Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA.
Rep Prog Phys. 2013 Sep;76(9):096801. doi: 10.1088/0034-4885/76/9/096801. Epub 2013 Sep 4.
Few areas of geophysics are today progressing as rapidly as basic geomagnetism, which seeks to understand the origin of the Earth's magnetism. Data about the present geomagnetic field pours in from orbiting satellites, and supplements the ever growing body of information about the field in the remote past, derived from the magnetism of rocks. The first of the three parts of this review summarizes the available geomagnetic data and makes significant inferences about the large scale structure of the geomagnetic field at the surface of the Earth's electrically conducting fluid core, within which the field originates. In it, we recognize the first major obstacle to progress: because of the Earth's mantle, only the broad, slowly varying features of the magnetic field within the core can be directly observed. The second (and main) part of the review commences with the geodynamo hypothesis: the geomagnetic field is induced by core flow as a self-excited dynamo. Its electrodynamics define 'kinematic dynamo theory'. Key processes involving the motion of magnetic field lines, their diffusion through the conducting fluid, and their reconnection are described in detail. Four kinematic models are presented that are basic to a later section on successful dynamo experiments. The fluid dynamics of the core is considered next, the fluid being driven into motion by buoyancy created by the cooling of the Earth from its primordial state. The resulting flow is strongly affected by the rotation of the Earth and by the Lorentz force, which alters fluid motion by the interaction of the electric current and magnetic field. A section on 'magnetohydrodynamic (MHD) dynamo theory' is devoted to this rotating magnetoconvection. Theoretical treatment of the MHD responsible for geomagnetism culminates with numerical solutions of its governing equations. These simulations help overcome the first major obstacle to progress, but quickly meet the second: the dynamics of Earth's core are too complex, and operate across time and length scales too broad to be captured by any single laboratory experiment, or resolved on present-day computers. The geophysical relevance of the experiments and simulations is therefore called into question. Speculation about what may happen when computational power is eventually able to resolve core dynamics is given considerable attention. The final part of the review is a postscript to the earlier sections. It reflects on the problems that geodynamo theory will have to solve in the future, particularly those that core turbulence presents.
目前，地球物理学中很少有领域像基础地磁学发展得如此迅速，基础地磁学旨在探究地球磁场的起源。卫星源源不断地传输着有关当前地磁场的数据，为从岩石磁场中推断出的过去更遥远时期的地磁场提供了越来越多的信息。本综述的第一部分总结了现有地磁数据，并对地核中电流层表面的大尺度地磁场结构做出了重要推断。在地核中，磁场起源于电流层。在该部分，我们认识到了进展的第一个主要障碍：由于地幔的存在，只能直接观测到磁场在核心内的宽而缓慢变化的特征。综述的第二部分（也是主要部分）从地球发电机假说开始：地磁场是由核心流作为自激发电机感应产生的。其电动力学定义了“运动发电机理论”。详细描述了涉及磁场线运动、它们在导电流体中的扩散以及它们的重联等关键过程。介绍了四个基本的运动模型，这对于后面关于成功的发电机实验的部分非常重要。接下来考虑了地核的流体动力学，地核中的流体由于地球从原始状态冷却而产生的浮力而被驱动成运动状态。由此产生的流动强烈受到地球自转和洛伦兹力的影响，洛伦兹力通过电流和磁场的相互作用改变流体运动。“磁流体力学（MHD）发电机理论”这一节专门讨论了这种旋转磁对流。地磁现象的 MHD 理论处理最终归结为对其控制方程的数值求解。这些模拟有助于克服进展的第一个主要障碍，但很快又遇到了第二个障碍：地核的动力学过于复杂，其作用的时间和长度尺度太宽，无法通过任何单一的实验室实验来捕捉，也无法在当今的计算机上解决。因此，对实验和模拟的地球物理相关性提出了质疑。对地核动力学的计算能力最终能够解决时可能会发生的情况的推测得到了相当大的关注。综述的最后一部分是前几部分的后记。它反映了未来地球发电机理论将必须解决的问题，特别是核心湍流带来的问题。
