  In geo- and astrophysics, low Prandtl number convective flows often interact
with magnetic fields. Although a static magnetic field acts as a stabilizing
force on such flow fields, we find that self-organized convective flow
structures reach an optimal state where the heat transport significantly
increases and convective velocities reach the theoretical free-fall limit, i.e.
the maximum possible velocity a fluid parcel can achieve when its potential
buoyant energy is fully converted into kinetic energy. Our measurements show
that the application of a static 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 temperature
gradients.
