Growth rate of the turbulent magnetic Rayleigh-Taylor instability.

The Rayleigh-Taylor instability is strongly modified in the presence of a vertical mean magnetic field. Perturbations are first stretched in the vertical direction with no mixing due to the inhibition of small-scale shear instabilities. Then smooth elongated fingers eventually break after transition to turbulence, and a strong anisotropy persists. For increasing Alfvèn velocities, the growth rate of the mixing zone in the fully turbulent regime is decreased due to the conversion of potential energy into turbulent magnetic energy. A new theoretical prediction for the growth rate based on turbulent quantities is proposed and assessed with high-resolution direct numerical simulations of the Boussinesq-Navier-Stokes equations under the magnetohydrodynamics approximation.

Spontaneous generation and reversal of helicity in anisotropic turbulence.

Helicity plays an important role in spectacular geophysical phenomena such as hurricanes or the generation of the terrestrial magnetic field. The present investigation shows how helicity can be created in a statistically homogeneous but anisotropic flow, driven by buoyancy. If the flow is close enough to a two-dimensional limit, spontaneous symmetry breaking leads to the generation of mean helicity. In particular, we explain these observations by identifying a simple linear mechanism, the relevance of which is illustrated by simulations of unstably stratified turbulence in a conducting fluid on which a magnetic field is imposed. Finally it is shown that the self-organized state displays dynamical reversals of the sign of the mean helicity.