Evolution of geomagnetic dipole moment and South Atlantic Anomaly

地球磁场保护我们的栖息地免受来自太空的太阳风和辐射的影响。由于磁场的几何形状，屏蔽一般在高纬度地区最弱。在南大西洋周围被称为南大西洋异常(SAA)的地区，它也异常弱，自1832年直接测量场强以来，全球偶极矩一直在减少近10%。由于我们对地核中的地球发电机过程的了解有限，不可能可靠地预测偶极矩和SAA在未来几十年的未来演化。然而，偶极矩和SAA区域磁场强度进一步减弱将导致缺乏磁场屏蔽，这将对现代技术造成越来越多的问题，特别是对易受辐射损害的卫星。为了估计磁场特性的未来发展，需要更好地理解潜在的过程。根据历史、考古和古地磁数据，在几个世纪到几千年的时间尺度上研究这些特征的过去演变，对于发现任何复发和周期性并与发电机数值模拟相比较或结合提供关于发电机过程的新见解至关重要。我们建议发展两个新的全球球谐地磁场模型，分别跨越1和10年，特别是为了研究1832年之前偶极矩的不间断衰减持续了多长时间，以及SAA是否是场的循环结构。我们将首次将所有可用的历史和考古地磁数据，包括方向和强度，结合在一个跨越过去1000年的球谐模型中。将相应调整现有的建模方法，并用新公布的数据补充现有的数据库。我们将进一步从佛得角群岛获取一些新的历史时期的古地磁数据，以更好地约束今天SAA的早期演变。为了研究该地区长期的磁场演化和类似弱场结构的可能再现，我们将从西非、巴西和智利沿海现有的海洋沉积岩芯中产生新的古地磁记录。在之前的千禧年尺度模型中，该地区受到的约束较弱。除了我们的主要目的是更好地了解以前偶极矩和SAA的演化外，这些模型还将用于研究偶极场和非偶极场贡献、半球对称性和核幔边界大尺度通量模式之间的关系。这些观测发现将提供与数值发电机模拟结果相比较的对地球发电机过程的新见解。此外，这些模型可以用来估计过去地球表面对太阳风的地磁屏蔽和银河系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.