Experimental study of super-rotation in a magnetostrophic spherical Couette flow

• Published in: Geophysical and astrophysical fluid dynamics Vol. 100; no. 4-5; pp. 281 - 298
• Main Authors: Nataf, H.-C., Alboussière, T., Brito, D., Cardin, P., Gagnière, N., Jault, D., Masson, J.-P., Schmitt, D.
• Format: Journal Article
• Language: English
• Published: Taylor & Francis Group 01-08-2006; Taylor & Francis
• Subjects: Earth Sciences; Environmental Sciences; Geophysics; Global Changes; Hartmann layers; Liquid sodium experiment; Magnetostrophic; Physics; Sciences of the Universe; Spherical Couette flow; Super-rotation; Ultrasonic Doppler velocimetry; ultrasonic Doppler velocimetry; super-rotation; liquid sodium experiment; Hartmann layers; magnetostrophic; Spherical Couette flow
• ISSN: 0309-1929; 1029-0419
• DOI: 10.1080/03091920600718426

We report measurements of electric potentials at the surface of a spherical container of liquid sodium in which a magnetized inner core is differentially rotating. The azimuthal angular velocities inferred from these potentials reveal a strong super-rotation of the liquid sodium in the equatorial region, for small differential rotation. Super-rotation was observed in numerical simulations by Dormy et al. (Dormy, E., Cardin, P. and Jault, D., MHD flow in a slightly differentially rotating spherical shell, with conducting inner core, in a dipolar magnetic field, Earth Planet. Sci. Lett., 1998, 160, 15--30). We find that the latitudinal variation of the electric potentials in our experiments differs markedly from the predictions of a similar numerical model, suggesting that some of the assumptions used in the model - steadiness, equatorial symmetry, and linear treatment for the evolution of both the magnetic and velocity fields - are violated in the experiments. In addition, radial velocity measurements, using ultrasonic Doppler velocimetry, provide evidence of oscillatory motion near the outer sphere at low latitude: it is viewed as the signature of an instability of the super-rotating region.