Influence of the Earth's inner core on geomagnetic fluctuations and reversals

Abstract

IN view of its relatively small size (one-third the radius of the outer core), many geodynamo models neglect the inner core entirely¹, or otherwise treat it as a non-conducting insulator^2,3. In a previous steady-state model⁴, we considered some effects of a finitely conducting inner core, in particular the resulting electromagnetic coupling between inner and outer core. Here we include a prescribed buoyancy force, which is geophysically more realistic, and also yields time-dependent rather than time-independent solutions. The field in the finitely conducting inner core does not then adjust instantaneously to the field in the outer core, but has a diffusive timescale of its own of a few thousand years. Rather large, rapid fluctuations in the outer core are then effectively averaged out by the inner core, producing a relatively stable external dipole field. We speculate that a geomagnetic reversal could only occur as a result of a particularly large fluctuation, large enough and lasting long enough to reverse the field throughout the inner core as well.