Pressure torque of torsional Alfvén modes acting on an ellipsoidal mantle

Abstract

We investigate the pressure torque between the fluid core and the solid mantle arising from magnetohydrodynamic modes in a rapidly rotating planetary core. A two-dimensional reduced model of the core fluid dynamics is developed to account for the non-spherical core-mantle boundary. The simplification of such a quasi-geostrophic model rests on the assumption of invariance of the equatorial components of the fluid velocity along the rotation axis. We use this model to investigate and quantify the axial torques of linear modes, focusing on the torsional Alfvén modes (TM) in an ellipsoid. We verify that the periods of these modes do not depend on the rotation frequency. Furthermore, they possess angular momentum resulting in a net pressure torque acting on the mantle. This torque scales linearly with the equatorial ellipticity. We estimate that for the TM calculated here topographic coupling to the mantle is too weak to account for the variations in the Earth's length-of-day.

accepted in Geophysical Journal International

References in corpus (4)

• Hydromagnetic quasi-geostrophic modes in rapidly rotating planetary cores
• Inviscid instabilities in rotating ellipsoids on eccentric Kepler orbits
• The coupling between inertial and rotational eigenmodes in planets with liquid cores
• pizza: An open-source pseudo-spectral code for spherical quasi-geostrophic convection

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• Did lunar tides sustain the early Earth's dynamo?