Earth's Core as a Layered System

确定地球磁场的起源对于了解行星的可居住性和演化至关重要，并且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不受太阳辐射的影响，现在有助于减轻空间天气事件的影响，这些事件可以通过破坏电信和电网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地核，即地表以下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，结合地震数据和物理模型的实地观测，为从地球形成到现在的地球深层内部的动力学和进化提供了独特的见解。在标准发电机模型中，整个堆芯经历湍流运动并混合成均匀成分。然而，地磁和地震证据表明，该模型缺乏基本的物理性质，需要重新考虑。最引人注目的磁场变化、偏移和极性逆转，最近通过41年前的最后一次偏移的首次全球呈现和跨越过去500万年的大量数据，以前所未有的细节被阐明。然而，我的团队的工作表明，基于标准模型的大型发电机模拟套件无法产生与这些新数据集的主要特征相匹配的反转行为。标准模型也无法解释异常地震结构——位于地核顶部100-400公里的E层和位于地核下方100-300公里的f层——它们被解释为稳定分层而不是湍流运动。这些层的存在具有深远的影响，因为f层将能量输入到产生磁场的核心，而E层过滤我们在地球表面观察到的信号。我建议，地震和地磁观测都可以通过将地核看作一个耦合层系统来解释，每个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，其中包括与地核相关的旋转磁条件下的两相和双扩散流动及其与湍流体的耦合的第一个模型。现有的工作尚未研究f层流体动力学，只考虑了E层的简单表示，因此我将首先对这些区域的过程进行详细分析。然后，我将使用CODEC来阐明在迄今尚未探索的物理条件下，F层和E层地震观测的动力学基础，这些层的磁特征以及磁场产生的过程。最后，我将使用CODEC对过去5~Myrs的主要磁场变化进行首次重建，从当前的快速动态到极性逆转，并研究CODEC预测未来磁场变化的能力。实现这些目标需要对现有的计算机代码进行重大改进，并首次解决新的和复杂的方程组。不能保证CODEC会产生与观测相符的磁性特征：基本原理是，通过结合正确的物理原理，期望的行为将自然出现，类似于单层发电机自然出现的反转。CODEC只能通过结合流体动力学，材料和计算科学的前沿研究来实现，并将在每个这些领域产生新的结果。这项工作将为地球系统的基本未知提供新的约束：地核组成、内核生长、地核和地幔之间的热量和质量传递。层状结构似乎在类地行星核心中无处不在，因此我们的研究结果将为这些天体的动力学和进化提供新的见解。

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.