from the inner core boundary right up to the polar region, but offset from the rotation axis. The plume does not remain at the same longitude, but migrates in a rather irregular fashion, generally westward. In nonmagnetic or weakly magnetic convection, however, the flow in the tangent cylinder takes the form of several tall thin columns whose radius is controlled by viscosity. We strongly believe that magnetoconvection in the form of a coherent plume gives rise to the observed polar vortex, and also expels magnetic flux from within the tangent cylinder. Identifying the transition between the viscous and magnetic modes of convection is the main aim of this study. Achieving this goal will greatly improve our current understanding of the dynamics of the Earth’s core.

Dr Binod Sreenivasan
Coventry University

Binod was awarded a Research Project Grant in March 2012; providing £91,997.