Tuning Dynamo Models to Paleomagnetic Observations

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Throughout Earth's history records of the geomagnetic field have been preserved in archeological artifacts, or in globally distributed sedimentary and igneous rocks. Compilations of archeomagnetic and paleomagnetic data with associated age constraints are now routinely used to make continuous time-varying models of the geomagnetic field that can extend from the present to 10 thousand years ago. The work proposed will link these advances in paleomagnetism with those in studies of the geodynamo to produce more realistic dynamo models and understand how they are linked to deep Earth processes including dynamics of its fluid outer core and core-mantle interactions. Results from our research will contribute to inter-disciplinary studies of the deep Earth and will be broadly disseminated to others through publications in peer-reviewed literature and via interactions with other deep-Earth researchers contributing to other NSF-funded initiatives, particularly the CIDER (Cooperative Institute for Deep Earth Research) and CIG (Computational Infrastructure for Geodynamics). We make a habit of systematic electronic archiving of tools and products in the EarthRef Digital Archive community database. These will be of use to other researchers in geo/paleomagnetism, and the images, models, and animations that we also archive will be useful for general research and educational purposes at multiple levels, providing tools for public education about the geomagnetic field. This project will also enhance future STEM activities and research through mentoring and training of a graduate student at Scripps. Education and outreach efforts will result from the PI's interactions with Scripps' Birch Aquarium and the ongoing NSF GK-12 program at Scripps.Our assessment of millennial geomagnetic models suggests that they are adequate to study large-scale geomagnetic field variations in many regions of the world. On thousand year time-scales we observe mobility of the main concentrations of magnetic flux on the core-mantle boundary, generally low flux over the poles, and drift of features at lower latitudes. On million year time-scales we can map the average field and its statistical variability and we have time-varying models of the dipole moment, including its dramatic fall during polarity excursions and reversals. Parallel progress in dynamo simulation allows routine production of predominantly dipolar fields that are capable of self-reversal and, with appropriate boundary conditions, can mimic spatial structures observed over recent centuries and provide encouraging correlations over longer time-scales. The intent of this proposal is to tune such models so that they do a better job of reproducing inferred properties of paleofield variations, thereby enhancing our understanding of core dynamics. This will be accomplished by using a suite of simulations with accessible values of the Ekman, Rayleigh, and Prandtl numbers and engaging in a systematic exploration of changes in both vertical and lateral buoyancy profiles in the outer core that are linked to models of the thermal evolution of the core. This regime will allow investigation of the influence of strongly stably stratified layers at the top of the liquid core on the observable magnetic fields, and will provide insight about the thermal and chemical processes that lend structure and stability to the geomagnetic field. The magnetic field is one of the few direct observables that carry information about the deep Earth: we therefore expect our results to influence mineralogical and seismological models of deep Earth structure and core-mantle interaction.