Persistent patterns in the geomagnetic field over the past 2.5 Myr

Abstract

HISTORICAL geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations ('lobes') at the surface of the liquid core and regions of rapid change extending from the Atlantic to the Indian Ocean¹. Palaeomagnetic data define a time-average over several thousand years² which might reflect the stationary parts of the present field, but unfortunately the historical record is too short to provide a satisfactory average³. Here we model palaeomagnetic directions from the past 2.5 Myr using the same methods as for modern data⁴, and find the two northern lobes in the same position as today, over Arctic Canada and Siberia. In southern regions, by contrast, the field appears to have been smoothed out, as might be expected from the current rapid secular variation¹. We propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years, as would occur if the solid mantle controls flow at the top of the core.