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

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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.