CMG Research: Constraints on the Earth's thermal history and probing the geomagnetic field beneath the core-mantle boundary using nonlinear inversions

CMG 研究：地球热历史的约束以及利用非线性反演探测核幔边界下方的地磁场

• 批准号: 0724331
• 负责人: Glenn Ierley
• 金额: $ 24.44万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-10-01 至 2010-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0724331&HistoricalAwards=false
• 关键词: CMG; Research; Constraints; Earth; thermal

Current thinking on the Earth's thermal history is marked by notable controversies on the age of the inner core and whether core radiogenic heating is required in order to maintain the magnetic field. A key factor in this balance is the power required to drive the geodynamo, dissipated internally as heat. Present estimates, primarily based on numerical dynamo simulations operating far from the relevant parameter range, vary significantly. The aim of this project is to constrain the Earth's thermal history by finding rigorous lower bounds for the dissipation of the present field. Building on an existing bound based on constraints from observation and models for the Earth's nutation, the proposed addition of the Taylor constraint will impose a significant dynamical restriction long known to be appropriate for the core field. If the resulting bound is sufficiently increased, then contentious scenarios for the lifetime of the inner core without radiogenic heating may well be excluded. Further, this variational formulation exposes some intriguing mathematical issues germane to consideration of the geodynamo as a dynamical system. The optimal fields resulting will be both three-dimensional and time-dependent and thus for the first time give evidence of whether or not the large body of recent observationally-derived findings of large-scale surface features are really representative of the internal field.Large scale computer simulation has fundamentally altered the way in which science is done. Yet for all the numerous advances, many of the most demanding computations in the area of geophysical fluid dynamics require that key properties, notably viscosity (the fluid "thickness"), be set to artificially high values. The effects of this adjustment on the numerical results are poorly understood and will remain so short of a better theory of turbulence than any presently in prospect. The so-called variational methods outlined in this proposal will both complement and critique numerical simulations of the Earth's dynamo. The results do not rely on artificial parameterizations and their interpretation as providing a bound on heat generated by the dynamo process is thus entirely rigorous. Key to this work will be a collaborative effort to link extensive long term observational data with a mathematically appropriate and numerically tractable formulation of the problem.

当前对地球热史的思考以内核年龄以及是否需要核心放射加热来维持磁场等问题存在着显着争议。 这种平衡的一个关键因素是驱动地球发电机所需的功率，并以热量的形式在内部消散。目前的估计主要基于远离相关参数范围的数值发电机模拟，差异很大。该项目的目的是通过找到当前场耗散的严格下限来限制地球的热历史。建立在基于观测约束和地球章动模型的现有界限的基础上，提议添加的泰勒约束将施加长期以来已知适合核心场的显着动力学限制。如果所得的界限充分增加，那么在没有放射加热的情况下内核寿命的有争议的情况就可以很好地排除。 此外，这种变分公式揭示了一些与将地球发电机视为动力系统密切相关的有趣数学问题。 由此产生的最佳场将是三维的且与时间相关，因此第一次提供了证据，证明最近通过观察得出的大规模表面特征的大量发现是否真正代表了内部场。大规模计算机模拟从根本上改变了科学研究的方式。然而，尽管取得了如此多的进步，地球物理流体动力学领域中许多最苛刻的计算都需要将关键属性，特别是粘度（流体“厚度”）设置为人为的高值。人们对这种调整对数值结果的影响知之甚少，而且仍然缺乏比目前任何一种更好的湍流理论。 该提案中概述的所谓变分方法将补充和批评地球发电机的数值模拟。结果不依赖于人工参数化，并且它们的解释因为对发电机过程产生的热量提供了限制，因此是完全严格的。这项工作的关键是共同努力，将广泛的长期观测数据与数学上适当且数字上可处理的问题表述联系起来。