Affiliation:
1. Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, UK
Abstract
SUMMARY
In a rapidly rotating Boussinesq fluid, buoyant anomalies radiate low-frequency inertial wave packets that disperse along the rotation axis. The wave packets lead to axially elongated vortices, which propagate negative (positive) kinetic helicity upwards (downwards) with respect to the rotation vector. The kinetic helicity carried by the inertial wave packets is near-maximal relative to the velocity and vorticity fields. In classical mean-field theory, kinetic helicity is often associated with the α-effect, which is thought to be an important ingredient for planetary dynamos. The modification of inertial wave packets in the presence of a transverse uniform magnetic field is investigated here, motivated by small-scale dynamics in planetary cores, where a large-scale magnetic field affects fluid motions. We study numerically the dispersion of wave packets from an isolated buoyant source and from a random layer of buoyant anomalies, while varying the Lehnert number Le—the ratio of the frequencies of Alfvén and inertial waves. We find that for Le < 0.1, the vortices are columnar and continue to segregate kinetic helicity so that it is negative (positive) above (below) the buoyant source. Importantly, the wave packets induce an α-effect, which remains strong and coherent for Earth-like values of the Lehnert number (Le < 0.1). The interaction of wave packets emitted by multiple neighbouring buoyant sources results in an α-effect that is stronger than the α-effect induced by wave packets launched from an isolated buoyant source, and we provide an analytical explanation for this. The coherence of the α-effect induced by the wave packets, for Earth-like values of the Lehnert number, lends support to the α2 dynamo model driven by helical waves.
Publisher
Oxford University Press (OUP)
Subject
Geochemistry and Petrology,Geophysics
Reference24 articles.
1. Approaching Earth’s core conditions in high-resolution geodynamo simulations;Aubert;Geophys. J. Int.,2019
2. Inertial–Alfvén waves as columnar helices in planetary cores;Bardsley;J. Fluid Mech.,2016
3. The dispersion of magnetic-Coriolis waves in planetary cores;Bardsley;Geophys. J. Int.,2017
4. Scaling properties of convection-driven dynamos in rotating spherical shells and application to planetary magnetic fields;Christensen;Geophys. J. Int.,2006
Cited by
1 articles.
订阅此论文施引文献
订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献