More than a half century ago, Sir Edward Bullard conjectured that the motions necessary to generate the Earth's magnetic field in the Earth's electrically-conducting fluid core might be driven by the luni-solar precession. All that has been unequivocally established in the intervening 55 years is that precession can in principle supply the geodynamo with abundant power. The question of whether it can draw on this power is still unanswered, and this provides the challenging target that is the basis of the intellectual merit of this proposal.
Much has been learned during the last decade about the structure and stability of precessionally-driven flows, but only when magnetic fields are absent. In analogy with parallel studies of convectively-driven system studies, it is anticipated that the character and strength of a precessionally-driven flow will be dramatically different in the presence of a "sufficiently-strong" magnetic field meaning, in the geophysical context, a field of strength comparable with that inferred for the Earth's core. The proponents to investigate this matter as part of a systematic study of such magnetoprecessional dynamos", and also to see if they provide a new example of so-called strong field dynamo action".
The main theoretical tool to be used in the proposed research is a computer code that has been under development at UCLA for the past 18 months. It employs the grid point based "method of overlapping grids". This is very suitable for rotationally flattened bodies of fluid such as Earth's core. The method has been generalized here so that it solves the full magnetohydrodynamic (MHD) dynamo equations in 3 dimensions. The code is on the verge of completion and this proposal mainly seeks support to parallelize it and to improve its efficiency so that Bullard's proposal can be thoroughly evaluated.
The broader impact of the completed code will exceed its application to precession. Without major modification, precessional driving can be replaced by buoyant forcing, so putting the model in competition with other codes currently under development for spherical convective dynamos. Without further change it could be used to study dynamos operating in rotationally flattened objects (Saturn, galactic discs,. . . ). The investigators plan to make these generalizations. It would also be possible to generalize to tidally distorted objects such as extra-solar planets. Even if Bullard's speculation has to be abandoned, it will be easy to investigate how a convectively-driven geodynamo is influenced by precession.
