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演化具有深远的影响。地球磁场保护地表环境和许多轨道卫星免受潜在有害的入射太阳辐射的影响；很久以前，这种屏蔽效应促进了可呼吸大气的形成。磁场强度远不是恒定的，在不同的地点和时间上也是不同的；事实上，在过去的150年里，磁场强度一直在下降，导致我们的防护盾牌减弱。在更具区域性的尺度上，可能会形成成片的弱磁场，例如位于南大西洋的当前低气压，众所周知，这种低气压会导致经过大西洋的卫星出现异常甚至故障，此外还与全球地磁场强度下降和当地气候变化有关。一些预测表明，这片薄弱领域将在未来100年内增长，鉴于社会对卫星和电子基础设施的依赖程度越来越高，这可能会产生重大后果。阐明导致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内部，即地球表面下约2800公里处的铁水区域。磁力线就像意大利面条一样，从外核发出，穿过整个地球，穿过地表，进入大气层。这个磁场是由铁水的剧烈运动产生的，它扭曲和拉伸磁力线，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(就像加热的锅中水的行为取决于炉子的温度一样)，而炉子的温度又决定了磁场的强度和结构。在一个重要的发展中，我最近的工作表明，目前可用于驱动铁水运动并由此产生磁场的能量比之前认为的要小2-3倍。这一结果意味着，地核中熔铁的行为可能与目前的预测截然不同(想象一下，在将炉子温度从沸腾降低到文火后，水会如何反应)，目前对导致我们的磁场在空间和时间上发生变化的过程的解释可能是不正确的。在更基本的层面上，我们目前不知道我们的星球是如何在其历史的大部分时间里支撑着磁场的，因为今天的能量减少给所有以前解释过去35亿年磁场存在的模型带来了重大问题。可用于地球外核的能量的急剧减少正在促使我们对地磁场的理解发生过去20年来最大的变化之一。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如何随着时间的推移而演变，从而达到现在的状态。我最近开发了一个新的外核演化数学模型，缓解了以前模型遇到的技术困难。在接下来的五年里，我将使用这个模型来理解过去35亿年来地球是如何支持其磁场的，从而为我们提供了对地球上最偏远和最神秘的地区的根本新视角。我将使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，并对未来的磁场行为做出预测，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.