A New Energy Budget for Earth's Core and Implications for the Geomagnetic Field

至少在过去的35亿年里，地球一直拥有磁场，这一事实对我们星球的进化有着深远的影响。地磁场保护地表环境和许多轨道卫星免受潜在的有害太阳辐射；很久以前，这种屏蔽作用促进了可呼吸大气的形成。场强远不是恒定的，它随地点和时间而变化；事实上，磁场强度在过去的150年里一直在下降，导致我们的防护盾变弱。在更大的区域尺度上，弱磁场可以形成斑块，例如位于南大西洋的当前低气压，已知它会导致通过它的卫星出现异常甚至故障，并且还与全球地磁场强度减弱和局部气候变率有关。一些预测表明，这片弱磁场将在未来100年内增长，鉴于社会对卫星和电子基础设施的日益依赖，这可能会产生重大后果。阐明产生全球和区域磁场变化的过程是预测未来行为的基础。地球磁场的来源位于地核外，这是一个位于地球表面以下约2800公里的熔融铁区域。磁力线，就像意大利面条一样，从外核发出，穿过整个地球，穿过地球表面，进入大气层。这个磁场是由熔融铁的剧烈运动产生的，它扭曲和拉伸了磁力线，这一过程需要大量的能量来维持。可用能量的多少决定了铁水的行为（就像加热的锅里的水的行为取决于炉子的温度），这反过来又决定了磁场的强度和结构。在一个重大的发展中，我最近的工作表明，可用的能量，使熔融铁进入运动，从而产生磁场，目前比以前认为的小2-3倍。这一结果表明，地核中熔融铁的行为可能与目前的预测非常不同（想象一下，把炉子的温度从沸腾降至小火后，水会发生什么反应），而且目前对导致磁场在空间和时间上变化的过程的解释可能是不正确的。在一个更基本的层面上，我们目前还不知道我们的星球是如何在其历史上的大部分时间里维持磁场的，因为现在能量的减少给之前解释过去35亿年磁场存在的所有模型带来了重大问题。地球外核可用能量的急剧减少促使我们在过去20年里对地磁场的理解发生了最大的变化之一。为了重新建立一个解释磁场长期存在的基本理论，需要一个模型来描述外核是如何随着时间的推移而演变，从而达到今天的状态。我最近开发了一个新的外核演化的数学模型，它减轻了以前模型遇到的技术困难。在接下来的五年里，我将用这个模型来了解地球在过去35亿年里是如何支撑它的磁场的，从而为我们这个星球上最遥远、最神秘的地区提供新的视角。我将利用这些信息对地球外核进行计算机模拟，这将建立负责产生我们观察到的复杂磁场行为的过程，并对磁场的未来行为做出预测，包括全球磁场强度的演变和弱磁场的斑块。

Earth has possessed a magnetic field for at least the last 3.5 billion years, a fact that has profound implications for the evolution of our planet. The geomagnetic field shields the surface environment and the many orbiting satellites from potentially harmful incoming solar radiation; long ago, this shielding effect facilitated the formation of a breathable atmosphere. The field strength is far from constant, varying from place to place and also in time; indeed, the field strength has been decreasing for the last 150 years, leading to a weakening of our protective shield. On a more regional scale, patches of weak field can develop, such as the current low located in the southern Atlantic, which is known to cause anomalies and even failures in satellites that pass through it and has additionally been linked to the global decrease in geomagnetic field strength and local climate variability. Some predictions suggest that this patch of weak field will grow over the next 100 years, which could have significant consequences given society's increasing reliance on satellites and electronic infrastructure. Elucidating the processes that produce global and regional changes in the magnetic field is fundamental for predicting future behaviour.The source of Earth's magnetic field lies inside the outer core, a region of molten iron some 2800km below Earth's surface. Magnetic field lines, like strands of spaghetti, emanate from the outer core and thread through the whole Earth, passing through the surface and off into the atmosphere. This field is generated by vigorous motion of the molten iron, which twists and stretches the magnetic field lines, a process that requires a significant amount of energy to maintain. The amount of available energy determines the behaviour of the molten iron (just like the behaviour of water in a heated pan depends on the temperature of the stove), which in turn dictates the strength and structure of the magnetic field. In a significant development, my recent work has shown that the energy available to power the molten iron into motion, and hence generate the magnetic field, is presently 2-3 times smaller than previously thought. This result implies that the behaviour of the molten iron in Earth's core may be very different to current predictions (imagine how the water reacts after turning the stove temperature down from boil to simmer), and that current interpretations of the processes causing our magnetic field to vary in space and change in time may be incorrect. At a more fundamental level, we do not currently know how our planet has managed to support a magnetic field for much of its history because the present-day energy reduction causes significant problems for all previous models that explain the existence of the field for the last 3.5 billion years. The dramatic reduction in energy available to Earth's outer core is prompting one of the biggest changes to our understanding of the geomagnetic field in the last 20 years. To reestablish a basic theory that explains the long-term existence of the magnetic field requires a model that describes how the outer core has evolved over time and therefore arrived its present-day state. I have recently developed a new mathematical model of outer core evolution that alleviates the technical difficulties encountered by previous models. Over the next five years I will use this model to understand how the Earth has supported its magnetic field for the last 3.5 billion years, thereby providing fundamental new sight into the most remote and enigmatic region of our planet. I will use this information to make computer simulations of the Earth's outer core, which will establish the processes responsible for producing the complex magnetic field behaviour we observe and make predictions about future behaviour of the field including the evolution of the global field strength and patches of weak magnetic field.