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

利用地磁场和地球自转的快速变化探测地球内部深处

• 批准号: NE/M012190/1
• 负责人: Richard Holme
• 金额: $ 34.3万
• 依托单位: University of Liverpool
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2015
• 资助国家: 英国
• 起止时间: 2015 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FM012190%2F1
• 关键词: Probing; Earth; deep; interior; rapid

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.

地磁场的时间尺度从毫秒到数十亿年不等，它的来源既有地球内部的，即在高导电性的液态铁核中产生主磁场的发电机，也有地球外部的，即在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（小于一年的周期）来自地球外部，而较长周期的变化来自地球内部。将信号从1年到10年分离是一项挑战，但也有可能告诉我们更多关于地球结构和过程的信息。通常被认为是内部起源的最快速的变化是所谓的“地磁急动”-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（如波和核心流体的上涌），还可以告诉我们介于两者之间的固体地幔。这个岩石区域的导电性不如铁芯，但它仍然可以微弱地导电。最近，另一种地球物理测量方法--地球自转速率--提供了对这种性质的强有力的限制。我们发现，磁场的急剧变化与地球自转速率的急剧变化几乎同时发生。这既提供了导致这些事件的线索，也强烈限制了地幔的传导性-如果这是更高的，那么磁信号将滞后于旋转信号，因为场需要时间从核幔边界的起源扩散到地表的固体地球。地幔电导率还受到测量变化的外部磁场所产生的感应磁场的限制，即所谓的地磁测深。地球上方的这一制约因素与深部地幔的新制约因素相结合，将用来提供电导率随深度变化的详细剖面图，而深度又制约着固体地球的成分和矿物状态。例如，如果预测硅酸盐岩石的相变会导致电导率急剧上升，那么地磁数据可以排除这种相变。本研究的大部分工作是对地磁和地球自转数据进行详细分析，以梳理出它们所包含的信号的更多信息。两次测量都证实了六年振荡，但更快的变化更难区分，因为它们与其他来源重叠：来自外部电流系统的磁场，以及来自与大气的角动量交换的地球自转。例如，地球自转的短周期（大气）变化与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。我们将构建空间和时间磁场变化的详细模型，以研究导致这些变化的原因。最近，对地球内部深处物质物理状态的量子力学计算已经修正了我们对地核电导率和相关热导率的假设值。这些新的数值改变了我们对地核如何工作的理解--我们现在相信，在地核顶部很可能存在一个稳定分层的流体层，而不是完全的剧烈对流。这一层将支持波动和不稳定性，而不是大规模的对流，因为我们的大气和海洋，类似的稳定分层，快速旋转的流体。最近的一个简单的模型，这些波可以解释的偶极场在地球上的变化的细节，我们的初步结果表明，他们也可以解释地磁急动。因此，我们的工作应该同时限制地幔的结构和地核的动力学。

项目成果

Investigation of the consistency of Swarm data and CHAOS-6 towards crustal magnetic field studies in China

Swarm数据与CHAOS-6对中国地壳磁场研究的一致性探讨

• DOI: 10.1007/s11770-021-0968-1
• 发表时间: 2022
• 期刊: Applied Geophysics
• 影响因子: 0.7
• 作者: Yi J

Evidence for MAC waves at the top of Earth's core and implications for variations in length of day

• DOI: 10.1093/gji/ggv552
• 发表时间: 2016-03-01
• 期刊: GEOPHYSICAL JOURNAL INTERNATIONAL
• 影响因子: 2.8
• 作者: Buffett, Bruce;Knezek, Nicholas;Holme, Richard
• 通讯作者: Holme, Richard

Long-wavelength lithospheric magnetic field of China

• DOI: 10.1093/gji/ggaa490
• 发表时间: 2020-12
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Yi Jiang;R. Holme;S. Xiong;Yong Jiang;Yan Feng;Hai Yang

Penetration of boundary-driven flows into a rotating spherical thermally stratified fluid

边界驱动流渗透到旋转球形热分层流体中

• DOI: 10.1017/jfm.2018.999
• 发表时间: 2019
• 期刊: Journal of Fluid Mechanics
• 影响因子: 3.7
• 作者: Cox G

Geomagnetic Virtual Observatories: monitoring geomagnetic secular variation with the Swarm satellites

• DOI: 10.1186/s40623-021-01357-9
• 发表时间: 2021-02-19
• 期刊: EARTH PLANETS AND SPACE
• 影响因子: 3
• 作者: Hammer, Magnus D.;Cox, Grace A.;Finlay, Christopher C.
• 通讯作者: Finlay, Christopher C.