Annu. Rev. Astron. Astrophys. 1996. 34: 155-206
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4.2. Large-Scale Fields

The main challenge in the theory of galactic magnetism is to explain the origin and structure of the observed large-scale field. In Figure 6 we sketch different routes by which large-scale magnetic fields may arise. Large-scale flows (shear, compression) together with turbulence effects (swirling motions and inverse cascade - see below) can amplify weak seed magnetic fields (Section 5), thereby converting small-scale fields into large-scale fields. The amplifying effect of swirling motions on the large-scale field is described by the alpha-effect (Parker 1955, Steenbeck et al 1966, Moffatt 1978). Such motions also lead to an inverse cascade from the conservation properties of the magnetic helicity (Frisch et al 1975, Pouquet et al 1976) and from the cross-helicity effect (Yoshizawa & Yokoi 1993).

Figure 6

Figure 6. Sketch illustrating the various routes by which large-scale magnetic fields can arise. Turbulence effects (inverse cascade and alpha-effect) combined with shear and compression (differential rotation) amplify weak magnetic fields to produce strong large-scale fields.

These concepts were originally applied to stellar turbulence, where the existence of dynamos can almost be considered as an observational fact. It is not clear, however, how much galactic turbulence has in common with thermal turbulence in stars. Nevertheless, statistical properties of turbulence in molecular clouds seem to be remarkably similar to those determined from numerical simulations of ordinary compressible turbulence (Falgarone et al 1994).

There have been attempts to explain the large-scale magnetic field without invoking dynamo action. The turbulence must then be regarded as unimportant, and a large-scale seed magnetic field has to be amplified by large-scale shearing and compression alone. The inevitable eventual decay is assumed then to be considerably postponed (e.g. Kulsrud 1986). A model of this type has recently been proposed by Chiba & Lesch (1994), who consider fields that are maintained by an unspecified mechanism at large distances. Because these processes describe field amplification by shearing and compression alone, it is quite uncertain whether they can give fields of the strength and topology required at ages of about 10 Gyr; however, in conjunction with a dynamo, such motions might be important in certain galaxies.

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