Magnetic Field Enhancement and Hydromagnetic Dynamics in an Experimental Model of the Earth’s Core

Abstract:

Magnetic field amplification and hydromagnetic dynamics relevant to the earth’s outer core are studied in the University of Maryland Three Meter Geodynamo, a rapidly rotating spherical Couette experiment that is geometrically similar to the earth’s core and filled with liquid sodium. Turbulent flow is driven in the sodium by differential rotation of the inner and outer spherical shells, while an external coil applies a magnetic field in order to study hydromagnetic effects such as dynamo action. An array of 31 external Hall sensors measures the Gauss coefficients of the resulting field. The flow state is strongly dependent on Rossby number Ro = (Ω_I - Ω_O)/Ω_O, where Ω_I and Ω_O are the inner and outer sphere rotation frequencies. The flow state is inferred from the torque required to drive the inner sphere. The generation of internal toroidal magnetic field through the Omega effect is measured by a Hall probe inserted into the sodium. A self-sustaining dynamo has not yet been observed at rotation speeds up to about half of the design maximum. However, continuous dipole amplification up to ~12% of a small applied field has been observed at Ro = -17.7 while bursts of dipole field have been observed up to 15% of a large external applied field at Ro = +6.0 and up to ~20% of a small applied field at Ro = +2.15.