Inner core translation and the hemispheric balance of the geomagnetic field

- We model the observable geomagnetic consequences of inner core translation.
- Fast translation best matches observed spectral characteristics of the past 400 years.
- Flux patches form at American longitudes in the long-term average magnetic field.
- New measures are proposed to characterise this hemispheric imbalance.

Bulk translation of the Earth's inner core has been proposed as an explanation of observed quasi-hemispheric seismic structure. An important consequence of inner core translation would be the generation of a spherical harmonic degree one heat flow anomaly at the inner core boundary (ICB) that would provide an inhomogeneous forcing for outer core convection. We use geodynamo simulations to investigate the geomagnetic signature of such heterogeneity. Strong hemispheric heterogeneity at the ICB is found to produce a hemispheric signature in both the morphology of the magnetic field and its secular variation; in particular, we note the formation of high-intensity flux patches at high-latitudes and American longitudes in our model with strong ICB heterogeneity. In our simulations, this model provides the best match to the Earth's field over the past 400 years according to previously proposed measures of field structure. However, these criteria do not include the hemispheric balance of the field. We propose new criteria to measure this balance and find that our model with strong ICB heterogeneity produces the poorest match to the hemispheric balance of the historical geomagnetic field. Resolution of the hemispheric balance of the magnetic field throughout the Holocene would provide a strong test of any proposal of rapid inner core translation.