Collaborative Research: CSEDI--Interdisciplinary Investigation of Geodynamo Reversal Mechanisms

这项合作研究结合了地球古代磁场的测量、地球深层结构的信息和地球发电机的数值模型，以研究地磁极性反转的原因和后果。这项研究的主要目标是建立液体外核中地球发电机反转其磁极性的物理机制，并确定对极性反转过程的其他地球物理控制，特别强调地球的固体地幔如何影响地质时期极性反转的频率。该研究使用发电机过程的第一原理数值模型。这些模型在与地核具有相同径向结构的旋转导电球体中同时求解流体力学、热和质量传输以及电磁学方程，并受到多种边界条件选择的影响。数值模型的结果正在使用先前建立的缩放关系缩放到地球的核心条件。研究人员正在将数值发电机模型的时变输出与极性变化和偏移期间的古磁场记录进行比较。发电机模型反转行为被用来识别液体外核和固体内核中的流体动力学条件，这些条件产生快速极性反转的特征古地磁特征，这些极性反转由具有相对均匀强度的偶极子主导磁场的长期稳定极性历元分隔开。该项目的第一部分是描述在核心-地幔边界具有均匀（均匀）条件的标准、基准数值发电机的极性反转机制。该项目的第二部分是确定发电机模型反转对模拟地幔随时间变化的变化边界条件的敏感性。特别是，研究人员正在测试异质的核心-地幔边界热条件，这些条件与下地幔地震成像三维结构的相互竞争的解释相对应。这一阶段的研究旨在确定导致地磁场在可逆和不可逆（超时间）状态之间交替的地球物理因素。第三部分旨在通过确定磁场结构和极性反转行为的变化，使数值模型更接近地球发电机，因为对流强度、旋转速率、扩散率和其他模型参数逐渐增加到地球核心的实际值。在开发与地震、古地磁、地磁观测一致的反转发电机模型时，本研究为驱动磁层和大气模型提供了一个基于第一原理的工具，可用于预测与过去和未来地磁场总体变化相关的环境影响。

This collaborative research combines measurements of the Earth's ancient magnetic field, information on deep Earth structure, and numerical models of the geodynamo, to investigate the causes and consequences of geomagnetic polarity reversals. The broad objectives of this research are to establish the physical mechanisms in the liquid outer core by which the geodynamo reverses its magnetic polarity, and identify other geophysical controls on the polarity reversal process, with special emphasis on how the Earth's solid mantle affects the frequency of polarity reversals through geologic time. The investigation uses first-principles numerical models of the dynamo process. These models solve the equations of fluid mechanics, heat and mass transport, and electromagnetism simultaneously in a rotating, electrically-conducting sphere with the same radial structure as the Earth's core, subject to a wide choice of boundary conditions. The results of the numerical models are being scaled to Earth's core conditions using previously-established scaling relationships. The investigators are comparing the time-variable output of the numerical dynamo models with the record of the paleomagnetic field during polarity changes and excursions. The dynamo model reversal behavior is being used to identify the fluid dynamical conditions in the liquid outer core and solid inner core that produce characteristic paleomagnetic signature of rapid polarity reversals separated by long stable polarity epochs with a relatively uniform intensity, dipole-dominant magnetic field. The first part of the project is to delineate the polarity reversal mechanism of a standard, benchmark numerical dynamo with homogeneous (uniform) conditions at the core-mantle boundary. The second part of the project is to determine the sensitivity of the dynamo model reversals to varying boundary conditions that simulate changes in the mantle through time. In particular, the investigators are testing heterogeneous core-mantle boundary thermal conditions that correspond to competing interpretations of the seismically-imaged three dimensional structure of the lower mantle. This phase of the research is aimed at establishing the geophysical factors that cause the geomagnetic field to alternate between reversing and nonreversing (superchron) states. The third part aims to bring the numerical model closer to the geodynamo, by determining the changes in magnetic field structure and polarity reversal behavior as the vigor of convection, the rate of rotation, diffusivities, and other model parameters are progressively increased toward realistic values for the Earth's core.In developing a reversing dynamo model that is consistent with seismic, paleomagnetic, geomagnetic observations, this study is providing a first-principles-based tool for driving magnetospheric and atmospheric models that can be used to predict environmental effects associated with gross changes in the geomagnetic field, past and future.