Global structure of magnetic fields in spiral galaxies.

Observational techniques, data, and data analysis procedures for characterizing the magnetic field configurations of spiral galaxies are described. Primary data are gathered with optical and radio observations, the former revealing polarization and the orientation of dust grains, the latter showing the field strength and the direction of the field. Analytical models are presented for deriving the field parameters. The characteristics of fields typical of spiral, irregular and peculiar galaxies are summarized and several images are provided of fields superimposed on photographs of galaxies. Theoretical aspects of ring and bisymmetric spiral magnetic fields, two models which fit observed fields enveloping nearby spiral galaxies, are discussed.