Quantifying changes in dipole polarity reversal frequency using a combination of geodynamo simulations and stochastic models
Abstract
Motion of liquid-iron in Earth's outer core generates the main part of the geomagnetic field through a self-sustaining dynamo. Using measurements of the paleomagnetic field recorded in geologic materials, we can construct time-varying models of past field behaviors. Field models, in turn, provide insight into conditions in the deep earth and Earth's thermal evolution. Unfortunately, paleomagnetic models have limited resolution, so we are restricted in what we can learn from paleomagnetic data alone. Numerical simulations are complementary because they do not share the limitations in resolution. However, simulations cannot yet be run in the parameter space of the geodynamo. An interdisciplinary approach linking paleomagnetic observations and numerical simulations is appealing because they complement one another. An interesting behavior of Earth's magnetic field that can be explored with such an approach is the frequency of dipole polarity reversals. During the past 10 Myr there have been ~4 polarity reversals per Myr. This value has varied over the past 200 Myr. The Cretaceous normal-polarity superchron was a period when the reversal rate was zero from 83.0 to 120.6 Myr ago. During the Jurassic (201.3-145.5 Myr ago) the magnetic field had a high rate of reversals (4 to 12 reversals per Myr). Here we present a suite of geodynamo simulations with a range of Rayleigh numbers traversing the non-reversing to reversing transition at Ekman numbers of 3e-4 and 5e-5. Dynamos with low Ekman numbers cannot be easily run for long time spans, so we explore the use of stochastic models to extend the simulated ADM variations of dynamo simulations. The idea is to use fluctuations in the dynamo simulations to construct the stochastic models and use long realizations of the stochastic models to compute more robust reversal statistics. The dynamos display a range of field morphologies becoming less dipolar with increasing Rayleigh number. The reversing dynamos are more dipolar at lower Ekman number. These field morphologies can be compared with a compilation of paleomagnetic datasets, and their dynamics explored.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP34A..01A
- Keywords:
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- 1513 Geomagnetic excursions;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1517 Magnetic anomalies: modeling and interpretation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1522 Paleomagnetic secular variation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- GEOMAGNETISM AND PALEOMAGNETISM