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.