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.
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