CUED Publications database

Segregation of helicity in inertial wave packets

Ranjan, A (2017) Segregation of helicity in inertial wave packets. Physical Review Fluids, 2. ISSN 2469-990X

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© 2017 American Physical Society. Inertial waves are known to exist in the Earth's rapidly rotating outer core and could be important for the dynamo generation. It is well known that a monochromatic inertial plane wave traveling parallel to the rotation axis (along positive z) has negative helicity while the wave traveling antiparallel (negative z) has positive helicity. Such a helicity segregation, north and south of the equator, is necessary for the α2-dynamo model based on inertial waves [Davidson, Geophys. J. Int. 198, 1832 (2014)GJINEA0956-540X10.1093/gji/ggu220] to work. The core is likely to contain a myriad of inertial waves of different wave numbers and frequencies. In this study, we investigate whether this characteristic of helicity segregation also holds for an inertial wave packet comprising waves with the same sign of Cg,z, the z component of group velocity. We first derive the polarization relations for inertial waves and subsequently derive the resultant helicity in wave packets forming as a result of superposition of two or more waves. We find that the helicity segregation does hold for an inertial wave packet unless the wave numbers of the constituent waves are widely separated. In the latter case, regions of opposite color helicity do appear, but the mean helicity retains the expected sign. An illustration of this observation is provided by (a) calculating the resultant helicity for a wave packet formed by superposition of four upward-propagating inertial waves with different wave vectors and (b) conducting the direct numerical simulation of a Gaussian eddy under rapid rotation. Last, the possible effects of other forces such as the viscous dissipation, the Lorentz force, buoyancy stratification, and nonlinearity on helicity are investigated and discussed. The helical structure of the wave packet is likely to remain unaffected by dissipation or the magnetic field, but can be modified by the presence of linearly stable stratification and nonlinearity.

Item Type: Article
Divisions: Div A > Fluid Mechanics
Depositing User: Cron Job
Date Deposited: 17 Jul 2017 20:02
Last Modified: 22 May 2018 08:21