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  On the generation and segregation of helicity in geodynamo simulations

Ranjan, A., Davidson, P. A., Christensen, U. R., & Wicht, J. (2020). On the generation and segregation of helicity in geodynamo simulations. Geophysical journal international, 221(2), 741-757. doi:10.1093/gji/ggaa011.

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Ranjan, A, Author
Davidson, P A, Author
Christensen, Ulrich R.1, Author              
Wicht, Johannes1, Author              
Affiliations:
1Department Planets and Comets, Max Planck Institute for Solar System Research, Max Planck Society, ou_1832288              

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 Abstract: Helicity, the inner product of velocity and vorticity, is considered an important ingredient for the maintenance of a dipolar magnetic field in the geodynamo. Outside the tangent cylinder—an imaginary cylinder which circumscribes the inner core—a spatial segregation of helicity has been observed in several simulations, being negative in the north and positive in the south. Such a segregation pattern is important for a dynamo that relies on the α-effect. However, the origin of this pattern in these simulations is poorly understood. In this paper, we use three strongly forced numerical dynamo solutions to study the various sources of helicity, including those due to buoyancy (HT)⁠, Coriolis, Lorentz and viscous forces. We find a strong spatial correlation between the segregation pattern of helicity and HT both in the instantaneous and the time-averaged results. Our results show that, outside the tangent cylinder, HT is dominated by the product −uz∂T/∂φ⁠, where uz is the vertical velocity component and T is the temperature perturbation. It is known that when inertial waves are launched from a localized buoyant anomaly, HT takes the same sign as the local helicity. We conjecture that this is the reason for the spatial correlation between HT and helicity in our simulation results. The flow in our simulations being strongly turbulent, this effect seems to be a statistical one and manifests itself most clearly in the averaged quantities.

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Language(s): eng - English
 Dates: 2020
 Publication Status: Published in print
 Pages: -
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1093/gji/ggaa011
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Title: Geophysical journal international
Source Genre: Journal
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Publ. Info: Oxford [u.a.] : Blackwell
Pages: - Volume / Issue: 221 (2) Sequence Number: - Start / End Page: 741 - 757 Identifier: ISSN: 0956-540X
CoNE: https://pure.mpg.de/cone/journals/resource/0956-540X