Dynamo-based limit to the extent of a stable layer atop Earth's core

Abstract

The existence of a stably stratified layer underneath the core-mantle boundary (CMB) has been recently revived by corroborating evidences coming from seismic studies, mineral physics and thermal evolution models. Such a layer could find its physical origination either in compositional stratification due to the accumulation of light elements at the top or the core or in thermal stratification due to the heat flux becoming locally sub-adiabatic. The exact properties of this stably-stratified layer, namely its size $\mathcal{H}_S$ and the degree of its stratification characterised by the Brunt-V\"ais\"al\"a frequency $N$, are however uncertain and highly debated. A stable layer underneath the CMB can have crucial dynamical impacts on the geodynamo. Because of the inhibition of the convective motions, a stable layer is expected to primarily act as a low-pass filter on the magnetic field, smoothing out the rapidly-varying and small-scale features by skin effect. To investigate this effect more systematically, we compute 70 global geodynamo models varying the size of the stably-stratified layer from 0 to 300~km and its amplitude from $N/\Omega = 0$ to $N/\Omega \simeq 50$, $\Omega$ being the rotation rate. We show that the penetration of the convective flow in the stably-stratified layer is controlled by the typical size of the convective eddies and by the local variations of the ratio $N/\Omega$. Using quantitative measures of the degree of morphological semblance between the magnetic field obtained in numerical models and the geomagnetic field at the CMB, we establish an upper bound for the stable layer thickness $\mathcal{H}_s < (N/\Omega)^{-1} d_c$, $d_c$ being the horizontal size of the convective flow at the base of the stable layer. This defines a strong geomagnetic constraint on the properties of a stably-stratified layer beneath the CMB.