Investigations of Faraday Rotation Maps of Extended Radio Sources ...

Chapter 1
Introduction
Magnetic fields are ubiquitous throughout the Universe. It is the magnetic field of the
planet earth which prevents the solar wind and the high energy particles ejected by the
Sun from reaching the atmosphere and thus, making life possible on Earth. It is the
magnetic field of the Sun which strongly influences the appearance of Sun’s surface
and generates stunning events such as sun spots, arches and flares. It is the magnetic
field of pulsars which drives the exactly timed radio pulses coming from these objects
on the sky.
However, magnetic fields are detected not only on small scales such as planets,
like Earth or Jupiter, or stars, but also on large scales in galaxies and on even larger
scales in clusters of galaxies. They are also observed on intermediate scales, i.e. in
molecular clouds which are the progenitor of proto stars, thus, magnetic fields are
detected in the interstellar medium (ISM) where the star forming takes place. Magnetic
fields are believed to be important for these star forming processes but to what extent
and during what stages the magnetic fields become important is still under debate.
However, magnetic fields remove angular momentum from proto stellar clouds making
star formation possible (for a review, see Bourke & Goodman 2004).
For the ISM, the magnetic energy density is at least comparable to the energy density
of the thermal gas, it is comparable to the energy of cosmic rays and it is also
comparable to the energy of the turbulent motion of the ISM. On larger scales in clusters
of galaxies, magnetic fields also introduce an additional pressure component to
the energy balance of the cluster gas, which is a hot plasma. The dynamical importance
of magnetic fields for this plasma is still under debate. However, a complete
description of the state of the plasma, and in particular the role of particle transport
processes, requires knowledge of the strength and the morphologies of intra-cluster
magnetic fields.
Related to these processes magnetic field constrain the heat conductivity. Transport
coefficients, e.g. heat conduction, are determined by the rate of change of (thermal)
electron momenta. In a fully magnetised plasma, particles experience momentum
changes not only through mutual collisions but perhaps primarily due to the Lorentz
force which leads to a confinement of their motion mainly along field lines. These
field lines may be tangled on scales less than the Coloumb mean free path, which is
about tens of kilo parsecs in a cluster atmosphere. Treating the field tangling scale in
a simple approximation as an effective mean free path, one can estimate the change
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