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

地磁场在地球系统中起着关键作用，它可以保护地球表面环境免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;场的强度和结构随时间变化很大。这可能会给国际电信造成问题，并在卫星通过弱场区域时干扰卫星。为了理解磁场为什么会变化，我们必须深入看看我们脚下的地球铁芯。正是在地核中，我们的磁场是由液态铁合金海洋产生的，液态铁合金海洋通过热量损失到上覆地幔而产生湍流运动。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，正是在这一点上，我们有了重要的新见解。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：地核顶部是否有一个稳定的流体层，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，因为稳定层的存在会从观测的角度隐藏在核心湍流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，它们提供的证据相互矛盾。地震研究发现，与湍流区域的预期速度相比，最上层核心的波速非常慢，这意味着核心顶部有一个稳定层。相反，地磁观测似乎需要在核心的顶部径向流体运动，运动将在一个稳定层缺席，关键的，未经检验的，在所有以前的工作中固有的假设是，任何稳定分层防止湍流对流的核心出现作为一个全球层。使用先进的计算机模拟，我们最近发现了一种新的情况是可能的，分层发生在区域范围内，而不是作为一个全球层。在我们的模拟中，稳定区域的出现是因为离开核心的热量在核幔边界周围变化：核心的径向运动受到太平洋中部和非洲异常热的地幔的抑制;相反，径向核心流增强，美国和东亚的冷地幔从核心吸收更多的热量。在这种情况下，地震和地磁观测显然表明不同的动态可以在一个单一的连贯框架内解决。我们对地幔温度变化的最佳估计表明，稳定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，它可以通过改进地震和地磁观测来检验。我们将通过将尖端的地震处理技术与最先进的核心动力学模拟和定量指标相结合来测试最上层核心的区域结构和动力学模型，以比较模拟和观测的磁场。通过使新的地震观测能够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像，从而更好地理解核如何产生磁场。

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.