Global Magnetic Structures in Spiral Galaxies: Evidence for Dynamo Action

Observational evidence for dynamo action in spiral galaxies is reviewed, and the capabilities of various theories in explaining the basic features of galactic magnetic fields are discussed. Mean-field dynamo models appear to be unique in providing a coherent explanation of a wide variety of magnetic features in spiral galaxies. Reliable modelling of global magnetic structures, such as the magnetic ring in M31, requires detailed knowledge of the rotation curve, the magnitude and radial profile of turbulent and noncircular systematic velocities, the scale height of the warm ionized layer, the total gas density, the turbulent scale and their variations with galactocentric radius. More detailed models involving the effects of the spiral arms on magnetic field require the knowledge of the arm-interarm contrasts in the above quantities.