Insights into the last 100 ky of geomagnetic field variability using numerical dynamo simulations

Abstract

Recent observational models of the paleomagnetic field have revealed new details about geomagnetic field variability, which have yet to be adequately explored in numerical dynamo simulations. Here we present results from a new suite of dynamo simulations with computationally accessible rotating rates and diffusivities, an Earth-like magnetic Reynolds number, and a force balance that is consistent with the expected regime of the geodynamo, allowing comparison of simulated data and observational models. We find that such simulations are able to simultaneously reproduce the observed extreme rates of change in intensity and direction as well as the general amplitude of field variability over the last 100 ky, if the mean dipolarity is in the range 0.4-0.5. We use the paleosecular variation (PSV) index to identify a broad spectrum of polarity excursions and show that the PSV index is closely linked to the dipolarity of the simulation. Simulated excursional events are mostly associated with a decrease in the axial dipole moment with generally modest changes in dipole tilt. The excursions range from global events characterised by a reduction in the field contribution from solely the axial dipole component and a decrease in mean VDM in the manner of the Laschamp excursion, to localised events with anomalous activity in small regions reminiscent of the Mono Lake/Auckland excursion. Global events are generally longer than regional excursions, and reflect a drop in the total magnetic energy of the dynamo.