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Eccentricity of the geomagnetic dipole caused by lopsided inner core growth


Seismic images of Earth’s inner core reveal an east–west dichotomy1,2,3. This dichotomy has been interpreted as lopsided growth, with faster solidification on one hemisphere of the inner core boundary, and slower solidification and perhaps melting on the other4,5. Today, the geomagnetic field that originates in the core is also slightly lopsided, with reconstructions revealing an offset of the dipole axis from Earth’s centre by more than 500 km. The geomagnetic dipole has moved into Earth’s Eastern Hemisphere over the past two centuries6,7 and has declined in intensity8. However, reconstructions of the magnetic field from the past 10,000 years9,10 show that the dipole previously had an offset to the west and reconstructions of the older palaeomagnetic field11,12 yield a dipole with an offset to the east. Here we use numerical simulations of the dynamo to show that lopsided inner core growth can cause persistent eccentricity of the geomagnetic field, with the best-fitting dipole axis offset in the direction of fastest inner core solidification. Our results therefore imply that faster growth may have occurred in the Western Hemisphere of the inner core during the past ten millennia. In contrast, offset of the palaeomagnetic dipole implies faster Eastern Hemisphere growth at earlier times. We suggest that a reorientation of the location of fastest inner core growth has occurred over the past 5 million years.

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Figure 1: Geomagnetic dipole eccentricity.
Figure 2: Dynamo boundary heterogeneity.
Figure 3: Dynamo eccentricity.
Figure 4: Eccentric dynamo internal structure.
Figure 5: Dipole eccentricity during a reversal.

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We gratefully acknowledge support from grant EAR-0909622 and Frontiers in Earth System Dynamics grant EAR-1135382 from the National Science Foundation.

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P.O. and R.D. jointly formulated the numerical dynamo calculations and P.O. analysed their output. R.D. formulated the offset dipole calculations and analysed the archaeomagnetic, palaeomagnetic and geomagnetic data to produce the dipole offset histories. P.O. and R.D. jointly wrote the paper.

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Correspondence to Peter Olson.

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The authors declare no competing financial interests.

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Olson, P., Deguen, R. Eccentricity of the geomagnetic dipole caused by lopsided inner core growth. Nature Geosci 5, 565–569 (2012).

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