Archeomagnetism of Southern Africa: Implications for Longevity of the South Atlantic Anomaly

地球最近磁场中的一个广泛的低强度区域横跨南大西洋，非洲和南美洲。这通常被称为南大西洋异常（SAA）。SAA允许相对接近地球的辐射带，影响航天器的操作。低磁场强度降低了该区域的磁屏蔽效率，这可能会影响大气臭氧。许多人认为SAA与过去160年中偶极子地磁场强度的急剧衰减以及南非下方地球核心上反向磁场区域的增长有关。有些人甚至推测地球正在走向地磁场逆转。但是，由于缺乏南半球的古地磁数据（也就是说，来自考古对象的数据被烧到高温，随后在冷却时保存了地球磁场的记录），因此在较长期的地磁历史背景下理解这些现象受到了限制。我们的研究和教育计划旨在了解南部非洲记录的SAA地区地磁场的历史。工作的主要重点是收集和随后分析来自铁器时代燃烧结构的古地磁数据，并测试模型，说明地核和地幔之间的边界性质如何引起地磁场的近期变化。1000-1650 AD）在1300 AD显示了急剧的强度下降，与现代南大西洋异常（SAA）的现代场变化相当，但值较低。低场值的重现可能反映了由地震学（特别是非洲大低速地震省，或LLVSP）定义的南部非洲下不寻常的核-幔边界成分和结构促进的来自核的磁通量排出。由于非洲LLVSP是一个长期存在的结构，这个地区可能是一个稳定的网站的通量驱逐，也许是触发网站的地磁反转，在数百万年的时间尺度。 如果正确的话，这个概念模型是变革性的，因为它表明逆转不会在随机位置开始，而是在核幔边界处成核，促进通量向上泄漏。该模型预测调制的时间尺度上的涡流的核心流的生命周期的字段，并将通过延长铁器时代南部非洲的古地磁记录进行测试。该研究计划与本科和研究生教育相结合，将与来自南非，博茨瓦纳和津巴布韦的地质学家和考古学家进行。 该项目由国际科学工程办公室的GVF基金共同资助。

A broad low intensity area in Earth's recent magnetic field spans the southern Atlantic Ocean, Africa and South America. This is commonly called the South Atlantic Anomaly (SAA). The SAA allows a relatively close approach of Earth's radiation belts, affecting spacecraft operations. The low magnetic intensity decreases the efficiency of magnetic shielding in the region, which can influence atmospheric ozone. Many believe the SAA is linked to the dramatic decay of the dipole geomagnetic field intensity during the last 160 years, and the growth of an area of reversed magnetic field on Earth's core beneath South Africa. Some have even speculated Earth is heading toward a geomagnetic field reversal. But understanding these phenomena within the context of longer-term geomagnetic history has been limited by a lack of Southern Hemisphere archeomagnetic data (that is, data from archeological objects that were fired to high temperature and subsequently preserved a record of Earth's magnetic field as they cooled). Our research and education program is aimed at understanding the history of the geomagnetic field in the SAA region as recorded in southern Africa. The main focus of the work is the collection and subsequent analysis of archeomagnetic data from Iron Age burnt structures, and tests of models addressing how the nature of the boundary between Earth's core and mantle may be giving rise to recent changes seen in the geomagnetic field.Archeomagnetic data from Iron Age sites of southern Africa (ca. 1000-1650 AD) show a sharp intensity drop at 1300 AD, at a rate comparable to modern field changes in the present-day South Atlantic Anomaly (SAA), but to lower values. The recurrence of low field values may reflect magnetic flux expulsion from the core promoted by the unusual core-mantle boundary composition and structure beneath southern Africa defined by seismology (specifically the African Large Low Velocity Seismic Province, or LLVSP). Because the African LLVSP is a longstanding structure, this region might be a steady site of flux expulsion, and perhaps the triggering site for geomagnetic reversals, on time scales of millions of years. If correct, this conceptual model is transformative because it suggests reversals do not initiate at random locations, but instead nucleate at core-mantle boundary sites that promote flux to leak upward. The model predicts modulation of the field on time scales of the lifetime of an eddy in the core flow, and will be tested by extending the archeomagnetic record of Iron Age southern Africa. The research program is integrated with undergraduate and graduate education and will be conducted with geologists and archeologists from South Africa, Botswana, and Zimbabwe. This project was co-funded by the GVF fund in the Office of International Science Engineering.