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

在过去的160年里，地球的偶极磁场强度迅速下降了2.9%，这与南半球被称为南大西洋异常的广泛低场强有关。当外推到地球的内部深处，到液态地核和上覆的固态地幔之间的边界时，这种微弱的地表磁场在南非和巴塔哥尼亚下面出现了明显的极性反转。这种模式，加上磁场强度的迅速下降，导致了激烈争论的猜测，即地球正处于地磁极性逆转的早期阶段。这场争论的主要局限性是缺乏过去1500年来南半球关键地区的数据，以及对地球深部控制磁场的理论理解。为了解决这些局限性，该项目将完善南半球的古地磁记录，开发数值模拟来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）下方的不寻常地幔是否对南大西洋异常的特征进行一阶控制。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，制定修订后的地球发电机代码，并向社区提供，并支持研究生和本科生，他们将接受该领域的广泛培训，实验岩石和古地磁，该项目将支持一项合作研究，该研究将汇集不同学科和子学科的工作人员，以进一步完善古地磁记录，更直接地检验了低切变速度区对地磁场特征的控制作用。具体而言，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古地磁记录，以便在地球深部和考古问题上加以应用;（ii）进行岩石磁性、分析显微镜和古地磁分析，并处理磁场测量数据，以确定采集点周围的磁场，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，目的是测试拟议工作的主要假设;以及（iii）使用数值发电机模拟来研究非洲大的低剪切速度区是否能够局部地缺陷核心流，驱动磁雷诺数朝向1，该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.