Free-fall velocities and heat transport enhancement in liquid metal magneto-convection

We investigate the effect of a static, horizontal magnetic field on a liquid metal Rayleigh-Bénard convection by means of laboratory experiments. Although a static magnetic field acts as a stabilizing force on the fluid, we find that self organized convective flow structures reach an optimal state where the heat transport significantly increases and convective velocities perpendicular to the magnetic field reach the theoretical free-fall limit. Our measurements show that the application of the magnetic field leads to an anisotropic, highly ordered flow structure and a decrease of the turbulent fluctuations. When the magnetic field strength is increased beyond the optimum, Hartmann braking becomes dominant and leads to a reduction of the heat and momentum transport. The results are relevant for the understanding of magneto-hydrodynamic convective flows in planetary cores and stellar interiors in regions with strong toroidal magnetic fields oriented perpendicular to the temperature gradient.