THE EARTH DYNAMO

Recent developments in geodynamo theory have advanced along two distinct tracks. On the one hand, a mean field approach is adopted. With reasonable assumptions about the nature of the a- and co-effects of mean field theory, axisymmetric dynamo solutions are constructed which include the nonlinear feed back of the Lorentz force on the motion. On the other hand, the nature of the asymmetric waves, convection and turbulence, which occur in the presence of a prescribed magnetic field are considered. The Earth's rapid rotation ω causes these asymmetric disturbances to be helical and so give rise to the α-effect. Recently numerical solutions of the complete set of equations governing a convectively driven dynamo have been obtained. Rotation inhibits convection but the constraint is relaxed by the presence of magnetic field. The relaxation is optimised when the Lorentz and Coriolis forces are comparable which occurs at field strengths of order (pω/σ)1/2, where p and σ are the fluid density and electrical conductivity respectively. It is often argued that this provides a lower limit on the field strength for dynamo operation. On this basis the Earth is an αω-dynamo with a magnetic field predominantly azimuthal in the fluid core. © 1991, Taylor & Francis Group, LLC. All rights reserved.