From X-ray and SZ observations we know all major characteristics of the hot intracluster medium (ICM) filling the entire volume of galaxy clusters - the largest virialized objects in our Universe. However, several important properties are still poorly known, including thermal conduction in the ICM, mediated by electrons.
To
explain the sharp temperature gradients in galaxy clusters, it is often
proposed that thermal conduction is suppressed both by the topology of
magnetic-field lines, which tangle electron trajectories, and by
variations of the field strength that can trap electrons. The latter
mechanism can be crucially important when the so-called mirror
instability generates fluctuations of the magnetic field strength: this
kinetic instability is triggered by pressure anisotropies in turbulent
plasma. Even if such fluctuations are present on truly microscopic
scales, they have the potential to completely shut down heat conduction.
Scientists at MPA have investigated such a possibility by analysing the
results of recent simulations and found that the suppression of thermal
conductivity is in fact rather modest, a factor of ~5 compared to
unmagnetized plasma. The effect operates in addition to other
suppression mechanisms and independently of them, and depends only
weakly on the macroscopic parameters of the intracluster medium.
The dominant baryonic component of a galaxy cluster is hot
tenuous plasma that has accreted into the deep gravitational well formed
by the dominant dark matter component. This makes galaxy clusters
unique laboratories for a variety of plasma phenomena on an extremely
wide range of scales. Intricate plasma processes on microscales, more
than ten orders of magnitude smaller than the size of the cluster,
affect the large-scale properties of the cluster; for example modifying
particle transport influences the temperature profile. Many puzzling
features of galaxy clusters, such as the stability of cool cores, sharp
local gradients or the substructure seen in temperature maps, are
closely tied to the problem of thermal conduction in the intracluster
medium (ICM).
From X-ray observations it is now clear that the ICM demonstrates a
variety of violent physical processes, such as cluster mergers,
infalling galaxies, shock waves, and active galactic nuclei. These
naturally render the plasma turbulent. In addition, radio observations
show evidence that the ICM is pervaded by magnetic fields.
The field
magnitude is sufficient to confine the motion of charged particles to
spiralling around field lines with a tiny Larmor radius, much smaller
than the mean free paths of the particles. This effectively shuts down
particle transport perpendicular to field lines. Moreover, such a plasma
turns out to be unstable to pressure anisotropies that are easily
generated by turbulent motions. These instabilities then grow rapidly on
Larmor scales.
When studying thermal conduction, the mirror instability is of
particular interest. In this case, the magnetic field strength is
perturbed with a significant amplitude on the order of the local mean
magnetic field. The correlation length of mirror perturbations is only
two orders of magnitude longer than the electron Larmor radius, but
about ten orders of magnitude smaller that the collisional mean free
path. This means that such perturbations are capable of magnetically
mirroring the electrons: a charged particle spiralling along a field
line is reflected from a region with a strong magnetic field. If
perturbations of the magnetic field are generated by turbulence on
scales above the collisional mean free path, magnetic trapping is
ineffective. The mirror fluctuations, in contrast, are at the scales
comparable to the ion Larmor radius, where magnetic mirrors can suppress
electron transport considerably.
The scale of mirror fluctuations is far smaller than the current
observational limits. Instead, one has to turn to numerical simulations.
Only recently have particle-in-cell codes become capable of studying
micro-instabilities driven by pressure anisotropies. In these
simulations, a region of plasma (with a linear size of the order of a
few hundreds of the ion Larmor radius) is subjected to a shear,
stretching the magnetic field lines, producing pressure anisotropy, and
triggering the instability.
Scientists at MPA have used the results of these simulations to
investigate the motion of electrons in mirror fluctuations (shown in
Fig. 1). By applying a Monte Carlo approach, the diffusion and thermal
conduction coefficients have been estimated for a representative field
line extracted from the simulation domain. The probability distribution
function of the magnetic field strength along the field line turns out
to have a cut-off at a field strength of several times the initial
value. This leads to only a moderate amount of particle transport
suppression. In the limit where the collisional mean free path is much
larger than the correlation length of the mirror fluctuations, diffusion
is suppressed by a factor of ~10 (Fig. 2). This value then has to be
converted into the suppression of thermal conduction.
Due to the additional presence of diffusion in energy space the
thermal conductivity is suppressed by approximately a factor of two less
effectively than the particle transport. The resulting suppression by a
factor of ~5 appears to depend only very weakly on macroscopic
parameters of the ICM as long as the ion Larmor radius remains much
smaller than the correlation scale of the mirror perturbations, which is
indeed well satisfied in the ICM. The effect operates on top of other
suppression mechanisms and independently of them.
Author: Sergey Komarov
Original Publication:
1. Komarov S. V., Churazov E. M., Kunz M. W., Schekochihin A. A.
Title: 2016, MNRAS, 460, 467