Liquid sodium model of Earth's outer core

Convective motions in Earth's outer core are responsible for the generation of the geomagnetic field. We present liquid sodium convection experiments in a spherical vessel, designed to model the convective state of Earth's outer core. Heat transfer, zonal fluid velocities, and properties of temperature fluctuations were measured for different rotation rates O and temperature drops DeltaT across the convecting sodium.; The small scale fluid motion was highly turbulent, despite the fact that less than half of the total heat transfer was due to convection. The typical length scale of convective motions decreases with rotation rate like O -1/3. These convective structures give rise to temperature fluctuations which decrease in amplitude with increasing rotation rate and grow linearly with the temperature drop; sigmaT ∼ O-1/3 DeltaT. Convective heat transfer was observed to increase with both temperature drop and rotation rate proportional to O 1/3DeltaT. Retrograde zonal velocities were measured at speeds up to 0.02 times the tangential speed of the outer wall of the vessel. These velocities scale linearly with rotation rate and imposed temperature gradient; U&phis; ∼ ODelta T. Power spectra of temperature fluctuations exhibit a well defined knee at a frequency which is characterized by ballistic velocities. The knee frequency is thought to be associated with the convective motions (i.e. the energy injection scale for the underlying fluid motion). We observe a sensitive dependence of heat flux on an applied magnetic field: heat transfer concentrates in the equatorial region with an applied magnetic field parallel to the rotation axis.; In the context of Earth's outer core, our observations imply a thermal Rayleigh number Ra = 1022 and a convective velocity near 10-5 m/s. There is likely a knee in the energy spectrum of outer core fluid motions associated with convective length and time scales of 100 m and 2 days. Heat flux measurements suggest that persistent inhomogeneity in the geomagnetic field may cause inhomogeneities in the formation of the inner core.