Bayesian inversion for a global apparent magnetic susceptibility model based on quiet-time satellite magnetic field measurements

岩石圈磁场的研究是Swarm任务的主要目标之一。它的结构被设计成特别有利于探测岩石圈磁信号的小波长。特别是，卫星阿尔法和查理以1.4度的纵向距离并排飞行，这使得磁场的东西向梯度近似。这一梯度主要缺乏由磁层环电流产生的大尺度和快速变化的磁场。目前，实现高分辨率岩石圈磁场模型的最大挑战之一是有效地从测量中去除环电流特征，这也是群磁场测量如此有价值的原因。然而，梯度不能有效地去除由流经极地区域的电流产生的磁场的有害特征，包括扰动极地2电流系统、极地电喷流和场向电流。在本文的第一部分，我们提出了一种处理岩石圈场模型的极性污染的创新方法。我们建议将SuperMAG指数和AMPERE卫星任务提供的数据结合使用，可以显著改善目前在极地地区采用的数据选择方案，从而允许构建一个磁安静的极地数据集。这个新的数据选择方案将应用于最新的Swarm和CHAMP 3级磁场数据。主成分分析的附加步骤将有助于建模和消除极点上任何剩余的外部场噪声。在项目的第二部分，将按照SPP1788一期项目“从全球卫星数据分析的磁性岩石圈结构”的思路，对生成的数据集进行全球可见磁化模型的反演。这部分的方法创新包括采用随机反演方法，提供模型不确定性的后验估计。在项目的第三部分，将磁化模型转换为视磁化率模型。这一步骤面临的主要挑战之一是反演磁场数据固有的非唯一性。然而，最近导出的方程第一次允许在感应磁化假设下估计磁化强度的零空间，从而估计视磁化率。磁化率是岩石的一个物理量，反映了岩石的年龄、矿物成分、温度和压力等属性。在这方面，它与许多学科直接相关，例如地质学、岩石学、地震学、重力和构造重建研究。

The study of the lithospheric magnetic field is one of the main goals of the Swarm mission. Its configuration has been designed to be especially beneficial for detecting the small wavelengths of the lithospheric magnetic signal. In particular, satellites Alpha and Charlie fly side by side at a longitudinal distance of 1.4 degrees, which allows for the approximation of the East-West gradient of the magnetic field. This gradient is mainly devoid of the large scale and rapidly varying magnetic field generated by the magnetospheric ring current. Effectively removing the ring current signature from the measurements is currently one of the biggest challenges in achieving high-resolution lithospheric field models and that is why Swarm magnetic field measurements are so valuable. However, gradients cannot effectively remove the unwanted signature of magnetic fields generated by currents flowing over the polar regions, including the Disturbance Polar 2 current system, the polar electrojet and Field-Aligned Currents. In the first part of this proposal, we suggest an innovative way to deal with the polar contamination of lithospheric field models. We suggest that the combined use of SuperMAG indices and data delivered by the AMPERE satellite mission can significantly improve the data selection scheme currently employed over the polar regions, therefore allowing the construction of a magnetically quiet polar data set. This new data selection scheme will be applied on the latest Swarm and CHAMP Level-3 magnetic field data. The additional step of a principal components analysis will help to model and remove any remaining external field noise over the poles. In the second part of the project, the generated data set will be inverted for a global visible magnetization model, along the lines of the project “Structure of the magnetic lithosphere from global analysis of satellite data” from the first phase of SPP1788. Methodological innovations for this part include employing a stochastic inversion approach that delivers posterior estimation of the model uncertainties. In the third part of the project, the magnetization model will be converted to an apparent magnetic susceptibility model. One of the main challenges towards this step is the inherent non-uniqueness of inverting magnetic field data for the apparent susceptibility. However, recently derived equations allow for the first time, under the assumption of induced magnetization, the estimation of the null space of the magnetization and therefore of the apparent magnetic susceptibility. Magnetic susceptibility is a physical quantity of the rocks that reflects properties like age, mineral composition, temperature and pressure. In this respect, it is of direct interest to a variety of disciplines, dealing, e.g., with geology, petrology, seismology, gravity, and tectonic reconstruction studies.