Collaborative Research: Archeomagnetism of southern Africa and dynamo modeling: Testing the hypothesis of South Atlantic Anomaly-Large Low Shear Velocity Province Agency

地球偶极子磁场的强度一直在迅速下降，在过去的160年里下降了9%，这种下降与南半球一大片被称为南大西洋异常的低场强有关。当推算到地球深处，到液核和上覆的固体地幔之间的边界时，这个微弱的表面场在南非和巴塔哥尼亚下面看起来是反转的突出斑块。这种模式，加上快速下降的磁场强度，导致了人们热议的猜测，即地球正处于地磁极性反转的早期阶段。这场辩论的主要局限是缺乏过去1500年南半球关键区域的数据，以及对地球深部对磁场的控制的理论理解。为了解决这些限制，该项目将完善南半球的古地磁记录，开发数值模拟以探索地球深部对磁场的影响，并将这两个组成部分结合起来，以探索南非和巴塔哥尼亚(非洲低剪切速度省)之下的异常地幔是否对南大西洋异常的特征施加了一级控制。该项目将促进与来自南非、博茨瓦纳和津巴布韦的考古学家的国际合作，开发将提供给社区的修订的地球发电机代码，并支持将在实地、实验岩石和古地磁学以及地球物理建模方面接受广泛培训的研究生和本科生。该项目将支持一项合作研究，该研究将汇集不同学科和分学科的工作人员，以进一步完善考古地磁记录，并更直接地测试低剪切速度省控制地磁场特征的有效性。具体地说，研究小组将：(I)进行实地研究，包括实地调查和收集材料，目的是完善铁器时代的考古地磁记录，潜在地应用于地球深处和考古问题；(Ii)对野外磁力测量数据进行岩石磁学、分析显微镜和古地磁分析和处理，以确定收集地点的环境磁场、收集的考古文物中的磁性载体以及方向和强度的古代记录，目的是验证拟议工作的基本假设；以及(Iii)使用数值发电机模拟来调查非洲大的低剪切速度省是否能够局部地缺陷地芯流，使磁雷诺数趋于一致，有利于磁扩散和磁通量排出，从而对在地球表面进行的地磁观测设置一级控制。

Earth’s dipole magnetic field has been rapidly decreasing in strength, by ∼9% over the past 160 years, with the decline linked to a broad swath of low field intensity in the Southern Hemisphere known as the South Atlantic Anomaly. When extrapolated to Earth’s deep interior, to the boundary between the liquid core and overlying solid mantle, this weak surface field appears as prominent patches of reversed polarity beneath South Africa and Patagonia. This pattern, together with the rapidly decreasing field strength, has led to hotly debated speculation that Earth is in the early stages of a geomagnetic polarity reversal. Principal limitations in this debate have been a lack of data for the last 1500 years from key regions in the Southern Hemisphere and theoretical understanding of deep Earth controls on magnetic fields. To address these limitations, this project will refine the archeomagnetic record of the southern Hemisphere, develop numerical simulations to explore the affect the deep Earth has on magnetic fields, and integrate these two components to explore whether the unusual mantle beneath South Africa and Patagonia (the African Low Shear Velocity Province) places a first-order control on the character of the South Atlantic Anomaly. This project will foster international collaboration with archeologists from South Africa, Botswana, and Zimbabwe, develop a revised geodynamo code that will be made available to the community, and support of graduate and undergraduate students who will receive a broad training in the field, experimental rock and paleomagnetism, and geophysical modeling.This project will support a collaborative study that will bring together workers in disparate disciplines and subdisciplines to further refine the archeomagnetic record and more directly test the efficacy of Low Shear Velocity Province control on the character of the geomagnetic field. Specifically, the research team will: (i) conduct field studies including field surveys and collection materials with the goal of refining the Iron Age archeomagnetic record with potential application to both deep Earth and to archeological problems; (ii) conduct rock magnetic, analytical microscopy, and paleomagnetic analyses and processing of field magnetic survey data to define ambient magnetic fields of the collection sites, magnetic carriers in the collected archeological artifacts, and ancient recordings of direction and intensity with the goal of testing primary hypothesis of the proposed work; and (iii) use numerical dynamo simulations to investigate whether the African Large Low Shear Velocity Province can locally defect core flow, drive the magnetic Reynolds number toward unity, favoring magnetic diffusion and flux expulsion, and thus place first-order controls on geomagnetic observations made at Earth’s surface.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.