Rapidly rotating spherical Couette flow in a dipolar magnetic field: an experimental study of the mean axisymmetric flow

Abstract

In order to explore the magnetostrophic regime expected for planetary cores, experiments have been conducted in a rotating sphere filled with liquid sodium, with an imposed dipolar magnetic field (the DTS setup). The field is produced by a permanent magnet enclosed in an inner sphere, which can rotate at a separate rate, producing a spherical Couette flow. The flow properties are investigated by measuring electric potentials on the outer sphere, the induced magnetic field in the laboratory frame, and velocity profiles inside the liquid sodium using ultrasonic Doppler velocimetry. The present article focuses on the time-averaged axisymmetric part of the flow. The Doppler profiles show that the angular velocity of the fluid is relatively uniform in most of the fluid shell, but rises near the inner sphere, revealing the presence of a magnetic wind, and gently drops towards the outer sphere. The transition from a magnetostrophic flow near the inner sphere to a geostrophic flow near the outer sphere is controlled by the local Elsasser number. For Rossby numbers up to order 1, the observed velocity profiles all show a similar shape. Numerical simulations in the linear regime are computed, and synthetic velocity profiles are compared with the measured ones. In the geostrophic region, a torque-balance model provides very good predictions. We find that the induced magnetic field varies in a consistent fashion, and displays a peculiar peak in the counter-rotating regime. This happens when the fluid rotation rate is almost equal and opposite to the outer sphere rotation rate. The fluid is then almost at rest in the laboratory frame, and the Proudman-Taylor constraint vanishes, enabling a strong meridional flow. We suggest that dynamo action might be favored in such a situation.

in press in Physics of the Earth and Planetary Interiors

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• Spherical Couette flow in a dipolar magnetic field

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• Zonal shear and super-rotation in a magnetized spherical Couette flow experiment
• Modes and instabilities in magnetized spherical Couette flow
• Oscillatory thermal-inertial flows in liquid metal rotating convection
• Magneto--Coriolis waves in a spherical Couette flow experiment
• On the peculiar nature of turbulence in planetary dynamos
• Experimental investigation of the return flow instability in magnetic spherical Couette flow
• Dynamic Domains of DTS: Simulations of a Spherical Magnetized Couette Flow
• Magnetic induction maps in a magnetized spherical Couette flow experiment
• On the saturation of non-axisymmetric instabilites of magnetized spherical Couette flow
• Kinematic dynamo in spherical Couette flow
• Bound of dissipation on a plane Couette dynamo