The Precessionally-Driven Dynamo

半个多世纪前，爱德华·布拉德爵士（Sir Edward Bullard）提出，在地球的导电流体核心中产生地球磁场所必需的运动可能是由太阳岁差驱动的。在这55年里，所有已经明确确立的是，岁差原则上可以为地球发电机提供充足的能量。关于它是否能够利用这种力量的问题仍然没有答案，而这也为我们提供了一个具有挑战性的目标，而这个目标正是这一提议的智力价值的基础。在过去的十年中，我们已经对旋进驱动流的结构和稳定性有了很多了解，但这只是在没有磁场的情况下。与对流驱动系统研究的平行研究相类比，可以预期，在存在“非常强”磁场的情况下，岁差驱动流的特征和强度将大不相同，在地球物理学背景下，这意味着磁场的强度与地球核心推断的强度相当。支持者将调查这一问题作为系统研究这种磁衰退发电机的一部分，并看看它们是否提供了一个所谓强场发电机作用的新例子。在拟议的研究中使用的主要理论工具是一个计算机代码，该代码在过去的18个月里一直在加州大学洛杉矶分校开发。它采用了基于网格点的“重叠网格法”。这非常适合于旋转扁平的流体，如地球的核心。该方法已被推广到求解三维磁流体动力学（MHD）发电机方程组，该程序即将完成，该程序主要寻求并行化和提高效率的支持，以便对Bullard的程序进行全面评估，该程序的广泛影响将超过其在旋进问题上的应用。在没有重大修改的情况下，旋进驱动可以被浮力强迫所取代，因此将该模型与目前正在开发的其他球形对流发电机代码进行竞争。如果没有进一步的变化，它可以被用来研究发电机在旋转扁平物体（土星，银河系盘，。. . ).研究人员计划作出这些概括。这也可以推广到潮汐扭曲的物体，如太阳系外的行星。即使布拉德的推测不得不放弃，研究一个对流驱动的地球发电机是如何受到岁差的影响也是很容易的。

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.