A New Generation of Forward and Inverse Geodynamo Models

你有没有想过为什么指南针指向北方？这个基本问题仍然没有解决，困扰着包括爱因斯坦在内的许多科学家。科学家们现在接受了地磁场的来源位于地球的核心，就像我们脚下数千英里处的巨大条形磁铁一样，辐射出触角状的磁场线，穿透地幔并环绕地球。虽然很明显，在地球表面，地磁场主要是偶极的（有北极和南极），但在核心边缘的磁场图像显示出许多更复杂的特征。此外，地磁场随时间缓慢变化。虽然在人类一生中这些变化很小，但地质学家已经表明，在地球历史的过程中，地磁场在全球范围内发生逆转：北极和南极大约每百万年交换几次位置。地磁场不仅对导航至关重要（许多动物，以及人类），而且为我们提供了一个电磁屏蔽，保护我们的星球免受有害的太阳和星际辐射，因此对气候有潜在的重要影响。特别令人感兴趣的是最近的证据表明，地球的偶极场正在失去力量，它可能在2000年内完全逆转。在逆转期间，电磁屏蔽被显著削弱，尽管我们的祖先在许多这样的事件中显然没有受到伤害，但我们不知道我们社会所依赖的许多卫星和其他技术基础设施将受到什么影响。充分了解地球磁场的产生及其逆转无疑将有助于我们限制可能发生的任何破坏性影响。地磁场是由所谓的地球发电机机制产生的，由地球核心的熔融铁运动提供动力。科学家们通过使用模拟地核过程的复杂计算机模型来研究地球发电机。然而，由于当前技术固有的严重计算困难，必须改变模拟地球的物理性质，以便使问题在当今的计算机上可解决。不可能忠实地模拟地球的核心，对这种地球发电机模型的许多解释，包括全球逆转的触发因素，仍然存在意见分歧。另一种技术是利用对地球表面磁场的观测来对磁场和核心边缘的熔融铁流结构进行成像。这在揭示许多有趣的动力学方面取得了巨大的成功，尽管与试图通过简单地检查海洋表面来了解世界海洋的内部流动一样，不可能对核心内部的过程说得太多。因此，地球内部的许多方面和地磁场的来源仍然是一个谜。今年早些时候，在一个令人兴奋的新发展中，我提出了一些重要的理论进展，为构建新一代地球发电机模型铺平了道路。这项新技术提供了一种全新的方法，可以直接在计算机上访问地核的真实属性，我将能够制作出第一个能够准确模拟地核的地球发电机模型。这将有助于对产生地磁场的机制进行强有力的定性，并详细分析相关现象，包括磁场逆转。此外，通过进一步利用新的理论发展，我将能够利用观测来探测地核内部的当今磁场结构，这也是以前从未有过的。这两个互补的项目将彻底改变地球发电机建模的最新技术，并将为地球核心的动态提供前所未有的见解。

Have you ever wondered why compasses point north? This fundamental question, still unsolved, has perplexed many scientists including the likes of Einstein. Scientists now accept that the source of the geomagnetic field lies inside Earth's core and, like a giant bar magnet thousands of miles beneath our feet, radiates tentacle-like magnetic field lines that penetrate the mantle and wrap around the Earth. Although it is apparent that, at the Earth's surface, the geomagnetic field is principally dipolar (having north and south poles), images of the field at the edge of the core show many more complex features. Furthermore, the geomagnetic field varies slowly with time. Although over the duration of a human lifetime these changes are small, geologists have shown that, over the course of the Earth's history, the geomagnetic field globally reverses: the north and south poles swap places a few times every million years or so. The geomagnetic field is not only crucial for navigation (by many animals, as well as humans) but provides us with an electromagnetic shield that protects our planet from harmful solar and interstellar radiation, and consequently has a potentially important influence on climate. Of particular interest is the recent evidence that the Earth's dipolar field is losing strength and that it may completely reverse within 2000 years. During reversals the electromagnetic shield is significantly weakened and, although our ancestors have survived many such events apparently without harm, it is unknown how the many satellites and other technological infrastructure, that our society relies upon, will be affected. A full understanding of the generation of the Earth's magnetic field and its reversals would undoubtedly help us to limit any destructive effects that might occur. The geomagnetic field is generated by the so-called geodynamo mechanism, powered by motions of molten iron in the Earth's core. Scientists study the geodynamo by using complex computer models that simulate the processes in the core. However, because of severe computational difficulties inherent in current techniques, the physical properties of the modelled Earth must be altered in order to make the problem solvable on present-day computers. It is not possible to faithfully simulate the Earth's core, and opinion remains divided over the many interpretations of such geodynamo models, including the trigger for global reversals. An additional technique is to use observations of the magnetic field on Earth's surface to image the field and the structure of the flow of molten iron at the edge of the core. This has been enormously successful in revealing many interesting dynamics although, rather like attempting to understand the internal currents of the world's oceans by simply inspecting the ocean surface, it is impossible to say very much about the processes inside the core. Consequently, many aspects of the Earth's interior and source of the geomagnetic field remain a mystery. Earlier this year in an exciting new development, I presented some crucial theoretical advances that pave the way for constructing a new generation of geodynamo models. The new technique supplies a fundamentally new approach that allows realistic properties of the core to be accessed directly on computers, and I will be able to produce the first geodynamo models able to simulate the Earth's core accurately. This will enable robust characterisation of the mechanism that generates the geomagnetic field and detailed analysis of the associated phenomena, including magnetic reversals. Additionally, by further exploitation of the new theoretical developments, I will be able to use observations to probe the present-day magnetic field structure inside the core, again something that has never before been possible. These two complementary projects will revolutionise the state-of-the-art in geodynamo modelling and will provide unprecedented insights into the dynamics of the Earth's core.