Global characteristics of geomagnetic field reversals from data and numerical simulations

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Understanding variabilities in the geomagnetic field reversal record is a central aspect in the priority programme DeepDyn, that aims at understanding Earth’s deep interior evolution throughout the geological history. However, to fully understand the reversal record and statistical properties of full reversals and short-lived excursions, which also sometimes include globally fully reversed field directions, a better understanding of the underlying processes of magnetic polarity changes is also necessary. Global models based on spherical harmonic basis functions offer the possibility to study global properties of the magnetic field during polarity transitions. The found characteristics can be directly compared to results from numerical simulations, which are based on considerations of the physical processes and force balances that drive the geodynamo. With the growing numbers of paleomagnetic records, both from sediment drill cores and lava sequences that are produced in paleomagnetic laboratories worldwide, global models have recently been developed for the Laschamps (41 000 years ago) and Norwegian-Greenland Basin (ca. 65 000 years ago) excursions, and a new model for the Matuyama-Brunhes reversal (ca. 780 000 years ago) is under construction. Here, we will develop the first global model of the Gauss-Matuyama reversal (ca. 2.6 million years ago). Together with the existing models this will enable us to study similarities and differences between at least four events, i.e. two full reversals and two excursions and provide a good understanding of robust statistical properties of some of the most recent magnetic field polarity changes. A growing number of numerical geodynamo simulations with different parameters and with or without boundary constraints, that e.g. mimic expected core-mantle boundary heat flux heterogeneities as inferred from seismic tomography models have reversals and excursions. We will perform a systematic study of different characteristics of polarity changes found in a large range of simulations. By comparisons between these and the results from the data based models we will gain a better understanding of the physical processes underlying the studied real events and the influence of the lower mantle on the geodynamo.