Nonlinear simulations of the heat-flux-driven buoyancy instability and its implications for galaxy clusters

In low-collisionality plasmas heat flows almost exclusively along magnetic field lines and the condition for stability to convection is modified from the standard Schwarzschild criterion. We present local two- and three-dimensional simulations of a new heat-flux-driven buoyancy instability ( the HBI) that occurs when the temperature in a plasma decreases in the direction of gravity. We find that the HBI drives a convective dynamo that amplifies an initially weak magnetic field by a factor of similar to 20. In simulations that begin with the magnetic field aligned with the temperature gradient, the HBI saturates by rearranging the magnetic field lines to be almost purely perpendicular to the initial temperature gradient. This magnetic field reorientation results in a net heat flux through the plasma that is less than 1% of the field-free (Spitzer) value. We show that the HBI is likely to be present in the cool cores of clusters of galaxies between similar to 0.1-100 kpc, where the temperature increases outward. The saturated state of the HBI suggests that inward thermal conduction from large radii in clusters is unlikely to solve the cooling flow problem. Finally, we also suggest that the HBI may contribute to suppressing conduction across cold fronts in galaxy clusters.