Andrew John Biggin, Chris Davies , Jonathan Mound, Richard Bono

A crucial characteristic of Earth’s magnetic field, known since 1600, is that it approximates a dipole (bar magnet) aligned with the planetary rotation axis. Previous studies have disagreed over the extent to which this situation has persisted through geological time which is important for determining solar wind shielding and building palaeogeographic reconstructions. It has recently become possible to quantify this axial dipole dominance back in time using the equatorial variability of the palaeomagnetic field recorded in rocks. Here we show that this aspect of palaeomagnetic field behaviour was extraordinarily stable across three periods spanning the last 3 billion years despite being an unstable characteristic in most numerical geodynamo simulations. We further demonstrate a means to reproduce this stability in new rapidly rotating and turbulent geodynamo simulations. Large, seismically-inferred, hot provinces in the lowermost mantle suppress core flow beneath them, impose large-scale order on the magnetic field emerging from the core at low latitudes, and stabilise the palaeomagnetic behaviour of the simulations enabling them to reproduce the observations. Our combined modelling and observational results suggest that features akin to these large low velocity provinces in the lowermost mantle today may have stabilised Earth’s magnetic field throughout much of geological time.