Torsional waves driven by convection and jets in Earth’s liquid core

Teed, R.J. , Jones, C.A. and Tobias, S.M. (2019) Torsional waves driven by convection and jets in Earth’s liquid core. Geophysical Journal International, 216(1), pp. 123-129. (doi: 10.1093/gji/ggy416)

Abstract

Turbulence and waves in Earth’s iron-rich liquid outer core are believed to be responsible for the generation of the geomagnetic field via dynamo action. When waves break upon the mantle they cause a shift in the rotation rate of Earth’s solid exterior and contribute to variations in the length-of-day on a ∼6-yr timescale. Though the outer core cannot be probed by direct observation, such torsional waves are believed to propagate along Earth’s radial magnetic field, but as yet no self-consistent mechanism for their generation has been determined. Here we provide evidence of a realistic physical excitation mechanism for torsional waves observed in numerical simulations. We find that inefficient convection above and below the solid inner core traps buoyant fluid forming a density gradient between pole and equator, similar to that observed in Earth’s atmosphere. Consequently, a shearing jet stream—a ‘thermal wind’—is formed near the inner core; evidence of such a jet has recently been found. Owing to the sharp density gradient and influence of magnetic field, convection at this location is able to operate with the turnover frequency required to generate waves. Amplified by the jet it then triggers a train of oscillations. Our results demonstrate a plausible mechanism for generating torsional waves under Earth-like conditions and thus further cement their importance for Earth’s core dynamics.

• Item Type: Articles
• Additional Information: This work was partly supported by the Natural Environment Research Council, grant NE/I012052/1.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Teed, Dr Robert
• Authors: Teed, R.J., Jones, C.A., and Tobias, S.M.
• College/School: College of Science and Engineering > School of Mathematics and Statistics > Mathematics
• Journal Name: Geophysical Journal International
• Publisher: Oxford University Press
• ISSN: 0956-540X
• ISSN (Online): 1365-246X
• Published Online: 08 October 2018
• Copyright Holders: Copyright © 2018 The Authors
• First Published: First published in Geophysical Journal International 216(1):123-129
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher