Probing Earth's deep interior with rapid changes in Geomagnetic field and Earth rotation

地球磁场在毫秒到数十亿年的时间尺度上变化，其来源既有地球内部的，从在高导电液态铁核中产生主场的发电机，也有地球外部，来自我们上方的电离层和磁层中流动的电流，反映了太阳风与我们星球的相互作用。一般来说，快速变化(不到一年的周期)起源于地球以外，而较长的周期变化来自地球内部。从一年到十年分离信号是一个挑战，但也有可能告诉我们更多关于地球结构和过程的信息。最快的变化通常被认为是由内部引起的，那就是所谓的“地磁突变”--磁场变化率的快速变化。它们的结构和演化不仅可以告诉我们地球流体核心的快速变化(例如地核流体的波动和上升流)，还可以告诉我们介于两者之间的固体地幔。这个多岩石的区域不像铁芯那样导电，但它仍然可能导电很弱。最近，另一项地球物理测量--地球自转速度--提供了对这一特性的强烈限制。我们发现，磁场的急剧变化与地球自转速度几乎同时发生的急剧变化相匹配。这既给出了导致事件的原因的线索，也强烈限制了地幔的传导性--如果传导性更高，那么磁信号将滞后于旋转信号，因为磁场需要时间从核幔边界的原点扩散到固体地球，才能在地表观察到。地幔的传导性还受到来自不同外部磁场的感应磁场测量的限制，即所谓的地磁深度测深。来自地球上方的这种约束和来自地幔深处的新约束相结合，将被用来给出作为深度函数的传导性的详细轮廓，这反过来又限制了固体地球的组成和矿物状态。例如，如果预测硅酸盐岩石的相变导致电导率急剧上升，则可以通过地磁数据排除这种相变。这项研究的主要工作是对地磁和地球自转数据进行详细分析，以梳理出关于它们包含的信号的更多信息。这两次测量都证实了六年的振荡，但更快的变化更难区分，因为它们与其他来源重叠：磁场、外部电流系统，以及地球自转与大气的角动量交换。例如，地球自转短周期(大气)变化的变化已被证明与ENSO气候信号有很强的联系。该项目的成功结果将取决于信号的成功分离。我们将构建磁场在空间和时间上变化的详细模型，以调查导致这些变化的原因。最近，对地球深部物质物理状态的量子力学计算修正了我们对核心电性和相关热导率的假设值。这些新的价值观改变了我们对核心如何工作的理解--我们现在认为，在核心的顶部很有可能存在稳定分层的流体层，而不是完全强烈的对流。这一层将支持波动和不稳定，而不是像我们看到的大气和海洋那样的大范围对流，同样是稳定分层、快速旋转的流体。最近这些波的一个简单模型可以解释地球偶极场变化的细节，我们的初步结果表明，它们也可以解释地磁突变。因此，我们的工作应该既限制地幔的结构，也限制其核心的动力学。

The geomagnetic field varies on time scales of milliseconds to billions of years, and has sources both inside the Earth, from the dynamo generating the main field in the highly conducting liquid iron core, and outside the Earth, from currents flowing above us in the ionosphere and magnetosphere, reflecting the interaction of the solar wind with our planet. In general, rapid variations (less than one year period) originate outside the Earth, while longer period variations come from inside. Separating signals from one to ten years is a challenge, but also has the potential to tell us much about Earth structure and processes. The most rapid variations generally identified as being of internal origin are so-called "geomagnetic jerks" - rapid changes in the rate of change of the magnetic field. Their structure and evolution can tell us not only about rapid changes in Earth's fluid core (such as waves and upwelling of core fluid) but also about the solid mantle in between. This rocky region is not as electrically conducting as the iron core, but it could still conduct weakly. A strong constraint on this property has recently been provided by another geophysical measurement, the rate of Earth rotation. We have found that sharp changes in the field are matched by almost contemporaneous sharp changes in the rate of Earth rotation. This both gives as clues as to what causes the events, but also strongly restricts the conductivity of the mantle - if this were higher, then the magnetic signal would lag the rotational signal as it would take time for the field to diffuse from its origin at the core-mantle boundary through the solid Earth to be observed at the surface. Mantle conductivity is also constrained by measurements of the induced magnetic field from varying external fields, so-called geomagnetic depth sounding. The combination of this constraint from above the Earth, and the new constraint from the deep mantle, will be used to give a detailed profile of conductivity as a function of depth, which in turn constrains the composition and mineral state of the solid Earth. For example, if a phase change of silicate rock were predicted which gives a sharp rise in conductivity, this phase change could be excluded by the geomagnetic data.The bulk of the work in this study is detailed analysis of both geomagnetic and Earth rotation data to tease out more information as to the signals they contain. A six-year oscillation has been confirmed in both measurements, but more rapid variations are even harder to distinguish, as they overlap with other sources: for the magnetic field, from external current systems, and for Earth rotation from angular momentum exchange with the atmosphere. For example, variations in short period (atmospheric) variations in Earth rotation have been shown to have a strong link to the ENSO climatic signal. A successful outcome of the project will rely on successful separation of the signals.We will construct detailed models of the magnetic field variation in space and time to investigate what is causing these changes. Recently, quantum mechanical calculations of the physical state of materials of the Earth's deep interior have revised our assumed value for the electrical and linked thermal conductivity of the core. These new values have changed our understanding of how the core works - we now believe that instead of full vigorous convection, it is highly likely that there is a stably stratified layer of fluid at the top of the core. This layer will support waves and instabilities rather than large scale convection, as is seen for our atmosphere and oceans, similarly stably stratified, rapidly rotating fluids. A recent simple model of these waves can explain the details of the variation of the dipole field in the Earth, and our preliminary results suggest that they may also explain the geomagnetic jerks. Thus our work should constrain both the structure of Earth's mantle, and the dynamics of its core.