  The grand-design face-on spiral galaxy M51 is an excellent laboratory for
studying magnetic fields in galaxies. We present new observations of M51 using
the VLA at the frequency range of S-band (2-4GHz), to shed new light on the
transition region between the disk and halo. We present images of the
distributions of the total intensity, polarized intensity, degree of
polarization, and rotation measure (RM). The RM distribution in S-band shows a
fluctuating pattern without any apparent large-scale structure. We discuss a
model of the depolarization of synchrotron radiation in a multi-layer
magneto-ionic medium and compare the model predictions to the polarization data
of M51 between 1-8GHz. Since the model predictions strongly differ within the
wavelength range of the S-band, the new data are essential. The parameters of
the model are adjusted to fit to the data of polarization fractions in a few
selected regions. In three spiral arm regions, the turbulent field in the disk
dominates with strengths between 18muG and 24muG, while the regular field
strengths are 8-16muG. In one inter-arm region, the regular field strength of
18muG exceeds that of the turbulent field of 11muG. The regular field strengths
in the halo are 3-5muG. The observed RMs in the disk-halo transition region are
probably dominated by tangled regular fields, as predicted from models of
evolving dynamos, and/or vertical fields, as predicted from numerical
simulations of Parker instabilities or galactic winds. Both types of magnetic
fields have frequent reversals on scales similar to or larger than the beam
size (550pc) that contribute to an increase of the RM dispersion and to
distortions of any large-scale pattern of the regular field. Our study devises
new ways of analyzing and interpreting broadband multi-frequency polarization
data that will be applicable to future data from, for example, the Square
Kilometre Array.
