3.7. Galactic Magnetic field Interpretations - dynamos or not ?

While the results of polarization observations are secure, repeatable, and do not vary with time, their interpretation has evolved for the better with time.

In the late 1970s, an interpretation in terms of magnetic control of gas and stellar systems by strong helicoidal magnetic fields was fashionable (e.g., Piddington 1981a; Piddington 1981b). Large-scale magnetic fields in this theory would be the result of twisting of a primordial cosmological universal magnetic field, in some ways (a gravitationnally contracting protogalaxy amplifies it by 2000; differential rotations in a galactic lifetime amplifies it by 30 (e.g. Beck et al. 1996). Strong magnetic fields used to be invoked only when there was a gap in our knowledge, rather like a strong 'magnetic field of the gaps' (Kahn and Brett, 1993). More data have been acquired since, revealing a weak galactic magnetic field, and an upper limit to a much weaker cosmological universal magnetic field.

In the late 1980s, an interpretation in terms of galactic dynamos (e.g., Parker 1976; Parker 1992) with weak azimuthal magnetic fields was preferred by most. The magnetic field structures 'preferred' by dynamo theory usually correspond to the most unstable linear eigenmodes of the system. Still, One can find occasionally a strong 'magnetic control' model in the litterature (e.g., Battaner & Florido 1995; Sánchez-Salcedo, 1996), despite the problem that the large magnetic field strength required would unacceptably expand the HI disk vertically (e.g., Melrose 1995).

In the 1990s, 'galactic dynamos' are currently preferred by most, but some authors have questioned its long-term ability and sustainability (see Sect. 3.7.2), and some others have proposed a non-dynamo theory of 'galactic fountains' based on MHD winds driven by cosmic rays (e.g., Kahn and Brett 1993; Kahn 1992; Zirakashvili et al. 1996).