NSFGEO-NERC: On the origin of extreme variations in Earth's magnetic field

地球磁场至少存在了35亿年，在不到几秒到数百万年的时间尺度上呈现出复杂的空间和时间变化频谱。平均而言，该磁场被认为采用了偶极主导的配置，这有助于保护地表环境和低轨道卫星免受太阳风的掠夺。显著的变化，例如最近南大西洋异常弱场区域的增长，以及漂移和极性反转，可能会改变场提供的屏蔽效应。这些表面观测记录了在液核中运行的发电机过程，并提供了对地球深部动力学和演化的独特洞察。然而，仅靠数据不能限制发生在堆芯内的磁场和流动之间的相互作用：这需要发电机的内部视图。因此，理解过去的场变化和对未来行为的预测需要在观测和模拟生成过程之间建立密切联系。地磁长期变化(SV)的标准图像由历史、全新世和更长期的场的随时间变化的全球模型提供。然而，古地磁数据也为异常快速的地磁事件(脉冲)提供了证据，其形式为快速的地磁强度峰值，以及大大超过这些模型中的值的方向性变化率。虽然这些冲动在当前的全球场模型中看不到，但我们最近表明，它们可以与数值发电机模拟中产生的最快变化(称为极端事件)相媲美，并与发电机过程的物理学相兼容。我们的结果还表明，极值强度和方向变化出现在不同的时间和地点，并与地核顶部不同的磁性特征的迁移有关。这些发现以一种新的、更复杂的视角将观测和模拟联系在一起，并为理解发电机过程和我们预测其未来变化的能力提供了新的方法。进展要求超越极端事件的简单定义，研究支撑冲动的动力学行为的光谱。对这一目标至关重要的是利用从古地磁全球磁场模型和地球发电机模拟中获得的补充信息。我们建议开发一系列新的全球随时间变化的地磁场模型，以捕捉快速变化。与此同时，我们将制作一套新的地球发电机模拟，访问被认为描述地核动态的快速旋转和强烈对流的区域。这些方法的综合将解决以下问题：1.冲动的定义空间和时间特征是什么？它们是发生在偏爱的地点还是在系统的时间尺度上？2.冲动的物理起源(S)是什么？3.冲动是否与漂移和逆转有关？4.冲动是否与地核和地幔之间的相互作用和/或地核顶部的分层有关？

Earth's magnetic field has existed for at least 3.5 billion years and exhibits a complex spectrum of spatial and temporal variations on timescales ranging from less than seconds to millions of years. On average the field is thought to adopt a dipole-dominated configuration, which helps protect the surface environment and low-orbiting satellites from the depredations of the solar wind. Significant variations, e.g., the recent growth of a region of anomalously weak field in the southern Atlantic, and excursions and polarity reversals, may alter the shielding effect provided by the field. These surface observations document a dynamo process operating in the liquid core and provide unique insight into the dynamics and evolution of Earth's deep interior. However, data alone cannot constrain the interactions between magnetic field and flow that occur within the core: that requires an internal view of the dynamo. Understanding past field variations and making predictions about future behaviour therefore requires an intimate link between observations and simulations of the generation process. The standard picture of geomagnetic secular variation (SV) is provided by time-dependent global models of the historical, Holocene and longer term field. However, paleomagnetic data also provide evidence for Unusually Rapid Geomagnetic Events (URGEs) in the form of rapid geomagnetic intensity spikes, and directional rates of change that greatly exceed values in these models. While these URGEs are not visible in current global field models, we have recently shown that they are comparable to the fastest changes (called extremal events) produced in numerical dynamo simulations and are compatible with the physics of the dynamo process. Our results also reveal that extremal intensity and directional changes arise in different times and places and are associated with migration of distinct magnetic features at the top of the core. These findings link observations and simulations in a new and more complex view of SV, and suggest new approaches for understanding the dynamo process and our ability to predict its future variations.Progress requires moving beyond simple definitions of extremal events to investigate the spectrum of dynamical behaviour that underpins URGEs. Critical to this goal is using complementary information drawn from paleomagnetic global field models and geodynamo simulations. We propose to develop a new series of global time-dependent geomagnetic field models that can capture rapid changes. In parallel we will produce a new suite of geodynamo simulations accessing the rapidly rotating and vigorously convecting regime thought to describe the dynamics of Earth's core. Synthesis across these approaches will address the following questions: 1. What are the defining spatial and temporal characteristics of URGEs? Do they occur in preferred locations or on systematic timescales? 2. What are the physical origin(s) of URGEs? 3. Are URGEs related to excursions and reversals? 4. Are URGEs related to interactions between the core and mantle and/or stratification at the top of the core?