CORONAL MAGNETIC-FIELDS FROM FARADAY-ROTATION OBSERVATIONS

In the first part of this communication we briefly summarize the resul ts of the first observation of linear polarization in the microwave em ission above a solar active region obtained with the Westerbork Synthe sis Radio Telescope, taking advantage of the very narrow bandwidths of a multi-channel spectral line receiver. The intensity of the Stokes p arameter U, measured at several points close to the line of zero circu lar polarization, showed a clear sinusoidal trend as a function of lam bda(2), in accordance to what is expected from Faraday rotation (Aliss andrakis and Chiuderi Drago, 1944). Combining the measured period of t he Faraday rotation with the observed deplacement of the depolarizatio n line with respect to the photospheric neutral line, the height above the photosphere of the depolarization point and the value of the elec tron density and the magnetic field at this point are computed. Althou gh the calculations are done in the very simplified assumptions of a b ipolar magnetic field and of a density following hydrostatic equilibri um, they represent the first estimate of the coronal magnetic field in an active region, far from sunspots.