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

这项合作研究结合了对地球古代磁场的测量、地球深部结构的信息和地球发电机的数值模型，以调查地磁极性反转的原因和后果。这项研究的广泛目标是建立液体外核中地球发电机用来反转其磁极的物理机制，并确定对反转过程的其他地球物理控制，特别强调地球固体地幔如何影响地质时间中极性反转的频率。这项研究使用了发电机过程的第一性原理数值模型。这些模型在一个旋转的导电球体中同时求解流体力学、热和质量传输以及电磁学的方程，该球体具有与地核相同的径向结构，受多种边界条件的限制。数值模型的结果正在使用先前建立的比例关系按地球的核心条件进行比例调整。研究人员正在将发电机数值模型的时变输出与极性变化和漂移期间的古地磁场记录进行比较。发电机模型的反转行为正被用来识别液体外核和固体内核中产生快速极性反转的特征古地磁特征的流体动力学条件，这些特征被具有相对均匀的强度、偶极主导的磁场的长的稳定的极性纪元隔开。该项目的第一部分是描述在核-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不同边界条件的敏感性。特别是，研究人员正在测试不同的核-地幔边界热条件，这些条件与对地震成像的下地幔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反转和非反转(超时)状态之间交替的地球物理因素。第三部分旨在使数值模型更接近地球发电机，通过确定随着对流强度、自转速度、扩散系数和其他模型参数逐渐增加到地核的实际值，磁场结构和极性反转行为的变化。在开发与地震、古地磁、地磁观测一致的反转发电机模型的过程中，这项研究提供了一个基于第一性原理的工具，用于驱动磁层和大气模型，可用于预测过去和未来与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.