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

Abstract

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 hypothesise 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.

New Paleointensities From the Skinner Cove Formation, Newfoundland, Suggest a Changing State of the Geomagnetic Field at the Ediacaran‐Cambrian Transition

“…While reversal frequencies in the Early Cambrian are still rather poorly constrained, Gallet et al. (2019) point out a similarity between the late Ediacaran and mid‐Cambrian hyperactivity episodes that could be connected to a reconfiguration of flow patterns in the outer core at a critical point of the aspect ratio of inner to outer core radii, where geodynamo simulations show a weaker field and higher reversal frequencies at the transition between small and large inner core regimes (Lhuillier et al., 2019). If the Ediacaran weak field state of the geodynamo does end with the rising paleointensities at the Ediacaran‐Cambrian transition, then the hyperactivity episodes (or their triggers) might be fundamentally different.…”

Multidisciplinary Constraints on the Thermal‐Chemical Boundary Between Earth's Core and Mantle

“…The present day inner-core:outer-core aspect ratio is 0.35. Lhuillier et al (2019) ran a suite of chemically driven dynamo simulations with a range of inner core sizes and found a transition from a "small inner-core" regime to a "large inner-core" regime at an aspect ratio of 0.2 where the field was weaker and polarity reversals more frequent.…”

Scaling behaviour in spherical shell rotating convection with fixed-flux thermal boundary conditions

“…Asymmetry between the inner and outer spherical boundaries leads to different aspect ratio systems having distinct temperature distributions with larger temperature drops occurring at the inner boundary relative to the outer boundary (Gastine et al 2015). The aspect ratio also changes the critical Rayleigh number at onset (Al-Shamali et al 2004) and can alter the morphology of convection driven magnetic fields (Lhuillier et al 2019). Fixed flux boundary conditions prefer longer wavelengths than the equivalent fixed temperature case at onset (Gibbons et al 2007) and lead to larger scale convective flows in the fully nonlinear regime (Sakuraba & Roberts 2009) although this diffence may not be present for very strongly supercritical dynamos (e.g.…”