Magnetic fields in the nearby spiral galaxy IC 342: A multi-frequency
  radio polarization study
  The total and polarized radio continuum emission of IC 342 was observed in
four wavelength bands with the Effelsberg and VLA telescopes. The frequency
dependence of the radial scalelength of synchrotron emission indicates
energy-dependent propagation of the cosmic-ray electrons, probably via the
streaming instability. The equipartition strength of the total magnetic field
is typically 15 muG, that of the ordered field 5 muG. Faraday rotation of the
polarization angles reveals an underlying regular field of only about 0.5 muG
strength with a large-scale axisymmetric spiral pattern, signature of a
mean-field dynamo, and an about 10x stronger field that fluctuates on scales of
a few 100 pc. The magnetic field around the bar in the central region of IC 342
resembles that of large barred galaxies; its regular spiral field is directed
outwards, opposite to that in the disk. The polarized emission in the disk is
concentrated in: (1) a narrow arm of about 300 pc width, displaced inwards with
respect to the eastern arm by about 200 pc, indicating magnetic fields
compressed by a density wave; (2) a broad arm of 300-500 pc width around the
northern arm with systematic variations in polarized emission, polarization
angles, and Faraday rotation measures on a scale of about 2 kpc, indicative of
a helically twisted flux tube generated by the Parker instability; (3) a
rudimentary "magnetic arm" in an interarm region in the north-west; (4) several
broad spiral arms in the outer galaxy, related to spiral arms in the total
neutral gas; (5) short features in the outer south-western galaxy, probably
distorted by tidal interaction. - The generation and development of "magnetic
arms" by a mean-field dynamo probably need a spiral pattern that is stable over
a few galactic rotation periods. The dynamo in IC 342 is slow and weak,
probably disturbed by the bar, tidal interaction, or a transient spiral
pattern.
