A heating mechanism for high-β plasmas in galaxy clusters

Turbulence driven by supermassive black hole activity, gravitational infall, and galaxy motions is an attractive energy source for heating the intracluster plasma (ICM) in galaxy clusters. However, how this energy dissipates into heat is unclear, since the ICM is collisionless¹. In this work, we perform particle-in-cell (PIC) simulations of a plasma subject to a periodic variation of the mean magnetic field, B(t), to show that particles can be heated by gyroviscosity via magnetic pumping. When B(t) grows (dwindles), a pressure anisotropy P_⊥>P_∥ (P_∥>P_⊥) builds up due to the adiabatic invariance of the particle's magnetic moment. When initially β=20, the plasma self-regulates its anisotropy by exciting the Mirror (P_⊥>P_∥) and Firehose (P_∥>P_⊥) instabilities. In this process, both instabilities pitch-angle scatter particles, breaking their adiabatic invariance and providing a channel to efficiently retain some energy in the plasma after one pump cycle, therefore effectively heating the system. The efficiency at which this mechanism acts depends on the level of macroscopic turbulence and how fast the instabilities can be excited and saturate. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM.

¹Kunz et al. 2011