Magnetic Fields in Spiral Galaxies
  Radio synchrotron emission is a powerful tool to study the strength and
structure of magnetic fields in galaxies. Unpolarized synchrotron emission
traces isotropic turbulent fields which are strongest in spiral arms and bars
(20-30 \mu G) and in central starburst regions (50-100 \mu G). Such fields are
dynamically important; they affect gas flows and drive gas inflows in central
regions. -- Polarized emission traces ordered fields, which can be regular or
anisotropic turbulent, where the latter originates from isotropic turbulent
fields by the action of compression or shear. The strongest ordered fields
(10-15 \mu G) are generally found in interarm regions. In galaxies with strong
density waves, ordered fields are also observed at the inner edges of spiral
arms. Ordered fields with spiral patterns exist in grand-design, barred and
flocculent galaxies, and in central regions. Ordered fields in interacting
galaxies have asymmetric distributions and are a tracer of past interactions
between galaxies or with the intergalactic medium. In radio halos around
edge-on galaxies, ordered magnetic fields with X-shaped patterns are observed.
-- Faraday rotation measures of the diffuse polarized radio emission from
galaxy disks reveal large-scale spiral patterns that can be described by the
superposition of azimuthal modes; these are signatures of regular fields
generated by mean-field dynamos. "Magnetic arms" between gaseous spiral arms
may also be products of dynamo action, but need a stable spiral pattern to
develop. Helically twisted field loops winding around spiral arms were found in
two galaxies so far. Large-scale field reversals, like the one found in the
Milky Way, could not yet be detected in external galaxies. -- The origin and
evolution of cosmic magnetic fields will be studied with forthcoming radio
telescopes like the Square Kilometre Array.
