Improved laboratory and computational models for thermal core–mantle interaction

Abstract

The Earth's magnetic field is generated by thermochemical convection within its fluid outer core. The near-stationary high-latitude magnetic flux concentrations in the present day field indicate that convective motion in the outer core is influenced by the thermal inho- mogeneity of the overlying mantle. Heat flow across the core{mantle boundary (CMB) follows the pattern of lateral variations in the lower mantle. In this thesis, thermal core{mantle inter- action and its consequences for dynamo action are investigated through computational and laboratory models. First, the onset of convection is studied in a rapidly rotating spherical shell subject to laterally varying boundary heat flux. It is shown that convection is organized in clusters of small-scale rolls in preferred longitudes. In addition, the lateral variations sub- stantially ease the onset of convection with both equatorially symmetric and antisymmetric lateral variations. Second, an experimental study is conducted that examines the role of laterally varying boundary heat flux on rapidly rotating convection in a cylindrical annulus. A radial temperature gradient is maintained across the annulus to generate buoyancy. Heat

flux is varied on the outer cylinder to mimic the thermal inhomogeneity at the CMB. The effect of large-scale boundary heat flux variations on rapidly rotating convection is demon- strated for the first time in a laboratory experiment. Finally, three-dimensional numerical dynamo simulations are performed in a rotating spherical shell to study the generation of the magnetic field by convective motions modified by lateral variations. It is shown that the lateral heat flux variations at the CMB favour the growth of a seed magnetic field when back- ground convection is weak. The results may explain the existence of the early geodynamo that operated on purely thermal convection.