Sensitivity of the geomagnetic axial dipole to thermal core-mantle interactions

Since the work of William Gilbert in 1600 (ref. 1), it has been widely beli eved that the Earth's magnetic field, when suitably time-averaged, is that of a magnetic dipole positioned at the Earth's centre and aligned with the rotational axis. This 'geocentric axial dipole' (GAD) hypothesis has been t he central model for the study of the Earth's magnetic field-it underpins a lmost all interpretations of palaeomagnetic data, whether for studies of pa laeomagnetic secular variation, for plate tectonic reconstructions, or for studies of palaeoclimate(2). Although the GAD hypothesis appears to provide a good description of the Earth's magnetic field over at least the past 10 0 Myr (ref. 2), it is difficult to test the hypothesis for earlier periods, and there is some evidence that a more complicated model is required for t he period before 250 Myr ago(3). Kent and Smethurst(3) suggested that this additional complexity might be because the inner core would have been small er at that time. Here I use a numerical geodynamo model and find that reduc ing the size of the inner core does not significantly change the character of the magnetic field. I also consider an alternative process that could le ad to the breakdown of the GAD hypothesis on this timescale, the evolution of heat-flux variations at the core-mantle boundary, induced by mantle conv ection. I find that a simple pattern of heat-flux variations at the core-ma ntle boundary, which is plausible for times before the Mesozoic era, result s in a strong octupolar contribution to the field, consistent with previous findings(3).