Mantle-induced temperature anomalies do not reach the inner core boundary

Christopher J Davies,Jon E Mound

doi:10.1093/gji/ggz254

Abstract

SUMMARY Temperature anomalies in Earth’s liquid core reflect the vigour of convection and the nature and extent of thermal core–mantle coupling. Numerical simulations suggest that longitudinal temperature anomalies forced by lateral heat flow variations at the core–mantle boundary (CMB) can greatly exceed the anomalies that arise in homogeneous convection (i.e. with no boundary forcing) and may even penetrate all the way to the inner core boundary. However, it is not clear whether these simulations access the relevant regime for convection in Earth’s core, which is characterized by rapid rotation (low Ekman number E) and strong driving (high Rayleigh number Ra). We access this regime using numerical simulations of non-magnetic rotating convection with imposed heat flow variations at the outer boundary (OB) and investigate the amplitude and spatial pattern of thermal anomalies, focusing on the inner and outer boundaries. The 108 simulations cover the parameter range 10−4 ≤ E ≤ 10−6 and Ra = 1−800 times the critical value. At each Ra and E we consider two heat flow patterns—one derived from seismic tomography and the hemispheric $Y_1^1$ spherical harmonic pattern—with amplitudes measured by the parameter q⋆ = 2.3, 5 as well as the case of homogeneous convection. At the OB the forcing produces strong longitudinal temperature variations that peak in the equatorial region. Scaling relations suggest that the longitudinal variations are weakly dependent on E and Ra and are much stronger than in homogeneous convection, reaching O(1) K at core conditions if q⋆ ≈ 35. At the inner boundary, latitudinal and longitudinal temperature variations depend weakly on Ra and q⋆ and decrease strongly with E, becoming practically indistinguishable between homogeneous and heterogeneous cases at E = 10−6. Interpreted at core conditions our results suggest that heat flow variations on the CMB are unlikely to explain the large-scale variations observed by seismology at the top of the inner core.

Highlights

Convection in Earth’s liquid core sustains Earth’s magnetic field through a dynamo process that converts kinetic energy into magnetic energy
Considering first the outer boundary (OB), the poles are anomalously hot compared to the mid-latitudes in all models since convection is suppressed inside the tangent cylinder, though the effect weakens as q increases
We have studied non-magnetic convection in a rotating spherical shell with lateral heat flow variations imposed at the OB

Summary

Introduction

Convection in Earth’s liquid core sustains Earth’s magnetic field through a dynamo process that converts kinetic energy into magnetic energy. Radial variations reflect the vigour of core convection and the relative strength of thermal and chemical driving (Lister & Buffett 1995). In longitude, may reflect coupling of core convection to temperature variations in the lowermost mantle (Buffett 2007). A key question is whether lateral variations in temperature imposed by mantle structure at the core–mantle boundary (CMB) persist to the inner core boundary (ICB) where a striking hemispheric variation in seismic properties is observed (Aubert et al 2008; Monnereau et al 2010; Gubbins et al 2011; Souriau & Calvet 2015).

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Conclusion