Statistical tests of galactic dynamo theory
  Mean-field galactic dynamo theory is the leading theory to explain the
prevalence of regular magnetic fields in spiral galaxies, but its systematic
comparison with observations is still incomplete and fragmentary. Here we
compare predictions of mean-field dynamo models to observational data on
magnetic pitch angle and the strength of the mean magnetic field. We
demonstrate that a standard $\alpha^2\Omega$ dynamo model produces pitch angles
of the regular magnetic fields of nearby galaxies that are reasonably
consistent with available data. The dynamo estimates of the magnetic field
strength are generally within a factor of a few of the observational values.
Reasonable agreement between theoretical and observed pitch angles generally
requires the turbulent correlation time $\tau$ to be in the range 10-20 Myr, in
agreement with standard estimates. Moreover, good agreement also requires that
the ratio of the ionized gas scale height to root-mean-square turbulent
velocity increases with radius. Our results thus widen the possibilities to
constrain interstellar medium (ISM) parameters using observations of magnetic
fields. This work is a step toward systematic statistical tests of galactic
dynamo theory. Such studies are becoming more and more feasible as larger
datasets are acquired using current and up-and-coming instruments.
