A Multidisciplinary Study of Thermal Core-Mantle Coupling in Geodynamo Models

地球的磁场大约和地球本身一样古老，它的存在对我们星球上的生命有着广泛的影响。几个世纪以来，磁场一直被人类和动物用作导航工具，它还保护地球表面免受潜在的有害辐射，并保护围绕地球运行的许多人造卫星。地球的磁场是在外核产生的，这是一个位于地球表面下约2800公里的熔铁区域，夹在固体内核和地幔之间。磁力线就像一串串意大利面条一样，从外核涌出，穿过固体地幔，最终出现在地球表面，并延伸到大气层。在地球表面观察时，这些场线形成一个偶极，在地理极附近有北极和南极。当观察到地核-地幔边界，即液核和地幔之间的界面时，就会出现一幅复杂得多的图景：磁场包含四个区域，磁力线聚集在一起，形成了一个斑块。这些斑块位于加拿大、南美、西伯利亚和澳大利亚的下方。地球磁场也表现出时间上的变化。在过去的400年里，斑块是静止的，与直接测量磁场的时间跨度一致，但在更长的时间段内似乎更具流动性。最戏剧性的事件是地磁反转，即北极和南极的磁极反转。在倒转期间，磁场的屏蔽作用减弱，对轨道卫星的影响可能是严重的；倒转的影响尚不清楚，但生命在迄今发生的多次倒转中幸存下来。完全了解地球磁场是如何产生的，以及如何发生逆转，对于减轻此类事件的潜在影响至关重要。这一挑战的关键是要确定目前的实地为什么会出现这种情况，即为什么含有补丁，补丁是否是实地的永久特征，以及在逆转期间和之后，目前的实地和实地之间是否存在联系。我们使用计算机模拟来模拟外核的物理过程。不幸的是，模拟地核条件的任务对于目前的计算机来说太大了，因此需要简化外核的物理属性才能取得进展。我已经证明，当从地核到地幔的热流跨越核-地幔边界变化时，简化的模型会产生真实的磁场，包括补丁。这就是地球上发生的情况：热量从外核传递到地幔，因为地球中部最热，但在非洲和太平洋下面损失的热量比在澳大利亚和墨西哥湾下面损失的更多。虽然这些简单模型的预测令人鼓舞，但当模拟中使用真实地球的物理属性时，维持斑块的机制不太可能适用。此外，模拟场不包含观测中看到的可变性(如反转)。今年早些时候，我观察到了一种新的过程，它可以通过核-地幔热流的变化来维持斑块。新的过程在理论上适用于真实的地球，并能够产生在观测中看到的时变行为。此外，我最近首次详细介绍了模拟接近地核的物理特性所需的计算能力。在这项研究计划中，我将利用这两个发现来产生第一个现实的地球磁场产生机制的模型，并以前所未有的方式阐明地球外核发生的复杂过程。

Earth's magnetic field is about as old as the Earth itself and its presence has broad implications for life on our planet. Used as a navigational aid by humans and animals for centuries, the magnetic field also shields Earth's surface from potentially harmful incoming radiation and protects the many man-made satellites orbiting the planet. Earth's magnetic field is generated in the outer core, a region of molten iron some 2800km below Earth's surface that is sandwiched between the solid inner core and mantle. Magnetic field lines, like strands of spaghetti, emerge from the outer core and thread through the solid mantle, finally appearing at Earth's surface and extending off into the atmosphere. When viewed at Earth's surface these field lines form a dipole, having north and south poles near the geographic poles. When viewed at the core-mantle boundary, the interface between the liquid core and the mantle, a much more complex picture emerges: the field contains four regions where magnetic field lines clump together, creating a patch. The patches are located under Canada, south America, Siberia and Australia. Earth's magnetic field also exhibits variations in time. Patches are stationary over the past 400 years, coinciding with the time-span over which the magnetic field has been directly measured, but appear to be more mobile over longer time periods. The most dramatic events are geomagnetic reversals, where the north and south magnetic poles flip. During reversals the shielding effect of the field is weakened and the impact on orbiting satellites could be severe; the effect of a reversal is unknown but life has survived the many reversals that have occurred to date. A complete understanding of how the Earth's magnetic field is generated and how reversals occur is of paramount importance for mitigating against the potential effects of such events. Crucial to this challenge is establishing why the present day field appears as it does, i.e. why is contains patches, whether patches are permanent features of the field, and whether there is a link between the present field and the field during and after a reversal. We model the physical processes in the outer core using computer simulations. Unfortunately, the task of simulating conditions in Earth's core is too great for current computers and so simplifications to the physical properties of the outer core are required in order to make progress. I have shown that simplified models produce realistic magnetic fields, including patches, when the flow of heat from the core to the mantle varies across the core-mantle boundary. This is the situation that occurs in the Earth: heat passes from the outer core to the mantle because the Earth is hottest in the middle, but more heat is lost beneath Africa and the Pacific than under Australia and the Gulf of Mexico. While predictions from these simple models are encouraging, the mechanism by which the patches are maintained is unlikely to apply when the physical properties of the real Earth are used in simulations. Moreover, the simulated fields do not contain the variability (such as reversals) seen in the observations. Earlier this year I observed a new process that can sustain patches through variations in core-mantle heat flow. The new process is theoretically applicable to the real Earth and is capable of producing the time-varying behaviour seen in observations. Additionally, I have recently presented the first detailed description of the computing power required to simulate physical properties approaching those of Earth's core. In this research proposal I will exploit these two discoveries to produce the first realistic models of the generation mechanism for Earth's magnetic field and elucidate the complex processes occurring in Earth's outer core in a manner never before possible.