## 1. IntroductionThe interpretation of radio-polarization data of spiral galaxies reveals the existence of large-scale magnetic fields with very special properties. Their explanation is of high interest because galaxies are the only astrophysical configurations with observable internal flow systems - in basic contrast to stars and planets whose magnetism therefore needs a more speculative theory for explanation. In all cases very similar versions of the theory of turbulent dynamos have been developed but the results are not without difficulties. In particular, there is little success to understand the relation between the considered flow field and the associated -effect. The observed butterfly diagram of the solar activity seems to indicate a very small in the solar convection zone but the application of the widely accepted mixing-length theory does not comply. On the other hand, the observed grand design of most of the known galactic magnetic fields requires a very small inducing role of the galactic differential rotation, or - with other words - a relatively large -effect. In the present paper, therefore, we present computations of the induction equation without dynamo terms, i.e. without alpha-effect. The fields are thus considered as generally decaying but with an unknown decay rate. While the galactic differential rotation always amplifies the magnetic fields, the interstellar turbulence will destroy them. The latter, however, is nonlinearly quenched by the magnetic field itself and its electromotive force (EMF) possesses a complex tensorial structure. It is thus not trivial to compute the resulting fields in order to compare them with the observations. The computations are performed in order to gain some insight into the field amplification caused by differential rotation, the lifetime of the magnetic field in the considered nonaxisymmetric geometry and also the existence of periods with the observed large values of both the magnetic amplitude and the pitch angles. The key properties of the galactic large-scale magnetic field pattern are: - [1.] Field amplitude is several G but not exceeding 10 G.
- 2. Field lines have pitch angles up to 35 .
- 3. Fields often exhibit a clear azimuthal modulation.
In the following these topics are discussed in more detail, in particular in relation to the theoretical concepts strengthened or weakened by them. ## 1.1. Field strengthThe observed magnetic field energy is of order of the energy of the interstellar turbulence. The equipartition field strength with density of order g/cm ## 1.2. Pitch anglesThe pitch angles reflect the ratio of the radial and the toroidal
magnetic field strengths. Dynamos of
-type are characterized by very small pitch
angles, e.g. 0.001 for the sun. For galaxies
pitch angles of 10-35 deg are reported. They are always decreasing
outwards (Beck 1993). Such observed values indicate that the
differential rotation in galaxies (basically )
does ## 1.3. NonaxisymmetryFor at least one case (M81) there is a clear bisymmetric azimuthal
structure so that in one magnetic arm the field spirals into the
center and the other one magnetically spirals outwards. It is not
trivial to explain such asymmetry of type BSS (i.e. Of special interest is the case of NGC 6946 possessing pitch angles
between 20 and 30 . As Beck
& Hoernes (1996) report, the large-scale magnetic fields are
It is a nonaxisymmetry without reversal of the magnetic polarity
(ASS, © European Southern Observatory (ESO) 1998 Online publication: December 16, 1997 |