Abstract
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, in the ICM the ion mean
free path between collisions is of the order of the dynamical scales, thus
requiring a collisionless MHD description. Unlike collisional MHD simulations,
our study uses an anisotropic plasma pressure with respect to the direction of
the local magnetic field, which brings the plasma within a parameter space
where collisionless instabilities should take place. 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. This relaxation acts to get the plasma distribution function
consistent with the empirical studies of collisionless plasmas. Our 3D
numerical simulations of forced transonic turbulence motivated by modeling of
the turbulent ICM are performed for different initial values of the magnetic
field intensity, and different relaxation rates of the pressure anisotropy. We
found that in the high beta plasma regime 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. Our
simulations that do not employ the anisotropy relaxation deviate significantly
from the collisional-MHD results, and show more power at the small-scale
fluctuations of both density and velocity representing the results of the
instabilities. For these simulations the large scale fluctuations in the
magnetic field are mostly suppressed and the turbulent dynamo fails in
amplifying seed magnetic fields.
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