Theoretical approaches to particle propagation and acceleration in turbulent intergalactic medium

Abstract

Abstract.Intercluster medium is expected to be turbulent with turbulence being superAlfvenic at large scales. Magnetic fields substantially modify the turbulent cascade when the turbulence reaches the scales at which the fluctuation velocity gets less than the Alfven velocity. At those scales it is possible to consider three cascades, of fast, slow and Alfven modes with little energy exchange between them. As Alfvenic and slow modes are anisotropic they marginally scatter and accelerate cosmic rays, while fast modes dominate the processes. However, in the presence of cosmic rays the turbulence is modified as cosmic rays transfer the energy of compressible motions (i.e. slow and fast modes) from large scales to the scale of cosmic ray Larmor radius. This results in generation of a new small-scale Alfvenic component which is not a part of the ordinary MHD cascade. This component does scatter and accelerate cosmic rays. In addition, magnetic reconnection in turbulent medium accelerates cosmic rays. The complexity of the interacluster turbulence calls for observational studies. A new technique Velocity Coordinate Spectrum (VCS) is particularly promising for studies of velocity fluctuations with a new generation of X-ray observatories.Key words: galaxies:clusters:general,turbulence, plasmas, intergalactic medium, galaxies: magnetic fields, MHD c 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Do we expect IGM to be turbulent?A fluid of viscosity ν becomes turbulent when the rate of viscous dissipation, which is ∼ ν/L 2 at the energy injection scale L, is much smaller than the energy transfer rate ∼ V L /L, where V L is the velocity dispersion at the scale L. The ratio of the two rates is the Reynolds number Re = V L L/ν. In general, when Re is larger than Re crit ∼ 10 − 100 the system becomes turbulent. Chaotic structures develop gradually as Re increases, and those with Re ∼ 10 3 are appreciably less chaotic than those with Re ∼ 10 8 .It is widely accepted that medium in clusters should be turbulent. Mergers of galactic subclusters may be one of the major energy injection mechanism (see Sarazin 2002, Brunetti 2003 and references therein). The details of the injection and the energy transfer are poorly understood. A crude picture includes the energy injection scale of 100-500 kpc and the injection velocity of the order of 10 3 km/s.A difficulty that one faces trying to understand turbulence is clusters is that the diffusivity is very different along and perpendicular to magnetic field. This is related to the fact that the mean free path of ions is substantial. For non-magnetized Correspondence to: lazarian@astro.wisc.edu intracluster gas the Re number is only marginally larger than Re crit and therefore formally is only just sufficient for initiating the turbulence. At the same time, even small magnetization makes the Reynolds number for the motions perpendicular to magnetic field very large (e.g. more than 10 10 ). This poses the question of what a reasonable choice of the Reynolds number is for intracluster medium....

Numerical Tests of Fast Reconnection in Weakly Stochastic Magnetic Fields

“…This particle may bounce back and forth between magnetic mirrors formed by oppositely directed magnetic fluxes moving towards each other with the velocity V R . Each bounce will increase the energy of a particle in a way consistent with the requirements of the first-order Fermi process 11 (de Gouveia dal Pino & Lazarian 2001, 2005Lazarian 2006). This is in contrast to the secondorder Fermi acceleration that is frequently discussed in terms of accelerating particles by turbulence generated by reconnection (La Rosa et al 2006).…”

The Milky Way’s Kiloparsec‐Scale Wind: A Hybrid Cosmic‐Ray and Thermally Driven Outflow

“…3 k k , individual cosmic rays sample many uncorrelated perturbations in k ? in each gyroperiod which effectively cancel each other out (Chandran 2000;Lazarian 2006). Thus, the background Galactic turbulent cascade has little effect on the cosmic rays compared to the waves that they generate themselves.…”

Turbulence and dynamo interlinks

“…Figure 4 shows a schematic representation on how interacting turbulent eddies can mix the gas and exchange parts of their magnetic flux tubes (through reconnection) favouring their diffusion. This theory predicts a turbulent reconnection diffusivity η t which is much larger than the Ohmic diffusivity at the turbulent scales: (Lazarian 2005;Santos-Lima et al 2010;Lazarian 2006;Lazarian et al 2012;Leão et al 2012):…”