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The existence of a stably stratified layer underneath the core-mantleboundary (CMB) has been recently revived by corroborating evidences coming fromseismic studies, mineral physics and thermal evolution models. Such a layercould find its physical origination either in compositional stratification dueto the accumulation of light elements at the top or the core or in thermalstratification due to the heat flux becoming locally sub-adiabatic. The exactproperties of this stably-stratified layer, namely its size $\mathcal{H}_S$ andthe degree of its stratification characterised by the Brunt-V\"ais\"al\"afrequency $N$, are however uncertain and highly debated. A stable layerunderneath the CMB can have crucial dynamical impacts on the geodynamo. Becauseof the inhibition of the convective motions, a stable layer is expected toprimarily act as a low-pass filter on the magnetic field, smoothing out therapidly-varying and small-scale features by skin effect. To investigate thiseffect more systematically, we compute 70 global geodynamo models varying thesize 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. Weshow that the penetration of the convective flow in the stably-stratified layeris controlled by the typical size of the convective eddies and by the localvariations of the ratio $N/\Omega$. Using quantitative measures of the degreeof morphological semblance between the magnetic field obtained in numericalmodels and the geomagnetic field at the CMB, we establish an upper bound forthe stable layer thickness $\mathcal{H}_s < (N/\Omega)^{-1} d_c$, $d_c$ beingthe horizontal size of the convective flow at the base of the stable layer.This defines a strong geomagnetic constraint on the properties of astably-stratified layer beneath the CMB.

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