Some fundamental questions about the earth's core and the geomagnetic field

Abstract

This paper presents a series of questions about the earth's core and the geomagnetic field, selected to emphasise disagreements that need to be resolved. The answers given represent a personal approach, intended to stimulate discussion. It is argued that the near whiteness of the spatial spectrum of the field at core level is a consequence of a white spatial spectrum of currents with a characteristic scale depth of about 160 km in the outermost part of the core. Dynamo action requires a magnetic Reynolds number exceeding about 10. This leads naturally to the 160 km scale, which is the size of the smallest vortices of magnetically controlled turbulence. Core turbulence is essential for mixing of the compositional (and perhaps thermal) heterogeneities that drive the dynamo. Available dynamo power is about 2 ´ 1011 W and is dissipated by an r.m.s. current density of 2 ´ 10?3A m?2 per harmonic degree. Current loops of this strength in the outer 160 km of the core precisely account for both the field strength and its spectrum. The field lines of the observed poloidal field are swept up into the stronger poloidal field at about the 160 km depth and the toroidal field is stronger by virtue of the much greater depth range of the current loops causing it, not by virtue of more intense currents. The arguments presented disallow a stably stratified layer at the top of the core or an explanation of the westward drift in terms of a bodily drift of core material. Heterogeneities at the base of the mantle are probably important to dynamo stability; seismic anisotropy of the inner core suggests that it grows primarily by accretion at the equator and continuously deforms towards its equilibrium shape. Thus the convective buoyancy in the core is strongly localized.