Strong magnetic fields are encountered on pulsar surfaces (1012 Gauss), on white dwarf surfaces (106 Gauss), on the solar chromosphere and Sunspots (103 Gauss), and on the Earth's surface (1 Gauss), but are limited to a small linear size (104 - 106 km). Weak magnetic fields are found in the interstellar and galactic spaces (a few 10-6 Gauss), but they are enormous in scale lengths (parsecs to Megaparsecs). The magnetic fields in spiral galaxies are related to the motions of interstellar gas in the spiral arms in the galactic disk and in the galactic halo; they still can significantly affect interstellar gas dynamics in the galactic disk and in the dense cores of molecular clouds within a galaxy.
Two salient points found in this review are the inclusion of medium-scale magnetic fields, and the choice of a fact-driven observational approach.
This review here includes the medium scale magnetic fields, on the scales of 1 pc to 1 kpc; these magnetic fields are not often found in galactic magnetic field reviews (e.g., Beck et al. 1996; Kronberg 1994; Sofue et al. 1986). Yet these medium-scale magnetic fields are necessary (i) for the study of molecular clouds (1-50 pc) and of magnetized superbubbles (50-500 pc), and (ii) for magnetic field reconnections used as energy input for the large-scale (> 1 kpc) magnetic fields (e.g., Parker 1992; Kahn 1992; Kahn and Brett 1993; Ferrière 1993a; Ferrière 1993b).
Many reviews have emphasized a predominently 'physical approach', being 'concept-driven', employing known physical laws to explain new discoveries in a 'theory-to-nature' type emphasizing the explanatory aspect. A few earlier reviews used a 'morphological approach', being 'fact-driven', employing empirical discoveries to spur new links with known physical laws in a 'nature-to-theory' type emphasizing the discovery aspect. This review uses the latter type, emphasizing the discovery aspect through its morphological, fact-driven, observational approach, with a larger emphasis on medium-scale areas of the Milky Way: interstellar objects, molecular clouds, superbubbles, nearby spiral arms, and the interstellar matter medium as a whole.
Fact-driven reviews on the subject of magnetism, covering the beginning of polarization observations since 1949, have been given elsewhere for the Milky Way (e.g., Verschuur 1970; Verschuur 1979; Vallée 1983a; Vallée 1997a) and for nearby spiral galaxies and cosmology (e.g., Vallée 1984a; Sofue et al. 1986; Vallée 1997b). More recent discussions have been made on the Milky Way (e.g., Asseo & Sol 1987; Heiles 1987; Wielebinski 1995; Heiles 1996). Recent concept-driven reviews have been made on nearby galaxies and cosmology (e.g., Asseo & Sol 1987; Kronberg 1994; Beck et al. 1996; Lesch & Chiba 1997).