Effect of thermal conductivity on the simultaneous formation of a stable region at the top of Earth's core and magnetic field generation over four billion years

TL;DR

This study investigates how thermal conductivity influences the formation of a stable stratified layer at Earth's core top and its role in sustaining Earth's magnetic field over billions of years, considering uncertainties in core properties.

Contribution

It provides a combined analysis of thermal conductivity and heat flow constraints, identifying conditions that support both stable stratification and magnetic field generation.

Findings

Stable stratification can coexist with magnetic field generation if CMB heat flow is 13-15 TW.

Thermal conductivity at CMB is estimated to be 77-121 W/m/K under these conditions.

The stratified layer is about 50 km thick, consistent with geomagnetic observations.

Abstract

The possibility of the emergence of a stratified region in the uppermost part of the Earth's outer core with long-term magnetic field generation is assessed, taking into account uncertainties in the thermal conductivity of the Earth's core and the present-day heat flow across the core-mantle boundary (CMB). The radial structures of the Earth's outer core are calculated for various values of thermal conductivity and CMB heat flow using a one-dimensional thermo-chemical model. The results show that there exist solutions that allow both emergence of stable stratification and long-term magnetic field generation although their thickness of stratified region is thinner than 100 km. In order to satisfy both emergence of stratified region and long-term magnetic field generation, possible value of the present-day CMB heat flow (13-15 TW) suggests a thermal conductivity of 77-121 W/m/K at CMB, which is in good agreement with the values estimated from the electrical conductivity measurements under the Earth' s core condition. The thickness of the stratified region in this case is about 50 km, which is also consistent with the thickness of the stratified region estimated from the geomagnetic secular variation. However, the proposed values of thermal conductivity obtained by this analysis could be smaller when the present-day CMB heat flow becomes smaller than the constraint used in this study.