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

来自地核的地磁信号：寻找波和稳定层结的证据

• 批准号: 1430526
• 负责人: Bruce Buffett
• 金额: $ 22.77万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-01 至 2018-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1430526&HistoricalAwards=false
• 关键词: Geomagnetic; Signals; Earth; Core; Searching

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.

地球磁场的波动为我们星球的内部动力学提供了独特的见解。液态金属芯中的湍流会产生地磁场，从而在地球表面引起可观察到的波动。 历史观察表明，几十年来的几个不同的周期性。这些波动的起源尚不清楚，但是它们的存在深刻影响了我们预测地磁领域变化的能力。拟议的研究旨在开发地球液体顶部的波的物理模型，目的是解释观察到的地磁波动。核心中的物理条件，包括密度分层的可能性，可以通过将模型预测拟合到观测值来恢复。该项目的一个潜在结果是我们解释地磁波动的方式发生了变化，因为某些波浪的存在可以大大改变我们对核心顶部动力学的理解。考虑到新的卫星任务（ESA的群体任务），以前所未有的分辨率监视地球的磁场，这项工作尤其及时。更好的观察和更现实的物理模型的结合应提高我们在接下来的几十年中预测地磁领域变化的能力。从更广泛的角度来看，我们的工作解决了地球的热和化学演化，因为内部的热量和物质的逐渐转移最终将当今地球的密度分层设定。 当液体的顶部通过热或组成梯度分层时，可能会出现几十年的波浪。这些波有时被称为MAC波，因为它们涉及磁性，阿基米德和科里奥利力的相互作用。初步结果表明，MAC的线性组合基于几个独立的核心流量估计值，可以很好地描述核心中的区域流。波浪的相同组合也可以解释偶极场中观察到的波动。在解释这些独特现象方面的成功激发了我们的假设，即历史记录中的主要周期性是由于MAC波。我们通过为波动运动开发更复杂的数值模型，并使用广泛的观测值（包括地磁世俗变化和一天的变化）来制定更全面的测试来探讨这一假设。我们建议将这些观察结果倒入核心中分层层的结构和特性。我们还将使用称为Calypso的新Geodynamo模型来探讨分层对核心长期动态的后果，该模型是在UC Berkeley开发的，并在先前的NSF支持下开发。 Calypso将用于定量评估核心中Mac波的激发，并确定可以支持或反驳我们假设的特定诊断。