Magnetic Field Amplification and Evolution in Turbulent Collisionless Magnetohydrodynamics: An Application to the Intracluster Medium (original) (raw)

The amplification and maintenance of the observed magnetic fields in the ICM are usually attributed to the turbulent dynamo action. This is generally derived employing a collisional MHD model. However, this assumption is poorly justified a priori since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless MHD description. To deal with this problem on the basis of solar wind and laboratory plasmas measurements we adopt a phenomenological model of scattering of particles by magnetic perturbations arising from instabilities within collisionless plasmas. In this model, we investigate the properties of magnetic turbulence and the dynamo action in the ICM. Unlike collisional MHD simulations, our study uses an anisotropic plasma pressure with respect to the direction of the local magnetic field and this anisotropy modifies the MHD linear waves bringing the plasma within a parameter space where collisionless instabilities should take place. However, within the adopted model these instabilities are contained at bay through the relaxation term of the pressure anisotropy which simulates the feedback of the mirror and firehose instabilities on the plasma under study. The relaxation term acts to get the plasma distribution function consistent with the empirical studies of collisionless plasmas. Our three-dimensional numerical simulations of forced transonic turbulence motivated by modeling of the turbulent ICM are performed for different initial values of the magnetic field intensity, as well as different relaxation rates of the pressure anisotropy. We found that in the high β plasma regime (where β is the ratio between thermal to magnetic pressures) corresponding to the ICM conditions, a fast anisotropy relaxation rate gives results which are similar to the collisional-MHD model as far as the statistical properties of the turbulence are concerned. Also, the amplification of seed magnetic fields due to the turbulent dynamo action is similar to the collisional-MHD model, especially in the limit of an instantaneous anisotropy relaxation. Our simulations that do not employ the anisotropy relaxation prescription (which are more like the standard so called CGL-collisionless models) deviate significantly from the collisional-MHD results, and in accordance with earlier studies, show more power at the small-scale fluctuations of both density and velocity representing the results of the kinetic instabilities at these scales. For these simulations the large scale fluctuations in the magnetic field are mostly suppressed and the turbulent dynamo fails in amplifying seed magnetic fields and the magnetic energy saturates at values several orders of magnitude smaller than the kinetic energy.