  Magnetic fields in nearby, star-forming galaxies reveal both large-scale
patterns and small-scale structures. A large-scale field reversal may exist in
the Milky Way but no such reversals have been observed so far in external
galaxies. The effects of star-forming regions of galaxies need to be included
when modelling the evolution of their magnetic fields, which can then be
compared to future radio polarization observations. The causes of large-scale
field reversals also need clarification. Our model of field evolution in
isolated disc galaxies includes a standard mean-field dynamo and continuous
injection of turbulent fields (the effect of supernova explosions) in discrete
star forming regions by implicit small-scale dynamo action. Synthetic maps of
radio synchrotronemission and Faraday rotation measures are computed. A
large-scale dynamo is essential to obtain regular large-scale spiral magnetic
fields, observed in many galaxies. These appear, on kpc scales in near energy
equilibrium with the turbulence, after 1-2 Gyr (redshift 4-3). Turbulent field
injection generates small-scale field structures. Depending on model
parameters, large-scale field reversals may persist over many Gyrs and can
survive until the present epoch. Significant polarized radio synchrotron
emission from young galaxies is expected at redshifts less than 4. Faraday
rotation measures (RM) are crucial to detect field reversals. Large-scale
patterns ofrotation measures can be observed at redshifts less than 3. Our
model can explain the general form of axisymmetric spiral fields with many
local distortions, as observed in nearby galaxies. For a slightly different
choice of parameters, large-scale field reversals can persist over the lifetime
of a galaxy. Comparison of our synthetic maps with future observations of
distant galaxies with the planned Square Kilometre Array (SKA) will allow
refinement of models.
