CAREER: Unraveling the Multiscale Asymptotic Dynamics of Planetary Dynamos

University Of Colorado At Boulder, Boulder CO

Investigators

Abstract

make revisions The magnetic field of the Earth, or the geomagnetic field, helps to shield Earth’s surface from solar radiation, and serves as an important reference for navigation. The geomagnetic field is dynamic: it shows a nearly steady westward movement; it can exhibit so-called excursions in which the magnetic north and south poles move toward the equator; and, on timescales ranging from hundreds of thousands of years to millions of years, it can undergo polarity reversals in which the magnetic poles swap positions entirely. Although these observations are well-known, the physics that controls them remains poorly understood. Massive winds present in the molten iron outer core of the Earth are responsible for the generation of the geomagnetic field, yet direct observation of these fluid motions is not possible. Instead, much of our understanding of the core and associated magnetic field, known as the geodynamo, is derived from computer models. While these models continue to provide valuable insight into the geodynamo, they cannot accurately represent the extreme conditions that characterize it. This project will address this limitation by using a combination of mathematical analysis and high-performance computer simulations to examine how fluid in the Earth’s outer core is pushed and pulled by different forces, and how these forces influence the resulting dynamics. The outcome of this work will be an improved understanding of the forces that control the evolution of the geomagnetic field, and the potential for building better models of it in the future. Additionally, most of the planets within the Solar System also have global-scale magnetic fields that are created through the same mechanism that creates the geomagnetic field; this work will also be of direct relevance for improving understanding of these planetary magnetic fields. In conjunction with the proposed research, a set of publicly available educational videos will be created with both undergraduate students and graduate students that highlights the underlying physics of planetary magnetics fields. Observations suggest that planetary and stellar dynamos are common throughout the universe. Progress on understanding these systems is made difficult due to the intrinsic nonlinearity and disparate spatiotemporal scales that characterize their dynamics. Current models utilize dynamical parameters that are many orders of magnitude different than those of planetary dynamos. However, the flows within the electrically conducting interiors of planets are thought to be balanced in the sense that two or more forces are dominant. The concept of balanced flows has been exploited with great success to efficiently model fluid flow in the Earth's atmosphere, oceans and sub-solidus mantle; such reduced models provide simplified physical understanding and substantially reduced computational cost in comparison to the un-approximated governing equations. No analogous model currently exists for understanding the spherical dynamo regions of planets. Moreover, the force balance that controls planetary dynamos is still debated. This project outlines a detailed investigation using both numerical modeling and perturbation theory that will systematically unravel the dynamical behavior of planetary dynamos by examining balances on multiple length scales. The work is based on the hypotheses that planetary dynamo physics is intrinsically multiscale, that three dominant length scales control their dynamics, and that distinct balances on these three scales exist. Preliminary results point to an alternative theory of planetary dynamo physics that has the potential to reconcile the current divergent viewpoints on planetary dynamo physics. To link the education and research activities of the principal investigator in a synergistic manner, a set of educational videos related to the proposed work will be developed with a team of undergraduate research students and graduate students and made available through the world wide web. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.