Evolution of geomagnetic dipole moment and South Atlantic Anomaly

地磁场保护我们的栖息地免受太阳风和来自太空的辐射。由于磁场的几何形状，高纬度地区的屏蔽一般最弱。在南大西洋周围被称为南大西洋异常（SAA）的区域，它也非常弱，自从1832年可以直接测量场强以来，全球偶极矩已经减少了近10%。由于我们对地核中的地球发电机过程的了解有限，因此不可能可靠地预测未来几十年的偶极矩和SAA的未来演变。然而，由于偶极矩和SAA区域场强的进一步减弱，缺乏磁场屏蔽将对现代技术，特别是容易受到辐射损害的卫星造成越来越多的问题。更好地了解的基本过程，需要估计磁场特性的未来发展。基于历史、考古和古地磁数据，在数百年至数千年的时间尺度上研究此类特征的过去演变，对于检测任何复发和周期性并提供发电机过程的新见解至关重要。与数值发电机模拟相结合。我们建议建立两个新的全球球谐地磁场模型，分别跨越1和10 kyrs，并特别设计来研究在1832年之前，偶极矩的不间断衰减已经持续了多长时间，以及SAA是否是场的重复结构。我们将首次联合收割机结合所有可用的历史和古地磁数据，包括方向和强度，在过去1000年的球谐模型中。现有的建模方法将相应调整，现有的数据库将得到新公布的数据的补充。我们将进一步从佛得角群岛的历史时期获得一些新的古地磁数据，以更好地约束现今SAA的早期演化。为了研究长期的磁场演变和类似的弱磁场结构在这一地区的可能重现，我们将产生新的古地磁记录从现有的海洋沉积物岩心西非，巴西和智利的海岸。该区域在以前的千年尺度模型中受到弱约束。除了我们的主要目的是更好地了解以前的偶极矩和SAA的演变，模型将被用来研究偶极和非偶极场的贡献，半球对称性和大规模的通量模式在核幔边界之间的关系。与数值发电机模拟结果相比，这些观测结果将为地球发电机过程提供新的见解，此外，这些模型可用于估计过去地球表面对太阳风的地磁屏蔽以及银河宇宙射线产生的核素。

The geomagnetic field shields our habitat against solar wind and radiation from space. Due to the geometry of the field, the shielding in general is weakest at high latitudes. It is also anomalously weak in a region around the south Atlantic known as South Atlantic Anomaly (SAA), and the global dipole moment has been decreasing by nearly 10 percent since direct measurements of field intensity became possible in 1832. Due to our limited understanding of the geodynamo processes in Earth’s core, it is impossible to reliably predict the future evolution of both dipole moment and SAA over the coming decades. However, lack of magnetic field shielding as would be a consequence of further weakening of dipole moment and SAA region field intensity would cause increasing problems for modern technology, in particular satellites, which are vulnerable to radiation damage. A better understanding of the underlying processes is required to estimate the future development of magnetic field characteristics. The study of the past evolution of such characteristics based on historical, archeo- and paleomagnetic data, on time-scales of centuries to millennia, is essential to detect any recurrences and periodicities and provide new insights in dynamo processes in comparison to or in combination with numerical dynamo simulations. We propose to develop two new global spherical harmonic geomagnetic field models, spanning 1 and 10 kyrs, respectively, and designed in particular to study how long the uninterrupted decay of the dipole moment has been going on prior to 1832, and if the SAA is a recurring structure of the field.We will combine for the first time all available historical and archeomagnetic data, both directions and intensities, in a spherical harmonic model spanning the past 1000 years. Existing modelling methods will be adapted accordingly, and existing data bases will be complemented with newly published data. We will further acquire some new archeomagnetic data from the Cape Verde islands from historical times to better constrain the early evolution of the present-day SAA. In order to study the long-term field evolution and possible recurrences of similar weak field structures in this region, we will produce new paleomagnetic records from available marine sediment cores off the coasts of West Africa, Brazil and Chile. This region is weakly constrained in previous millennial scale models. Apart from our main aim to gain better insights into the previous evolution of dipole moment and SAA, the models will be used to study relations between dipole and non-dipole field contributions, hemispheric symmetries and large-scale flux patterns at the core-mantle boundary. These observational findings will provide new insights into geodynamo processes when compared with numerical dynamo simulation results.Moreover, the models can be used to estimate past geomagnetic shielding above Earth’s surface against solar wind and for nuclide production from galactic cosmic rays.