Lopsided growth of Earth’s inner core could cause the geomagnetic field to be offset from the planet’s centre, reports a study published online this week in Nature Geoscience. Asymmetry along the boundary between the outer core and the mantle is thought to influence the structure of the magnetic field, but this study demonstrates that asymmetry of the inner core could be important too.

Peter Olson and Renaud Deguen use a numerical model to simulate the driving forces that generate Earth’s magnetic field. The inner core is thought to grow asymmetrically, with solidification occurring in one hemisphere and melting in the other. The authors show that incorporating this asymmetry into the model causes the axis of the magnetic field to be shifted away from the centre of the Earth, into the hemisphere where fastest solidification is occurring. Reconstructions of Earth’s magnetic field show that the axis of the dipole field was offset into the Western Hemisphere over the past approximately 10,000 years, and into the Eastern Hemisphere before that. The authors’ model results imply that the location of fastest inner core growth may also have shifted in the past few million years.

In an accompanying News and Views article, Christopher Finlay says: “Extrapolation of the details of simple numerical dynamo calculations to the Earth's core is controversial, but the prospect of fresh insights into the mechanism by which Earth's magnetic field operates is tantalizing.”