Testing the extrapolated Coronal Magnetic Fields using Propagation Effects at Microwaves

When a bipolar Active Region (A.R.) is observed at microwaves near the limb, its polarization may appear reversed, in some portions of the A.R., with respect to that expected from the polarity of the underlying photospheric field. This reversal can take place at the site where the wave vector k crosses the magnetic field almost perpendicularly, if a particular condition between the coronal parameters (N-e and B) and the observing frequency is fulfilled. Using a suitable observation of an A.R. performed at Westerbork on July 1993, it was possible to check for the first time the reliability of the force-free extrapolation of the coronal fields up to about 10^(5) km above the photosphere and to determine the best value of parameters capable of reproducing the observed polarization pattern.