The Precessionally-Driven Dynamo

半个多世纪前，爱德华·布拉德爵士推测，在地球导电流体核中产生地球磁场所必需的运动可能是由太阳进动驱动的。在其间的55年里，所有明确确立的是，进动在原则上可以为地球发电机提供充足的能量。它能否利用这种能量的问题仍然没有得到回答，这提供了一个具有挑战性的目标，这是这一提议的理论价值的基础。在过去的十年里，人们已经了解到许多关于以前驱动的流动的结构和稳定性的知识，但只有在没有磁场的情况下才会如此。与对流驱动系统研究的平行研究类似，预计在存在“足够强的”磁场的情况下，前驱流的特征和强度将有很大不同，在地球物理背景下，这是一个与对地核推断的磁场相当的强场。这项研究的主要理论工具是加州大学洛杉矶分校在过去18个月里一直在开发的计算机代码。它采用了基于网格点的“网格法”。这非常适合于像地核这样的旋转扁平的流体物体。本文将该方法推广到求解三维全磁流体动(MHD)发电机方程。程序即将完成，本方案主要寻求支持对其进行并行化并提高其效率，以便对Bullard的方案进行彻底的评估。完成的程序的更广泛的影响将超过其应用于进动。在没有重大修改的情况下，进动驱动可以被浮力驱动取代，因此使该模型与目前正在开发的球面对流发电机的其他代码竞争。如果没有进一步的改变，它可以被用来研究在旋转平坦的物体(土星、银盘)中运行的发电机。。。)。调查人员计划做出这些概括。也有可能推广到潮汐扭曲的物体，如太阳系外行星。即使布拉德的推测不得不放弃，研究对流驱动的地球发电机是如何受到进动影响的也是很容易的。

More than a half century ago, Sir Edward Bullard conjectured that themotions necessary to generate the Earth's magnetic field in theEarth's electrically-conducting fluid core might be driven by theluni-solar precession. All that has been unequivocally establishedin the intervening 55 years is that precession can in principlesupply the geodynamo with abundant power. The question of whetherit can draw on this power is still unanswered, and this providesthe challenging target that is the basis of the intellectual merit of thisproposal.Much has been learned during the last decade about the structureand stability of precessionally-driven flows, but only whenmagnetic fields are absent. In analogy with parallel studies ofconvectively-driven system studies, it is anticipated that the characterand strength of a precessionally-driven flow will be dramaticallydifferent in the presence of a "sufficiently-strong" magnetic fieldmeaning, in the geophysical context, a field of strength comparablewith that inferred for the Earth's core. The proponents to investigate thismatter as part of a systematic study of such magnetoprecessionaldynamos", and also to see if they provide a new example of so-calledstrong field dynamo action".The main theoretical tool to be used in the proposed researchis a computer code that has been under development at UCLA forthe past 18 months. It employs the grid point based "method ofoverlapping grids". This is very suitable for rotationallyflattened bodies of fluid such as Earth's core. The methodhas been generalized here so that it solves thefull magnetohydrodynamic (MHD) dynamo equations in 3 dimensions.The code is on the verge of completion and this proposalmainly seeks support to parallelize it and to improveits efficiency so that Bullard's proposal can be thoroughly evaluated.The broader impact of the completed code will exceed itsapplication to precession. Without major modification, precessionaldriving can be replaced by buoyant forcing, so putting the model incompetition with other codes currently under development forspherical convective dynamos. Without further change it could beused to study dynamos operating in rotationally flattened objects(Saturn, galactic discs,. . . ). The investigators plan to makethese generalizations. It would also be possible to generalizeto tidally distorted objects such as extra-solar planets.Even if Bullard's speculation has to be abandoned, it will beeasy to investigate how a convectively-driven geodynamo isinfluenced by precession.