A simple physical model for the terrestrial dynamo.

Abstract

The strength of the earth's magnetic field results from an equilibration between rates of buoyant energy production and Ohmic dissipation. Changes in magnetic field, in particular the long term changes in dipole moment, provide an indication of changes in core energy sources, and so become critical data for understanding the evolution of both the core and deep mantle. A simple physical model is proposed to establish a connection between dipole moment behaviour and production of buoyancy within the core. The model rests on two hypotheses: 1) magnetism is generated by small scale, rotation dominated turbulence consisting of a field of propagating inertial waves, and 2) the turbulence is supported by a flux of buoyancy, thermal or compositional, originating either at the core-mantle or inner core boundary. The efficiency with which wave kinetic energy is converted to magnetic energy is determined by the wave helicity - the correlation between velocity and vorticity. The wave helicity is non-zero if there exists a preferred propagation direction. Negative buoyancy generated at the mantle-core boundary leads to propagation radially inward; positive buoyancy generated at the inner core boundary leads to radially outward propagation. Using parameters appropriate for the earth's core, we find that the inertial wave dynamo dissipates 8X1011W in generating a magnetic field equal to the present terrestrial field. Four energy sources are considered: decay of potassium 40, secular cooling, inner core growth, and differentiation of the core from the mantle. Any of these sources can reasonably support the turbulent dynamo for much of the earth's history.-Author