Impact of inner-core size on the dipole field behaviour of numerical dynamo simulations

From a suite of 56 chemically driven dynamo simulations with aspect ratio (inner to outer core radii) ranging from 0.10 to 0.44, we conduct the first systematic investigation of the impact of inner-core size on the reversing behaviour of dynamos. We show that the growth of the inner core leads to a transition between a small inner-core' regime ( 0.18), when the field produced is intermediately strong and dipolar, and a large inner-core' regime ( > 0.26), when the field is stronger and more dipolar. During that transition the field is weaker and slightly less dipolar. For aspect ratios 0.20 0.22, reversal frequencies may be more sensitive to changes in the vigour of the convection, allowing high frequencies to be reached much more easily. Although other factors than the size of the inner core likely contribute to controlling the reversal frequency of the Earth's dynamo, we hypothesize that the occurrence of such a transition for the Earth's core between the end of the Precambrian and the end of the Devonian could possibly account for the manifestation of an unusual long-lasting episode of predominantly reversal hyperactivity and complex low intensity fields during that still poorly documented period of time.