Cosmic Magnetic Fields: Observations and Prospects
  Synchrotron emission, its polarization and its Faraday rotation at radio
frequencies of 0.2-10 GHz are powerful tools to study the strength and
structure of cosmic magnetic fields. The observational results are reviewed for
spiral, barred and flocculent galaxies, the Milky Way, halos and relics of
galaxy clusters, and for the intergalactic medium. Polarization observations
with the forthcoming large radio telescopes will open a new era in the
observation of cosmic magnetic fields and will help to understand their origin.
At low frequencies, LOFAR (10-250 MHz) will allow us to map the structure of
weak magnetic fields in the outer regions and halos of galaxies and galaxy
clusters. Polarization at higher frequencies (1-10 GHz), as observed with the
EVLA, ASKAP, MeerKAT, APERTIF and the SKA, will trace magnetic fields in the
disks and central regions of nearby galaxies in unprecedented detail. Surveys
of Faraday rotation measures of pulsars will map the Milky Way's magnetic field
with high precision. All-sky surveys of Faraday rotation measures towards a
dense grid of polarized background sources with the SKA and its precursor
telescope ASKAP are dedicated to measure magnetic fields in distant intervening
galaxies, galaxy clusters and intergalactic filaments, and will be used to
model the overall structure and strength of the magnetic field in the Milky
Way.
