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On flow magnitude and field-flow alignment at Earth's core surface

Finlay, Christopher C. ; Amit, Hagay

In: Geophysical Journal International, 2011, vol. 186, no. 1, p. 175-192

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    Titre
    • On flow magnitude and field-flow alignment at Earth's core surface
    Auteur
    • Finlay, Christopher C.. Institut für Geophysik, Sonneggstrasse 5, ETH Zürich, Zürich, CH-8092, Switzerland. E-mail: cfinlay@erdw.ethz.ch
    • Amit, Hagay. CNRS UMR 6112, Université de Nantes, Laboratoire de Planétologie et de Géodynamique, 2 Rue de la Houssinière, Nantes, F-44000, France
    Type de document
    • Postprint
    Identifiant OAI-PMH
    • oai:doc.rero.ch:300347
    Summary
    We present a method to estimate the typical magnitude of flow close to Earth's core surface based on observational knowledge of the geomagnetic main field (MF) and its secular variation (SV), together with prior information concerning field-flow alignment gleaned from numerical dynamo models. An expression linking the core surface flow magnitude to spherical harmonic spectra of the MF and SV is derived from the magnetic induction equation. This involves the angle γ between the flow and the horizontal gradient of the radial field. We study γ in a suite of numerical dynamo models and discuss the physical mechanisms that control it. Horizontal flow is observed to approximately follow contours of the radial field close to high-latitude flux bundles, while more efficient induction occurs at lower latitudes where predominantly zonal flows are often perpendicular to contours of the radial field. We show that the amount of field-flow alignment depends primarily on a magnetic modified Rayleigh number Raη=αg0ΔTD/ηΩ, which measures the vigour of convective driving relative to the strength of magnetic dissipation. Synthetic tests of the flow magnitude estimation scheme are encouraging, with results differing from true values by less than 8 per cent. Application to a high-quality geomagnetic field model based on satellite observations (the xCHAOS model in epoch 2004.0) leads to a flow magnitude estimate of 11-14 km yr−1, in accordance with previous estimates. When applied to the historical geomagnetic field model gufm1 for the interval 1840.0-1990.0, the method predicts temporal variations in flow magnitude similar to those found in earlier studies. The calculations rely primarily on knowledge of the MF and SV spectra; by extrapolating these beyond observed scales the influence of small scales on flow magnitude estimates is assessed. Exploring three possible spectral extrapolations we find that the magnitude of the core surface flow, including small scales, is likely less than 50 km yr−1