Electrical conductivity of the lowermost mantle explains absorption of core torsional waves at the equator

TL;DR

This study demonstrates that an electrically conducting layer at the Earth's mantle bottom can fully absorb core torsional waves at the equator, explaining their observed disappearance without requiring high mantle dissipation.

Contribution

It introduces a model showing how mantle conductivity causes complete absorption of torsional waves at the equator, advancing understanding of core-mantle interactions.

Findings

Total absorption occurs at specific conductance G when G = 1.3e8 S.

Reflection of waves depends on magnetic field strength and conductance.

Absorption explains wave disappearance without high mantle dissipation.

Abstract

Torsional Alfv{\'e}n waves propagating in the Earth's core have been inferred by inversion techniques applied to geomagnetic models. They appear to propagate across the core but vanish at the equator, exchanging angular momentum between core and mantle. Assuming axial symmetry, we find that an electrically conducting layer at the bottom of the mantle can lead to total absorption of torsional waves that reach the equator. We show that the reflection coefficient depends on G Br , where Br is the strength of the radial magnetic field at the equator, and G the conductance of the lower mantle there. With Br = 7e-4 T., torsional waves are completely absorbed when they hit the equator if G = 1.3e8 S. For larger or smaller G, reflection occurs. As G is increased above this critical value, there is less attenuation and more angular momentum exchange. Our finding dissociates efficient core-mantle coupling from strong ohmic dissipation in the mantle.