The Rayleigh-Taylor instability shown here is caused when a heavy fluid sits on top of a lighter fluid in the presence of gravity. A small perturbation to the interface of these two fluids will start the growth of the instability.
ρ top = 2
ρ bottom = 1
Pgas = 2.5
Gravity = 0.1
The movie below shows a single mode perturbation.
β = 2 * Pgas/ B 2
The simulation below shows the single mode MHD case with β = 500. In this case Kelvin-Helmholtz instabilities present in the hydro version are smoothed out by the magnetic field.
The simulation below shows the single mode MHD case with β = 125. In this case the magnetic field is strong enough to prevent the instability from developing.