Island geomagnetic observatories and upper mantle electric conductivity profiles

地球的电导率分布可以通过分析在地表观测到的磁场和电场来研究。全球地面地磁观测站网络提供高质量和高分辨率的数据，可随时对地磁场进行观测。然而，没有提供可比质量和数量的同时电场测量。最近，有人提出了一种仅从地磁观测获得一维电导率剖面的方法。这种方法是基于倾斜器的观测，倾斜器将观测到的地磁场变化的垂直分量和水平分量联系起来。此外，这种方法已被证明是解决1-D电导率剖面的地壳和上地幔利用已知的横向电导率对比海水和基岩附近的岛屿观测站。使用其他方法，由于与海上观测相关的后勤挑战，很难获得这样的电导率剖面。在本项目中，我们将把最近提出的方法应用于岛屿观测站。为此，我们将选择合适的观测站，计算从地磁数据的倾斜，并组装详细的模型的岛屿测深。最后，我们将反演这些倾斜的地壳和上地幔的局部1-D电导率剖面。这种电导率分布包含有价值的信息，化学和热输运性质，但也将有助于提高我们的理解所测得的表面磁场的结果，从各种来源。这些源很难分离，本研究项目将有助于提取上地幔中的诱发源信号。因此，该研究项目也有助于我们了解电离层和磁层中的电流系统，并促进卫星和地面地磁观测的联合解释。

The electrical conductivity profile of the Earth can be investigated by analyzing the observed magnetic and electric fields at the surface. Observations of the geomagnetic field are readily available from a worldwide network of ground-based geomagnetic observatories which provide data of high quality and of high resolution. However, simultaneous electric field measurements of comparable quality and quantity are not provided. Recently, a method to obtain 1-D electric conductivity profiles from geomagnetic observations alone has been suggested. This method is based on the observation of tippers, which relate the vertical and horizontal components of the observed geomagnetic field variations. Further, this method has been demonstrated to resolve 1-D conductivity profiles of the crust and upper mantle by exploiting the known lateral conductivity contrasts of seawater and bedrock near island observatories. With other methods, such conductivity profiles are difficult to obtain due to the logistical challenges associated with observations at sea.In this project, we will apply the recently suggested method to island observatories. For this purpose, we will select suitable observatories, calculate tippers from the geomagnetic data, and assemble detailed models of the island bathymetry. Finally, we will invert these tippers for local 1-D conductivity profiles of the crust and upper mantle. Such conductivity profiles contain valuable information on chemical and thermal transport properties, but also will help to improve our understanding of the measured surface magnetic field which results from a variety of sources. These sources are difficult to separate, and this research project will help to extract the signal of the induced sources in the upper mantle. In consequence, this research project may as well contribute to our knowledge of the electric current systems in the ionosphere and magnetosphere and facilitate the joint interpretation of satellite and ground based geomagnetic observations.