A new model of the geodynamo: large-scale vortices in the Earth's core

几个世纪以来，人类已经意识到地球上磁场的存在，因为它对磁化物体的作用，比如指南针的指针。测量其强度和方向的科学仪器表明，地球磁场（称为地磁场）在地球表面主要是偶极的，就像棒状磁铁产生的磁场一样。仪器进一步揭示了地磁场表现出更复杂的特征，例如区域斑块（半径约1000公里）的极性反转。地球磁场在人的一生中变化缓慢，但在地球历史的进程中，地球物理学家已经表明，它变化很大，有时会经历全球极性逆转，即南北磁极互换位置。这种全球倒转大约每百万年发生几次。每次全球倒转只需要大约5000年，在这段时间里，地磁场很弱，很可能是混乱的。地磁场不仅对导航至关重要（许多动物和人类都利用它），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辐射。在全球倒转期间，这种电磁屏蔽被大大削弱，如果今天发生倒转，它将对太空卫星和电网造成巨大损害。上一次全球倒转发生在78万年前，远在现代科技出现之前。近150年来，地球磁场强度一直呈下降趋势，这与地磁倒转斑块的出现是一致的。这些逆转的斑块是否是全球逆转的前兆还不清楚，全球逆转的原因也不清楚。为了预测地磁场的变化，这将有助于我们限制潜在的破坏性影响，我们需要更好地了解产生地磁场的过程。地磁场是在地球深处产生的，在由熔融铁组成的外核。熔融铁的运动通过一种叫做地球发电机的物理过程产生电流，从而产生磁场。地球发电机是由非线性数学方程控制的，这些方程只能借助计算机来求解。然而，即使是最强大的计算机也难以模拟核心中普遍存在的极端条件及其确切的物理性质。因此，计算机模型使用了强烈改变的物理性质，以便在当今的计算机上解决问题，这可能导致在将模型结果重新缩放到核心性质时出现不一致。特别是，目前的模型发现，地球发电机是由大约100米的铁水运动产生的。然而，关于地磁场产生的理论争论暗示，这些运动发生在更大的空间尺度上，这一结论被对直径约1000公里的极性反转斑块的观测所加强。因此，在目前的模型中缺少一个解释大规模流体运动形成的关键机制。在我的研究期间，我将通过研究一种新的机制来解决这个问题，该机制将解释在地核普遍存在的条件下如何形成大规模的流动。这个新机制是基于我最近使用一个简化的计算机模型的工作：我演示了从湍流的小尺度运动中形成大规模、长寿命的气旋（有点类似于在大气中观察到的热带气旋）。我将研究这些旋风是否可以在地核中存在，通过扩展我之前的结果到一个现实的地核模型，以及它们是否可以产生类似地球的磁场。然后，我将研究极性反转的斑块是否与这些大尺度气旋有关，以及它们是否是全球反转的前兆。

For centuries, humans have been aware of the presence of a magnetic field on Earth because of its action on magnetised objects, such as the needle of a compass. Scientific instruments that measure its strength and direction show that the Earth's magnetic field (called the geomagnetic field) is predominantly dipolar at the Earth's surface, like the magnetic field produced by a bar magnet. The instruments further reveal that the geomagnetic field displays more complex features, such as regional patches (of about 1000km in radius) of reversed polarity. The geomagnetic field varies slowly on a human lifetime, but over the course of the Earth's history, geophysicists have shown that it varies considerably and sometimes undergoes global polarity reversals, where the north and south magnetic poles swap places. These global reversals occur a few times every million years or so. Each global reversal takes only about 5000 years, and during this time, the geomagnetic field is weak and probably disorganised. The geomagnetic field is not only crucial for navigation (used by many animals, as well as humans) but provides us with an electromagnetic shield that protects our planet from harmful solar radiation. During a global reversal, this electromagnetic shield is significantly weakened, and if a reversal occurred today it would cause tremendous damage to space satellites and electrical power grids. The last global reversal occurred about 780,000 years ago, long before the advent of our modern technologies. The magnetic field strength has been decreasing for the last 150 years, coinciding with the appearance of the regional patches of reversed polarity. Whether these reversed patches are precursors for a global reversal is unknown, as is the cause of the global reversals. To predict the changes in the geomagnetic field, which would help us limit potential destructive effects, we need to better understand the processes that generate the geomagnetic field.The geomagnetic field is generated deep inside the Earth, in the outer core, which is composed of molten iron. Motions of molten iron generate electric currents that induce the magnetic field, through a physical process called geodynamo. The geodynamo is governed by nonlinear mathematical equations, which can only be solved with the help of computers. However, even the most powerful computers struggle to model the extreme conditions that prevail in the core and its exact physical properties. Thus the computer models use strongly altered physical properties in order to make the problem solvable on present-day computers, potentially leading to inconsistencies when rescaling the results of the models to the core properties. In particular, current models find that the geodynamo is produced by motions of molten iron of only about 100m. However, theoretical arguments about the generation of the geomagnetic field imply that these motions occur on much larger spatial scales, and this conclusion is reinforced by the observation of the patches of reversed polarity that measure about 1000km across. A key mechanism is therefore missing in the current models to explain the formation of large-scale fluid motions. During my fellowship, I will address this problem by studying a new mechanism that explains how large-scale flows can form under the conditions that prevail in the Earth's core. This new mechanism is based on my recent work using a simplified computer model: I demonstrated the formation of large-scale, long-lived cyclones (somewhat similar to the tropical cyclones observed in the atmosphere) from turbulent smaller scale motions. I will study whether these cyclones can be present in the Earth's core by extending my previous results to a realistic model of the core, and whether they can produce Earth-like magnetic fields. I will then investigate whether the patches of reversed polarity are associated with these large-scale cyclones and whether they are precursors for the global reversals.