Eccentricity of the geomagnetic dipole caused by lopsided inner core growth

Seismic images of Earth's inner core reveal an east-west dichotomy(1-3). 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 other(4,5). 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 centuries(6,7) and has declined in intensity(8). However, reconstructions of the magnetic field from the past 10,000 years(9,10) show that the dipole previously had an offset to the west and reconstructions of the older palaeomagnetic field(11,12) 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.