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

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• 作者：Nathanaël Schaeffer ; Dominique Jault
• 刊名：Geophysical Research Letters
• 出版年：2016
• 出版时间：28 May 2016
• 年：2016
• 卷：43
• 期：10
• 页码：4922-4928
• 全文大小：411.0KB
• ISSN：1944-8007

文摘

Torsional Alfvén 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 ta-equation-construct="true" class="math-equation-construct">ta-equation-image="true" class="math-equation-image">ta-equation-mathml="true" class="math-equation-mathml" style="display:none">th xmlns:mml="http://www.w3.org/1998/Math/MathML">ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">Gttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">t="true">B~rth>, where ta-equation-construct="true" class="math-equation-construct">ta-equation-image="true" class="math-equation-image">ta-equation-mathml="true" class="math-equation-mathml" style="display:none">th xmlns:mml="http://www.w3.org/1998/Math/MathML">ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">t="true">B~rth> is the strength of the radial magnetic field at the equator, and G the conductance of the lower mantle there. With ta-equation-construct="true" class="math-equation-construct">ta-equation-image="true" class="math-equation-image">ta-equation-mathml="true" class="math-equation-mathml" style="display:none">th xmlns:mml="http://www.w3.org/1998/Math/MathML">ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">t="true">B~rttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">=ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">7ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">×ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">1ttp://www.wiley.com/namespaces/wiley" xmlns:wiley="http://www.wiley.com/namespaces/wiley/wiley" xmlns:cr="urn://wiley-online-library/content/render" xmlns="http://www.w3.org/1998/Math/MathML">0−4th> T, torsional waves are completely absorbed when they hit the equator if G≃1.3 × 10⁸ 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.