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

地球磁场的波动为我们提供了对地球内部动态的独特见解。液态金属核中的湍流产生了地磁场，在地球表面引起了明显的波动。历史观察表明，在几十年的范围内，有几个明显的周期性。这些波动的起源尚不清楚，但它们的存在深刻地影响了我们预测地磁场变化的能力。这项拟议的研究旨在开发地球液核顶部的波的物理模型，目的是解释观察到的地磁波动。核心的物理条件，包括密度分层的可能性，可以通过将模型预测与观测相匹配来恢复。该项目的一个潜在结果是改变了我们解释地磁波动的方式，因为某些类型的波的存在可以极大地改变我们对地核顶部附近动力学的理解。考虑到一项新的卫星任务(欧空局的蜂群任务)以前所未有的分辨率监测地球磁场，这项工作尤其及时。更好的观测和更现实的物理模型相结合，应该会提高我们预测未来几十年地磁场变化的能力。更广泛地说，我们的工作涉及地球的热和化学演化，因为热量和物质在地球内部的逐渐转移最终决定了当今地球的密度分层。当液核的顶部被温度梯度或成分梯度分层时，可能会产生周期为几十年的波。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，基于对岩心流动的几个独立估计，MAC的线性组合能够很好地描述岩心中的地带性流动。同样的波组合也可以解释偶极场中观察到的波动。成功地解释了这些不同的现象，激发了我们的假设，即历史记录中的主导周期是由于MAC波。我们通过开发一个更复杂的波动数值模型和设计更全面的测试来探索这一假设，这些测试使用了广泛的观测，包括地磁的长期变化和日长的变化。我们建议对这些观测结果进行反转，以确定地核中分层层的结构和性质。我们还将使用一种名为Calypso的新地球发电机模型来探索分层对核心长期动力学的影响，该模型是在加州大学伯克利分校开发的，之前得到了NSF的支持。Calypso将被用来定量评估核心中MAC波的激发，并识别可能支持或反驳我们的假设的特定诊断。

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.