Diffusion of the geomagnetic secular variation through the heterogeneous mantle

使用外部源(周期长达11年的太阳周期变化)的感应研究很难限制下地幔的电导率。因此，本研究试图利用地磁长期变化作为内源来补充这些结果。然而，在这种方法中，我们面临着源的时间和空间结构未知的问题，必须用统计描述来代替。这一建议主要集中在两个方面：第一，假设电导率呈径向对称分布，上半部分约束良好，下半部分稀疏，我们试图估计震源统计描述中的未知参数。这项工作可以看作是Mc Donald(1957)经典方法的推广。第二：人们经常认为，地幔深部电导率的横向变化将在地球表面的地磁场中表现出来，例如通过优先的VGP路径或通过大西洋和太平洋半球长期变化的不同模式。考虑到非均匀球面薄片地幔电导率的横向变化，我们开发了一个数值模拟程序，并研究了深部地幔非均质性对地球表面长期变化模式的可能影响。

The electrical conductivity in the lower mantle is poorly constrained by induction studies using external sources (with periods up to the solar cycle variation of 11 years). Therefore the present study tries to complement these results by using in addition the geomagnetic secular variation as an internal source. However, in this approach we are faced with the problem that the temporal and spatial structure of the source is unknown and has to be replaced by a statistical description. The proposal focuses on two topics.Firstly: Assuming a radially symmetric conductivity distribu- tion with a well-constrained upper part and a sparsely para- metrized lower part, we try to estimate the unknown parameters in the statistical description of the source. The results of induction studies with external sources are used as con- straints.This work can be considered as an extension of the classical approach of Mc Donald (1957).Secondly: It is often argued that lateral variations of deep mantle electrical conductivity will find their manifestation in the geomagnetic field at the Earth`s surface, e.g. by preferred VGP paths or by the different pattern of the secular variation in the Atlantic and Pacific hemisphere. Taking lateral variations of mantle conductivity into account by a nonuniform spherical thin sheet, we develop a code for numerical modelling and investigate possible impacts of deep mantle heterogeneity onto the secular variation pattern at the surface of the Earth.