NSFGEO-NERC: Deciphering the Dynamics of Geomagnetic Excursions

Earth’s internal magnetic field is produced by fluid motions in the liquid, and its sustained presence is of great societal importance because, in addition to providing navigational guidance, it effectively shields the surface environment from the solar wind and space weather. The magnetic field varies on a wide range of time scales and the most striking variations occurring on geological time scales are polarity excursions and reversals. These events can lead to significantly weakened the global field strength, have the societal impacts arising from reduced magnetic shielding of the surface environment, and may be central to understanding the processes that generate and sustain the geomagnetic field in the liquid core. Excursions have received relatively little attention compared to reversals despite occurring more frequently and lasting long enough to cause potentially significant societal disruption. Recent observational models spanning the past hundred thousand years are illuminating the evolution of excursional fields in previously inaccessible detail. When combined with advances in numerical simulation of the physical field generation process, this paves the way for a new approach to understand such enigmatic events. An integral aspect of the work will be building models enabling further synergy across the geomagnetic, paleomagnetic, and numerical geodynamo and, by extension, the international Studies of Earth’s Deep Interior and planetary science communities. This is a collaborative proposal between scientists at the University of California San Diego and Leeds University, and is therefore co-funded by the National Science Foundation (NSF) and the United Kingdom’s Natural Environment Research Council (NERC). Several global, spatial, and temporal representations of Earth’s magnetic field cover 0-100 ka or subsections thereof, and document up to 5 geomagnetic excursions with qualitative similarities as well as distinct differences. Formal definition of excursional field perturbations, duration, local asynchroneity, etc., can be made via the Paleosecular Variation Index allowing comparisons with numerical geodynamo simulations. Expanding the time interval to 0-120 ka, we will produce the first high-resolution models with quantified uncertainty for the Post-Blake excursion at ~95 ka and the Hilina Pali excursion at ~20 ka. In parallel, numerical simulations will build on research at Leeds University (UK) that is defining the requirements for “Earth-like” field behavior. The focus of this proposal is to improve understanding of the nature and origin of geomagnetic excursions using these two synergistic interlinked components. We will (1) Use new and existing global and time-dependent observational models of several excursions during the past 120 kyr to characterize field behavior before, during, and after excursions; (2) Analyze the excursion mechanism and predictability in new geodynamo simulations conducted with a dominant force balance that reflects expected behavior in Earth’s liquid outer core; (3) Develop formal criteria for defining excursions and reconcile paleofield behavior with current and emerging views on Earth-like numerical dynamo simulations. A vital aspect of this proposal is exploiting existing synergy between the Scripps Institution of Oceanography (SIO) observational geomagnetic field modelling and paleosecular variation analysis and Leeds simulations of core dynamics. Observational models will contribute to new Earth-like standards for simulations, which will in turn be used to make high-resolution predictions of observable field variations that can be sought in the lower resolution observational data. We expect to enhance understanding of the dynamics of geomagnetic excursions, including their predictability and relation to polarity reversals. 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.