Liquid sodium model of geophysical core convection

Abstract

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 planetary cores such as the Earth’s. Heat transfer, azimuthal fluid velocities, and properties of temperature fluctuations were measured for different rotation rates and temperature drops across the convecting sodium. We observed small-scale convective motions with strong, large-scale azimuthal winds and developed turbulence despite the fact that convective heat transport was weak and the temperature profile was close to diffusive. In the context of Earth’s outer core, our observations suggest models which imply a thermal Rayleigh number R a ≈ 6 × 1 0 23 and a convective velocity near 2 × 1 0 − 4 m/s. Also, the energy spectrum of outer core may exhibit important structure down to length and time scales of 1 km and 30 days. Furthermore, we calculate an estimate of Ohmic dissipation, 0.1 TW, in the core based on the shape of experimentally observed power spectra.