Inertial–Alfvén waves as columnar helices in planetary cores

We consider a rapidly rotating, Boussinesq fluid stirred by buoyant anomalies. In such a system it is known that, in the absence of a magnetic field, inertial waves whose wave vectors lie normal to the rotation axis play a key role in establishing quasi-geostrophic motion. In particular, buoyant ano...

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Bibliographic Details
Published in:Journal of fluid mechanics 2016-10, Vol.805, Article R2
Main Authors: Bardsley, O. P., Davidson, P. A.
Format: Article
Language:English
Subjects:
Anomalies
Boussinesq approximation
Boussinesq equations
Buoyancy
Cores
Dynamics
Earth
Elongation
Fluid dynamics
Fluid mechanics
Fluids
Group velocity
Helices
Helicity
Inertial waves
Kinetic energy
Magnetic field
Magnetic fields
Planetary cores
Planetary rotation
Quasi-geostrophic motion
Rapids
Rotating fluids
Rotating generators
Rotation
Spontaneous emission
Vectors
Velocity
Vortices
Wave dispersion
Wave packets
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Summary:We consider a rapidly rotating, Boussinesq fluid stirred by buoyant anomalies. In such a system it is known that, in the absence of a magnetic field, inertial waves whose wave vectors lie normal to the rotation axis play a key role in establishing quasi-geostrophic motion. In particular, buoyant anomalies radiate low-frequency inertial wave packets which disperse along the rotation axis, leading to axially elongated columnar vortices. Here we focus on the influence of an ambient magnetic field on this process, motivated by the dynamics of planetary cores. We find that, once again, the waves responsible for establishing quasi-geostrophic structures have wave vectors normal to the rotation axis; however, these are not conventional inertial waves, but rather hybrid ‘inertial–Alfvén waves’. Their frequency equals that of an Alfvén wave but their axial group velocity is half that of the equivalent inertial wave. They have maximal kinetic, magnetic and cross-helicity, carry magnetic and kinetic energy in equal amounts, and are particularly potent in establishing columnar, helical vortices through the spontaneous emission of axially elongated wave packets. Although our hybrid inertial–Alfvén waves have been overlooked in dynamo literature to date, we speculate that they in fact play a central role in planetary dynamos.
ISSN:0022-1120
1469-7645
DOI:10.1017/jfm.2016.577