Magnetic fields and halos in spiral galaxies
  Radio continuum observations allow to reveal the magnetic field structure in
the disk and halo of nearby spiral galaxies, their magnetic field strength and
vertical scale heights. The spiral galaxies studied so far show a similar
magnetic field pattern which is of spiral shape along the disk plane and
X-shaped in the halo, sometimes accompanied by strong vertical fields above and
below the central region of the disk. The strength of the halo field is
comparable to that of the disk. The total and turbulent magnetic field strength
is (weakly) increasing with the star formation. There are, however, indications
that stronger star formation reduces the magnetic field regularity globally.
The magnetic field in spiral galaxies is generally thought to be amplified and
maintained by dynamo action. During the galaxy's formation and evolution the
turbulent dynamo amplifies the field strength to energy equipartition with the
turbulent gas, while the large-scale (mean-field) dynamo mainly orders the
magnetic field. Hence, the large-scale magnetic field pattern evolves with
time. Supernova explosions causes a further continuous injection of turbulent
magnetic fields. Assuming that this small-scale field injection is situated
only within the spiral arm region where star formation mostly occurs lead to a
large-scale field structure in which the magnetic field regularity is stronger
in the interarm region as observed in several nearby spiral galaxies. The
detection of similar scale heights in several spiral galaxies of different
Hubble type star formation implies a relation between the galactic wind, the
total magnetic field strength and the star formation in the galaxy. A galactic
wind may be essential for an effective dynamo action. Strong tidal interaction,
however, seems to disturb the balance leading to deviating and locally
different scale heights as observed in M82 and NGC~4631.
