Earth's Core as a Layered System

确定地球磁场的起源对于了解行星的宜居性和演化至关重要，并被广泛认为是地球科学的基本目标。数十亿年来，该领域一直在保护地表环境免受太阳辐射的影响，现在有助于减轻空间天气事件的影响，这些事件可能会扰乱电信和电网，从而产生重大的人为影响。然而，产生磁场的发电机过程发生在铁核，即地表下2800公里处的液态金属海洋。发电机与上覆地幔的条件密切相关，因此，现场观察结合地震数据和物理模型，提供了从地球形成到今天的地球内部深处的动力学和演化的独特见解。在标准发电机模型中，整个核芯经历湍流运动，并混合成均匀的成分。然而，地磁和地震证据表明，这个模型缺乏基本的物理学，需要重新考虑。最近，最引人注目的磁场变化、漂移和极性反转得到了前所未有的详细说明，这是41年前最后一次漂移的第一次全球表示，以及过去500万年的大量数据精选。然而，我的团队的工作表明，基于标准模型的大量发电机模拟未能产生与这些新数据集的主要特征相匹配的逆转行为。标准模型也无法解释异常地震结构--位于岩心顶部100-400公里处的E‘层和位于较低100-300公里处的F层--它们被解释为稳定分层，而不是处于湍流运动中。这些层的存在具有深远的影响，因为F层将能量输入传递到产生磁场的主体地核，而E‘层过滤我们在地球表面观察到的信号。我认为，地震和地磁观测都可以通过将地心视为一个耦合层系统来解释，每个耦合层都有自己独特的动力学。为了验证这一假设，我将开发地核耦合动力学(CODEC)框架，它包括与地核相关的旋转磁场条件下的两相和双扩散流的第一个模型，以及它们与湍流主体的耦合。现有的工作还没有研究F层的流体动力学，只考虑了E‘层的简单表示，所以我将首先对这些区域的过程进行详细的分析。然后，我将使用编解码器来阐明支持F层和E‘层地震观测的动力学、这些层的磁特征以及在迄今未探索的物理条件下的场产生过程。最后，我将使用编解码器对过去5~MYR的主要场变化进行第一次重建，从现在的快速动力学到极性反转，并研究编解码器预测未来场变化的能力。要实现这些目标，需要对现有的计算机代码进行重大改进，并首次对新的复杂方程组的解进行求解。不能保证编解码器将产生与观测结果相匹配的磁性特征：其基本原理是，通过结合正确的物理原理，所需的行为将自然出现，类似于单层发电机中自然出现的反转。编解码器只能通过结合流体动力学、材料和计算科学的尖端研究来实现，并将在这些领域中的每个领域产生新的结果。这项工作将对地球系统中的基本未知因素产生新的制约：核心组成、核心增长以及核心和地幔之间的热量和质量转移。分层似乎在类地行星核心中普遍存在，因此我们的结果将为了解这些天体的动力学和演化提供新的见解。

Establishing the origin of Earth's magnetic field is crucial for understanding planetary habitability and evolution and is widely recognised as a fundamental goal in Earth Science. The field has shielded the surface environment from solar radiation for billions of years and now helps mitigate against space weather events, which can have significant anthropogenic impacts by disrupting telecommunications and power grids. Yet the dynamo process that generates the field occurs in the iron core, an ocean of liquid metal 2800 km below the surface. The dynamo is intimately linked to conditions in the overlying mantle and so field observations, combined with seismic data and physical models, provide unique insight into the dynamics and evolution of Earth's deep interior from the formation of the planet to the present day. In the standard dynamo model the whole core undergoes turbulent motion and is mixed to uniform composition. However, geomagnetic and seismic evidence shows that this model lacks essential physics and needs to be reconsidered.The most striking magnetic field variations, excursions and polarity reversals, have recently been illuminated in unprecedented detail by the first global representations of the last excursion 41 kyrs ago and vast curation of data spanning the past 5 million years. However, my group's work has shown that a large suite of dynamo simulations based on the standard model fail to produce reversing behaviour that matches the main features of these new datasets. The standard model also fails to explain anomalous seismic structures - the E'-layer in the top 100-400 km and the F-layer in lower 100-300 km of the core - that are interpreted as stably stratified and not in turbulent motion. The presence of these layers has far-reaching consequences since the F-layer mediates the power input to the bulk core where the field is generated, while the E'-layer filters the signals that we observe at Earth's surface.I propose that both seismic and geomagnetic observations can be explained by viewing the core as a system of coupled layers, each with their own unique dynamics. To test this hypothesis I will develop the COupled Dynamics of Earth's Core (CODEC) framework, which comprises the first models of two-phase and double-diffusive flows in the rotating magnetic conditions relevant to Earth's core and their couplings to the turbulent bulk. Existing work has not yet studied F-layer fluid dynamics and has only considered simple representations of the E'-layer so I will first conduct detailed analysis of the processes in these regions. I will then use CODEC to elucidate the dynamics underpinning seismic observations of the F- and E'-layers, the magnetic signature of these layers, and the process of field generation, at hitherto unexplored physical conditions. Finally, I will use CODEC to produce the first reconstruction of the major field variations over the past 5~Myrs, from rapid present-day dynamics to polarity reversals, and investigate the capacity of CODEC for predicting future field variations.Achieving these goals requires major enhancements to existing computer codes and solutions of new and complex systems of equations for the first time. There is no guarantee that CODEC will produce magnetic features that match observations: the rationale is that by incorporating the correct physics the desired behaviour will emerge naturally, similar to how reversals emerged naturally in single-layer dynamos. CODEC can only be realised by combining cutting-edge research in fluid dynamics, materials and computational science and will yield novel results in each of these domains. The work will deliver new constraints on fundamental unknowns in the Earth system: core composition, inner core growth, and transfer of heat and mass between the core and mantle. Layering seems to be ubiquitous in terrestrial planetary cores and so our results will provide new insight into the dynamics and evolution of these bodies.