NSFGEO-NERC: Deciphering the Dynamics of Geomagnetic Excursions

地球的内部磁场是由液体中的流体运动产生的，它的持续存在具有重要的社会意义，因为它除了提供导航指导外，还有效地保护地表环境不受太阳风和空间天气的影响。磁场在广泛的时间尺度上变化，而在地质时间尺度上发生的最显著的变化是极性漂移和反转。这些事件可能导致全球磁场强度显著减弱，产生因减少地表环境的磁屏蔽而产生的社会影响，并可能是理解在液核中产生和维持地球磁场的过程的核心。与逆转相比，短途旅行受到的关注相对较少，尽管发生的频率更高，持续的时间也足够长，足以造成潜在的重大社会混乱。最近的观测模型跨越了过去的几十万年，正在以以前无法理解的细节阐明了偏移场的演变。当与物理场产生过程的数值模拟的进展相结合时，这为理解这种神秘事件的新方法铺平了道路。这项工作的一个重要方面将是建立模型，使地磁、古地磁和数值地球发电机以及国际地球内部和行星科学界的研究能够进一步协同。这是加州大学圣地亚哥分校和利兹大学的科学家合作提出的建议，因此由国家科学基金会(NSF)和英国自然环境研究理事会(NERC)共同资助。地球磁场的几种全球、空间和时间表示涵盖0-100ka或其子部分，并记录了多达5次地磁漂移，既有性质上的相似之处，也有明显的差异。可以通过古分子变化指数对偏移场扰动、持续时间、局部异步性等进行正式定义，从而与数值地球发电机模拟进行比较。将时间间隔扩展到0-120ka，我们将产生第一个具有量化不确定度的高分辨率模式，分别用于95ka的后布莱克漂移和20ka的Hilina Pali漂移。同时，数值模拟将建立在英国利兹大学的研究基础上，该研究正在定义“类似地球”的领域行为的要求。这项建议的重点是利用这两个相互关联的协同组成部分增进对地磁漂移的性质和来源的了解。我们将(1)使用新的和现有的全球和与时间相关的过去120KYR期间几次漂移的观测模型来描述漂移之前、期间和之后的现场行为；(2)分析新的地球发电机模拟中的漂移机制和可预测性，该模拟采用反映地球液态外核预期行为的主导力量平衡；(3)制定定义漂移的正式标准，并将古场行为与当前和新兴的地球类数值发电机模拟观点相协调。这项建议的一个重要方面是利用斯克里普斯海洋研究所(SIO)观测地磁场模拟和古分子变化分析以及核心动力学的利兹模拟之间的现有协同作用。观测模型将有助于建立新的类似地球的模拟标准，这反过来又将用于对可观测到的场变化进行高分辨率预测，这可以在较低分辨率的观测数据中寻找到。我们期望加强对地磁漂移动态的了解，包括它们的可预测性和与极性反转的关系。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Earth’s internal magnetic field is produced by fluid motions in the liquid, and its sustained presence is of great societal importance because, in addition to providing navigational guidance, it effectively shields the surface environment from the solar wind and space weather. The magnetic field varies on a wide range of time scales and the most striking variations occurring on geological time scales are polarity excursions and reversals. These events can lead to significantly weakened the global field strength, have the societal impacts arising from reduced magnetic shielding of the surface environment, and may be central to understanding the processes that generate and sustain the geomagnetic field in the liquid core. Excursions have received relatively little attention compared to reversals despite occurring more frequently and lasting long enough to cause potentially significant societal disruption. Recent observational models spanning the past hundred thousand years are illuminating the evolution of excursional fields in previously inaccessible detail. When combined with advances in numerical simulation of the physical field generation process, this paves the way for a new approach to understand such enigmatic events. An integral aspect of the work will be building models enabling further synergy across the geomagnetic, paleomagnetic, and numerical geodynamo and, by extension, the international Studies of Earth’s Deep Interior and planetary science communities. This is a collaborative proposal between scientists at the University of California San Diego and Leeds University, and is therefore co-funded by the National Science Foundation (NSF) and the United Kingdom’s Natural Environment Research Council (NERC). Several global, spatial, and temporal representations of Earth’s magnetic field cover 0-100 ka or subsections thereof, and document up to 5 geomagnetic excursions with qualitative similarities as well as distinct differences. Formal definition of excursional field perturbations, duration, local asynchroneity, etc., can be made via the Paleosecular Variation Index allowing comparisons with numerical geodynamo simulations. Expanding the time interval to 0-120 ka, we will produce the first high-resolution models with quantified uncertainty for the Post-Blake excursion at ~95 ka and the Hilina Pali excursion at ~20 ka. In parallel, numerical simulations will build on research at Leeds University (UK) that is defining the requirements for “Earth-like” field behavior. The focus of this proposal is to improve understanding of the nature and origin of geomagnetic excursions using these two synergistic interlinked components. We will(1) Use new and existing global and time-dependent observational models of several excursions during the past 120 kyr to characterize field behavior before, during, and after excursions;(2) Analyze the excursion mechanism and predictability in new geodynamo simulations conducted with a dominant force balance that reflects expected behavior in Earth’s liquid outer core;(3) Develop formal criteria for defining excursions and reconcile paleofield behavior with current and emerging views on Earth-like numerical dynamo simulations.A vital aspect of this proposal is exploiting existing synergy between the Scripps Institution of Oceanography (SIO) observational geomagnetic field modelling and paleosecular variation analysis and Leeds simulations of core dynamics. Observational models will contribute to new Earth-like standards for simulations, which will in turn be used to make high-resolution predictions of observable field variations that can be sought in the lower resolution observational data. We expect to enhance understanding of the dynamics of geomagnetic excursions, including their predictability and relation to polarity reversals.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.