  Polarization observations at 21cm and 18cm towards the local Taurus molecular
cloud complex were made with the Effelsberg 100-m telescope. Highly structured,
frequency-dependent polarized emission features were detected. We discuss
polarization minima with excessive rotation measures located at the boundaries
of molecular clouds. The multi-frequency polarization data have been
successfully modeled by considering magneto-ionic Faraday screens at the
surface of the molecular clouds. Faraday rotated background emission adds to
foreground emission towards these screens in a different way than in its
surroundings. The physical size of the Faraday screens is of the order of 2 pc
for 140 pc distance to the Taurus clouds. Intrinsic rotation measures between
about -18 rad/m2 to -30 rad/m2 are required to model the observations.
Depolarization of the background emission is quite small (compatible with
zero), indicating a regular magnetic field structure with little turbulence
within the Faraday screens. With observational constraints for the thermal
electron density from H-alpha observations of less than 0.8 cm^-3 we conclude
that the regular magnetic field strength along the line of sight exceeds 20
muG. We discuss some possibilities for the origin of such strong and well
ordered magnetic fields. The modeling also predicts a large-scale, regularly
polarized emission in the foreground of the Taurus clouds which is of the order
of 0.24 K at 21cm. This amount of synchrotron emission is clearly excessive
when compared to previous estimates of the local synchrotron emissivity.
