CORONAL MAGNETIC-FIELDS FROM FARADAY-ROTATION OBSERVATIONS

In the first part of this communication we briefly summarize the results of the first observation of linear polarization in the microwave emission above a solar active region obtained with the Westerbork Synthesis Radio Telescope, taking advantage of the very narrow bandwidths of a multi-channel spectral line receiver. The intensity of the Stokes parameter U, measured at several points close to the line of zero circular polarization, showed a clear sinusoidal trend as a function of lambda(2), in accordance to what is expected from Faraday rotation (Alissandrakis and Chiuderi Drago, 1944). Combining the measured period of the Faraday rotation with the observed deplacement of the depolarization line with respect to the photospheric neutral line, the height above the photosphere of the depolarization point and the value of the electron density and the magnetic field at this point are computed. Although the calculations are done in the very simplified assumptions of a bipolar magnetic field and of a density following hydrostatic equilibrium, they represent the first estimate of the coronal magnetic field in an active region, far from sunspots.