Geomagnetic Signals from the Earth's Core: Searching for Evidence of Waves and Stable Stratification

地球磁场的波动提供了对我们星球内部动态的独特见解。液态金属核心中的湍流产生地磁场，导致地球表面可观察到的波动。 历史观察显示，在几十年的范围内有几个不同的周期。这些波动的起源还不太清楚，但它们的存在深刻地影响了我们预测地磁场变化的能力。拟议的研究旨在建立地球液核顶部波的物理模型，目的是解释观测到的地磁波动。核心的物理条件，包括密度分层的可能性，可以恢复通过拟合模型预测的观察。这个项目的一个潜在结果是改变我们解释地磁波动的方式，因为某些类型的波的存在可以大大改变我们对地核顶部附近动力学的理解。鉴于一个新的卫星使命（欧空局的Swarm使命）以前所未有的分辨率监测地球磁场，这项工作特别及时。更好的观测和更现实的物理模型相结合，应该可以提高我们预测未来几十年地磁场变化的能力。更广泛地说，我们的工作解决了地球的热和化学演化，因为内部热量和物质的逐渐转移最终决定了当今地球的密度分层。 当液核的顶部被热梯度或成分梯度分层时，可以产生几十年周期的波。这些波有时被称为MAC波，因为它们涉及磁力、阿基米德力和科里奥利力的相互作用。初步结果表明，MAC的线性组合提供了一个很好的描述，在核心的纬向流的核心流量的几个独立的估计的基础上。同样的波的组合也可以解释偶极场中观测到的波动。成功地解释这些不同的现象激发了我们的假设，即在历史记录中占主导地位的周期性是由于MAC波。我们探索这一假设，开发一个更复杂的数值模型的波动，并通过设计更全面的测试，使用广泛的观测，包括地磁长期变化和变化的一天。我们建议将这些观察结果反演为核中分层层的结构和性质。我们还将探索分层的长期动态的核心使用一个新的geodynamo模型称为Calypso，这是在加州大学伯克利分校开发与先前的NSF支持的后果。 Calypso将用于定量评估核心中MAC波的激发，并确定可能支持或反驳我们假设的特定诊断。

Fluctuations in the Earth's magnetic field provide unique insights into the internal dynamics of our planet. Turbulent flow in the liquid metal core generates the geomagnetic field, causing observable fluctuations at the Earth's surface. Historical observations reveal several distinct periodicities in the range of several decades. The origin of these fluctuations is not well understood, but their existence profoundly affects our ability to forecast changes in the geomagnetic field. The proposed research seeks to develop physical models for waves at the top of the Earth's liquid core with the goal of explaining the observed geomagnetic fluctuations. Physical conditions in the core, including the possibility of density stratification, can be recovered by fitting the model predictions to the observations. One potential outcome of this project is a change in the way we interpret geomagnetic fluctuations because the presence of certain types of waves can substantially alter our understanding of dynamics near the top of the core. This work is particularly timely given a new satellite mission (ESA's Swarm mission) to monitor the Earth's magnetic field with unprecedented resolution. The combination of better observations and more realistic physical models should improve our ability to forecast of changes in the geomagnetic field over the next few decades. More broadly, our work addresses the thermal and chemical evolution of the Earth because the gradual transfer of heat and matter in the interior ultimately sets the density stratification in the present-day Earth. Waves with periods of several decades can arise when the top of the liquid core is stratified by either thermal or compositional gradients. These waves are sometimes called MAC waves because they involve the interplay of Magnetic, Archimedes and Coriolis forces. Preliminary results show that a linear combination of MAC provides a good description of zonal flow in the core, based on several independent estimates of core flow. The same combination of waves can also account for observed fluctuations in the dipole field. Success in explaining these distinct phenomena motivates our hypothesis that the dominant periodicities in the historical record are due to MAC waves. We explore this hypothesis by developing a more sophisticated numerical model for the wave motion and by devising more comprehensive tests using a wide range of observations, including geomagnetic secular variation and changes in the length of day. We propose to invert these observations for the structure and properties of a stratified layer in the core. We will also explore the consequences of stratification on the long-term dynamics of the core using a new geodynamo model called Calypso, which was developed at UC Berkeley with prior NSF support. Calypso will be used to quantitatively assess the excitation of MAC waves in the core and to identify specific diagnostics that could support or refute our hypothesis.