The Precessionally-Driven Geodynamo

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Magnetism is everywhere, in planets, stars and galaxies. Usually it is the by-product of electric currents created in the same way as in a commercial power plant, i.e., by dynamo action. This is the name given to the process through which the motion of an electrical conductor across a magnetic field creates electric currents that can maintain that magnetic field against energy losses through electrical resistance. Dynamos require an energy source to offset those losses. For the geodynamo operating in the Earth's liquid core there are two main possibilities: buoyancy and the luni-solar precession. Buoyancy is created by the release of latent heat and light constituents as core fluid solidifies onto the surface of the solid inner core in the general cooling of the Earth, the buoyant power release being proportional to the rate of solidification and therefore to the rate of growth of the solid core. Precession is the name given to the motion of the Earth's polar axis round a cone with a semi-angle of about 23.5 degrees; it sweeps out the complete cone approximately every 26,000 years. Precession is caused by the gravitational torques that the Sun, Moon and planets exert on the equatorial bulge of the Earth. The Earth's mantle and core are coupled by viscosity and topography, the topographic torque existing because of the slight oblateness of the core-mantle interface. Through this coupling, precession creates fluid motions in the core. Of the two mechanisms, the buoyancy explanation is the better studied and the more widely accepted. Nevertheless, there is a difficulty: the power requirements of the geodynamo imply such a rapid growth of the inner core that it is, on current estimates, at most about 2 billion years old. Paleomagnetism has established however that the geomagnetic field has existed at about its present strength for more than 3.4 billion years. The precessional explanation does not encounter this particular difficulty but it may face others. This will not be known until the precessional dynamo has been studied in greater detail than has so far been attempted. This is the main objective of this project. There is a subsidiary objective too, that of understanding better the flows generated in planets such as Mercury and satellites such as Europa by their libration. This is the term used when a body does not keep the same face towards another body about which it orbits. For example, the Moon does not present exactly the same face towards the Earth as it orbits about it. Librationally-driven motions are being actively studied experimentally at UCLA, and our techniques for solving precessionally-driven flows can be easily modified to apply to these experiments and to librating bodies in the solar system. Most studies of dynamo action in large naturally-occurring bodies have had to assume that those bodies are spherical. A radically new method is required to study precessionally-driven flows in non-spherical bodies. We have recently developed and tested a numerical technique for solving the equations of magnetohydrodynamics, governing fluid flow and magnetic field, in an oblate spheroidal container. The computer code advances the flow in time using finite differences on overlapping grids; in this way the grids map the spheroidal container and the numerical difficulty known as the pole problem is completely avoided. Already some unexpected, and so far inexplicable, results have been obtained. We intend to carry out many exploratory studies to try to elucidate these flows in order to determine how important precession is in driving motions in Earth's core and in creating the main magnetic field of the Earth.