The Precessionally-Driven Geodynamo

磁性无处不在，在行星、恒星和星系中。通常，它是以与商业发电厂相同的方式产生的电流的副产品，即，通过发电机的作用。这是一个过程的名称，通过该过程，电导体在磁场中的运动产生电流，电流可以维持磁场，防止通过电阻损失能量。发电机需要一种能源来抵消这些损失。对于在地球液体内核中运行的地球发电机来说，有两种主要的可能性：浮力和日月岁差。浮力是由内核流体在地球整体冷却过程中凝固到固体内核表面时释放出的潜热和光成分产生的，浮力释放与凝固速度成正比，因此与固体内核的增长速度成正比。岁差是地球极轴围绕一个约23.5度的圆锥体运动的名称;它大约每26，000年扫过一次完整的圆锥体。岁差是由太阳、月球和行星施加在地球赤道隆起上的引力力矩引起的。地幔和地核之间存在粘性和地形耦合，由于地核-地幔界面的轻微扁率而存在地形力矩。通过这种耦合，进动在核心中产生流体运动。在这两种机制中，浮力解释是研究得较好和被广泛接受的。然而，有一个困难：地球发电机的能量需求意味着内核的快速增长，根据目前的估计，它最多只有20亿年的历史。然而，古地磁学已经确定，地磁场以目前的强度存在了34亿多年。岁差解释不会遇到这种特殊的困难，但可能会遇到其他困难。这一点只有在对岁差发电机进行比目前更详细的研究之后才能知道。这是这个项目的主要目标，还有一个次要目标，即更好地了解水星等行星和木卫二等卫星因天平动而产生的流动。这是一个术语，当一个物体不保持相同的面对另一个物体时，它绕着另一个物体运行。例如，月球在绕地球运行时并不完全朝向地球。加州大学洛杉矶分校正在积极地实验研究由天秤驱动的运动，我们解决岁差驱动流的技术可以很容易地修改，以适用于这些实验和太阳系中的振动物体。出现的物体都必须假设这些物体是球形的。需要一种全新的方法来研究非球形物体中的旋进驱动流。我们最近开发并测试了一种数值方法，用于求解扁球形容器中的磁流体动力学方程，控制流体流动和磁场。计算机代码使用重叠网格上的有限差分及时地推进流动;以这种方式，网格映射球形容器，并且完全避免了称为极点问题的数值困难。已经取得了一些意想不到的、迄今为止令人费解的结果。我们打算进行许多探索性研究，试图阐明这些流动，以确定岁差在驱动地核运动和创造地球主磁场方面的重要性。

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.