  Both observations and modelling of magnetic fields in the diffuse
interstellar gas of spiral galaxies are well developed but the theory has been
confronted with observations for only a handful of individual galaxies. There
is now sufficient data to consider statistical properties of galactic magnetic
fields. We have collected data from the literature on the magnetic fields and
interstellar media (ISM) of 20 spiral galaxies, and tested for various
physically motivated correlations between magnetic field and ISM parameters.
Clear correlations emerge between the total magnetic field strength and
molecular gas density as well as the star formation rate. The magnetic pitch
angle exhibits correlations with the total gas density, the star formation rate
and the strength of the axisymmetric component of the mean magnetic field. The
total and mean magnetic field strengths exhibit noticeable degree of
correlation, suggesting a universal behaviour of the degree of order in
galactic magnetic fields. We also compare the predictions of galactic dynamo
theory to observed magnetic field parameters and identify directions in which
theory and observations might be usefully developed.
