Turbulence and mixing in the intracluster medium

The intracluster medium (ICM) is stably stratified in the hydrodynamic sense with the entropy s increasing outwards. However, thermal conduction along magnetic field lines fundamentally changes the stability of the ICM, leading to the "heat-flux buoyancy instability" when dT/dr > 0 and the "magnetothermal instability" when dT/dr < 0. The ICM is thus buoyantly unstable regardless of the signs of dT/dr and ds/dr. On the other hand, these temperature-gradient-driven instabilities saturate by reorienting the magnetic field (perpendicular to r̂ when dT/dr > 0 and parallel to r̂ when dT/dr < 0), without generating sustained convection. We show that after an anisotropically conducting plasma reaches this nonlinearly stable magnetic configuration, it experiences a buoyant restoring force that resists further distortions of the magnetic field. This restoring force is analogous to the buoyant restoring force experienced by a stably stratified adiabatic plasma. We argue that in order for a driving mechanism (e.g, galaxy motions or cosmic-ray buoyancy) to overcome this restoring force and generate turbulence in the ICM, the strength of the driving must exceed a threshold, corresponding to turbulent velocities ≳10-100 km/s. For weaker driving, the ICM remains in its nonlinearly stable magnetic configuration, and turbulent mixing is effectively absent.We discuss the implications of these findings for the turbulent diffusion of metals and heat in the ICM.