Thermal convection experiments in a liquid gallium layer subject to a uniform rotation and a uniform vertical magnetic field are carried out as a function of rotation rate and magnetic field strength. Our purpose is to measure heat transfer in a low-Prandtl-number (Pr = 0.023), electrically conducting fluid as a function of the applied temperature difference, rotation rate, applied magnetic field strength and fluid-layer aspect ratio. For Rayleigh-Bénard (non-rotating, non-magnetic) convection we obtain a Nusselt number-Rayleigh number law Nu = 0.129 Ra0.272±0.006 over the range 3.0 x 103 < Ra < 1.6 x 104. For non-rotating magnetoconvection, we find that the critical Rayleigh number RaC increases linearly with magnetic energy density, and a heat transfer law of the form Nu ∼ Ra1/2. Coherent thermal oscillations are detected in magnetoconvection at ∼ 1.4RaC. For rotating magnetoconvection, we find that the convective heat transfer is inhibited by rotation, in general agreement with theoretical predictions. At low rotation rates, the critical Rayleigh number increases linearly with magnetic field intensity. At moderate rotation rates, coherent thermal oscillations are detected near the onset of convection. The oscillation frequencies are close to the frequency of rotation, indicating inertially driven, oscillatory convection. In nearly all of our experiments, no well-defined, steady convective regime is found. Instead, we detect unsteady or turbulent convection just after onset.

A mechanism is proposed to explain the seismologically-inferred prograde rotation of the Earth's solid inner core in terms of the structure of convection in the fluid outer core. Numerical calculations of convection and dynamo action in the outer core exhibit excess temperatures inside the tangent cylinder surrounding the inner core. We show that this temperature difference generates a prograde thermal wind and a strong azimuthal magnetic field inside the tangent cylinder. Electromagnetic torques on the inner core derived from induced azimuthal magnetic fields and the ambient poloidal field equilibrate when the inner core angular velocity lags the nearby tangent cylinder fluid angular velocity by approximately 14%. The inferred prograde rotation of the inner core (1.1-3°/year relative to the mantle) can be produced by a very small (≃ 0.001 K) temperature anomaly within the tangent cylinder and indicates strong toroidal magnetic fields with peak intensities of 24-66 mT in that region of the core. Copyright 1996 by the American Geophysical Union.

Calculations of electromagnetic torques between the fluid outer core and the heterogeneous lower mantle demonstrate that the geomagnetic field can lock with the mantle during polarity transitions. We show that the Virtual Geomagnetic Pole (VGP) is attracted toward mantle regions with low electrical conductivity and repelled from mantle regions with high conductivity. Using seismically-determined structure of the lower mantle to infer the pattern of conductivity variations at the base of the mantle, we find two preferred reversal routes for the VGP, one beneath the Americas and another beneath Asia. These paths, particularly the one beneath the Americas, lie close to the VGP paths seen in some sedimentary and lava records of polarity reversals, suggesting that the core and mantle are electromagnetically coupled during reversals. Copyright 1996 by the American Geophysical Union.