Can conduction induce convection? On the non-linear saturation of buoyancy instabilities in dilute plasmas

We study the effects of anisotropic thermal conduction on low-collisionality, astrophysical plasmas using two- and three-dimensional magnetohydrodynamic simulations. Dilute, weakly magnetized plasmas are buoyantly unstable for either sign of the temperature gradient: the heat-flux-driven buoyancy instability (HBI) operates when the temperature increases with radius while the magnetothermal instability (MTI) operates in the opposite limit. In contrast to previous results, we show that the MTI can drive strong turbulence and operate as an efficient magnetic dynamo, akin to standard, adiabatic convection. Together, the turbulent and magnetic energies may contribute up to similar to 10 per cent of the pressure support in the plasma. In addition, the MTI drives a large convective heat flux, up to similar to 1.5 per cent x c3(s). These findings are robust even in the presence of an external source of strong turbulence. Our results for the non-linear saturation of the HBI are consistent with previous studies but we explain physically why the HBI saturates quiescently, while the MTI saturates by generating sustained turbulence. We also systematically study how an external source of turbulence affects the saturation of the HBI: such turbulence can disrupt the HBI only on scales where the shearing rate of the turbulence is faster than the growth rate of the HBI. The HBI reorients the magnetic field and suppresses the conductive heat flux through the plasma, and our results provide a simple mapping between the level of turbulence in a plasma and the effective isotropic thermal conductivity. We discuss the astrophysical implications of these findings, with a particular focus on the intracluster medium of galaxy clusters.