Dissipation in Intercluster Plasma

We discuss dissipative processes in strongly gyrotropic, nearly collisionless plasma in clusters of galaxies (the ICM). First, we point out that Braginskii's theory, which assumes that collisions are more frequent that the system's dynamical timescale, is inapplicable to fast, subviscous ICM motion. Most importantly, the electron contribution to collisional magnetoviscosity dominates over that of ions for short-scale Alfvénic motions with wavelength satisfying l ≤ (λ/)(m_e/m_p)^1/4 ~ 1 kpc (where λ is the particle's mean free path, β is the plasma pressure parameter, and m_e and m_p are electron and proton masses). Thus, if a turbulent cascade develops in the ICM and propagates down to scales ≤1 kpc, it is damped collisionally not on ions, but on electrons. Second, in high-β plasma of the ICM, small variations of the magnetic field strength, of relative value ~1/β, lead to the development of anisotropic pressure instabilities (firehose, mirror, and cyclotron). Unstable wave modes may provide additional resonant scattering of particles, effectively keeping the plasma in a state of marginal stability. We show that in this case the dissipation rate of a laminar, subsonic, incompressible flows scales as the inverse of the plasma β parameter. We discuss application to the problem of ICM heating.