Beyond 1D Structure of Earth's Core - Reconciling Inferences from Seismic and Geomagnetic Observations

地球磁场在地球系统中起着关键作用，它保护地表环境不受太阳发出的带电粒子风的影响。然而，这种屏蔽效应远不是恒定的；磁场的强度和结构在时间上有很大的变化。这可能会给国际电信带来问题，并在卫星通过弱场区域时中断卫星。为了理解磁场变化的原因，我们必须深入到我们脚下的地球铁核。正是在地核，我们的磁场是由液态铁合金海洋产生的，这些液态铁合金被上覆地幔的热量损失驱动成湍流运动。来自卫星和永久观测站的数据可以用来确定地核顶部的磁场，但不能直接提供有关地核内部的信息。因此，我们对地球磁场的了解与我们对地核表面的了解一样好，正是在这里，我们有了重大的新见解。围绕地核顶部动力学的争论已经持续了40多年，并围绕着一个关键问题：地核顶部是否有稳定的流体层，或者整个地核是否处于湍流运动中？这种区别是至关重要的，因为稳定层的存在将从观测角度隐藏在地核动荡的内部产生磁场的关键过程。研究地核的两个主要工具是地震学和地磁学，不幸的是，它们提供了相互矛盾的证据。地震研究发现，与湍流区域的预期波速相比，最高地核的波速异常缓慢，这意味着在地核顶部有一个稳定的层。相反，地磁观测似乎需要地核顶部的径向流体运动，这种运动在稳定的层中是不存在的。所有先前工作中固有的关键的、未经检验的假设是，任何阻止地心内湍流对流的稳定层结都是作为一个全球层出现的。使用先进的计算机模拟，我们最近发现一种新的情景是可能的，即分层发生在区域范围内，而不是作为全球层面。在我们的模拟中，稳定区域的出现是因为离开地核的热量在核幔边界附近不同：地核中的径向运动受到中太平洋和非洲下方异常热的地幔的抑制；相反，在美洲和东亚经度的冷地幔从地核吸收更多热量的地方，径向地核流动得到加强。在这种情况下，地震和地磁观测显然表明不同的动力学可以在一个连贯的框架内解决。我们对地幔温度变化的最佳估计表明，稳定区域和不稳定区域都应该存在于地球最外层的核心，下一步是确定它们是否存在。这种区域分层情景的一个关键方面是，它可以通过地震和地磁观测的改进来测试。我们将通过将尖端地震处理技术与最先进的岩心动力学模拟以及用于比较模拟和观测磁场的定量指标相结合，来测试最高核心区的区域结构和动力学模型。通过使新的地震观测驱动新的动力学模拟，反之亦然，我们将获得外核动力学的自洽图景，从而更好地理解地核是如何产生磁场的。

The geomagnetic field plays a key role in the Earth system by shielding the surface environment from a wind of charged particles emanating from the sun. However, this shielding effect is far from constant; the strength and structure of the field varies significantly in time. This can cause problems for international telecommunications and disrupt satellites as they pass through regions of weak field. To understand why the field changes we must look deep beneath our feet to Earth's iron core. It is in the core that our magnetic field is produced by an ocean of liquid iron alloy that is powered into turbulent motion by heat loss to the overlying mantle. Data from satellites and permanent observatories can be used to determine the magnetic field at the top of the core, but cannot directly provide information about the core's interior. Our understanding of Earth's magnetic field is therefore only as good as our knowledge of the core surface, and it is here that there have been significant new insights.Debate surrounding the dynamics at the top of Earth's core has persisted for over 40 years and has centred around one key question: is there a stable layer of fluid at the top of the core or is the whole core in turbulent motion? The distinction is critical because the existence of a stable layer would hide from observational view the key processes that generate the magnetic field in the core's turbulent interior. The two main tools for studying Earth's core are seismology and geomagnetism, unfortunately they provide conflicting evidence. Seismic studies find anomalously slow wave speeds in the uppermost core compared to what is expected for a turbulent region, implying there is a stable layer at the top of the core. Conversely, geomagnetic observations appear to require radial fluid motions at the top of the core, motions that would be absent in a stable layer.The crucial, and untested, assumption inherent in all previous work is that any stable stratification preventing turbulent convection in the core arises as a global layer. Using advanced computer simulations, we have recently discovered a new scenario is possible, that stratification occurs on a regional scale and not as a global layer. In our simulations, stable regions arise because the amount of heat leaving the core varies around the core-mantle boundary: radial motion in the core is suppressed by the unusually hot mantle under the central Pacific and Africa; conversely, radial core flow is enhanced where the cold mantle at American and east Asian longitudes draws more heat from the core. In this scenario, seismic and geomagnetic observations that apparently suggest different dynamics can be resolved within a single coherent framework. Our best estimates of temperature variations in the mantle suggest that both stable and unstable regions should exist in Earth's outermost core, the next step is to establish whether they do.A key aspect of this regional stratification scenario is that it can be tested using improvements in seismic and geomagnetic observations. We will test this model of regional structure and dynamics in the uppermost core by combining cutting-edge seismic processing techniques with state-of-the-art simulations of core dynamics and quantitative metrics for comparing simulated and observed magnetic fields. By enabling new seismic observations to drive new dynamical simulations and vice versa we will obtain a self-consistent picture of outer core dynamics and hence an improved understanding of how the core generates the magnetic field.