Experimental and Numerical study of Rotating Magneto-Convection

Nicolas Gillet ,  Grenoble Observatory - LGIT

We have experimentally and numerically studied finite-amplitude magneto-convection
in a rotating spherical shell filled with gallium (Prandtl number Pr=0.027).
We impose an azimutal magnetic field (up to 10^-2 T) on convective motions where Rayleigh number is a few times critical.
The thermal Ekman number Et reaches 0(10^-6), and the Elsasser number 0(10^-2);
thus we explore the "weak field" case where Elsasser is of order Et^1/3.
    Ultrasonic Doppler velocimetry is used to measure quantitatively the velocities in the equatorial plane:
a strong decrease of the motion is observed as the intensity of the imposed field increases.
    We present a comparative evolution of zonal and convective flows as the Elsasser number varies.
    Then with the help of quasi-geostrophic
    numerical simulations we propose a scaling of the velocities including the effect of the magnetic field on the onset parameters.
A physical interpretation that link together the velocity distributions and the heat transport is proposed.
    Prospects for further experiments and numerical simulations are then discussed.