A Heating Mechanism via Magnetic Pumping in the Intracluster Medium

Abstract

Turbulence driven by active galactic nuclei activity, cluster mergers, and galaxy motion constitutes an attractive energy source for heating the intracluster medium (ICM). How this energy dissipates into the ICM plasma remains unclear, given its low collisionality and high magnetization (precluding viscous heating by Coulomb processes). Kunz et al. proposed a viable heating mechanism based on the anisotropy of the plasma pressure under ICM conditions. The present paper builds upon that work and shows that particles can be heated by large-scale turbulent fluctuations via magnetic pumping. We study how the anisotropy evolves under a range of forcing frequencies, what waves and instabilities are generated, and demonstrate that the particle distribution function acquires a high-energy tail. For this, we perform particle-in-cell simulations where we periodically vary the mean magnetic field B (t). When B (t) grows (dwindles), a pressure anisotropy P ⊥ > P ∥(P ⊥ < P ∥) builds up (P ⊥ and P ∥ are, respectively, the pressures perpendicular and parallel to B (t)). These pressure anisotropies excite mirror (P ⊥ > P ∥) and oblique firehose (P ∥ > P ⊥) instabilities, which trap and scatter the particles, limiting the anisotropy, and providing a channel to heat the plasma. The efficiency of this mechanism depends on the frequency of the large-scale turbulent fluctuations and the efficiency of the scattering the instabilities provide in their nonlinear stage. We provide a simplified analytical heating model that captures the phenomenology involved. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in the ICM.

Electron Reacceleration via Ion Cyclotron Waves in the Intracluster Medium

“…The compression factor ∼6 at the end of our simulation exceeds the expected density contrast of both weak ICM shocks and subsonic compressive ICM turbulence (e.g., Gaspari & Churazov 2013). More realistic, nonadiabatic decompression may excite firehose modes that should also resonantly scatter CRe and alter ΔU revert (Melville et al 2016;Riquelme et al 2018;Ley et al 2023). In 2D or 3D simulations, the low-k drift of IC wave power may not persist, and mirror modes may weaken IC waves; both effects will weaken the energy gain from IC wave pumping.…”

“…Here we derive Equation (6) from a set of moment equations, similar to the drift-kinetic models of Zweibel (2020) and Ley et al (2023); a more general form is given by Chew et al…”

Pressure anisotropy and viscous heating in weakly collisional plasma turbulence

“…Thus, it may not be appropriate to consider scattering only in unstable regions – rather, the continual driving will create a ‘soup’ of magnetic fluctuations that drive scattering across the entire plasma, putting it in the weakly collisional or Braginskii regime (Ley et al. 2023). This situation applies to almost any foreseeable kinetic numerical simulation, including those of A22, but is at odds with the assumptions of our collisionless simulations, where we set in stable regions.…”

Balanced Turbulence and the Helicity Barrier in Black Hole Accretion

“…The RIAF systems most relevant for the EHT are much better described as Coulomb collisionless, but when the ratio of gas pressure to magnetic pressure is large (which is a likely description of much of the accretion flow), perturbations in the magnetic field can drive sufficient deviations from local thermodynamic equilibrium to trigger kinetic microinstabilities, which are nonlocal in nature and increase the effective collisionality beyond what the naïve Coulomb collision picture implies (Kunz et al 2014;Sironi & Narayan 2015;Melville et al 2016;Riquelme et al 2016;Squire et al 2017;Bott et al 2021;Arzamasskiy et al 2023;Ley et al 2023;Tran et al 2023;A. F. A. Bott et al 2023, in preparation).…”