CORONAL MAGNETIC-FIELDS FROM MICROWAVE POLARIZATION OBSERVATIONS

The solar active region (AR) 7530 was observed at 6 cm on July 3 and 4 , 1993 with the Westerbork Synthesis Radio Telescope, using a multi-ch annel receiver with very narrow bandwidth. We compare the radio data w ith Yohkoh SXT observations and with the magnetic field extrapolated f rom the Marshall vector magnetograms in the force-free and current-fre e approximations. The comparison with soft X-rays shows that, although a general agreement exists between the shape of the radio intensity m ap and the X-ray loops, the brightness temperature, T-b, Obtained usin g the parameters derived from the SXT is much lower than that observed . The comparison with the extrapolated photospheric fields shows inste ad that they account very well for the observed T-b above the main sun spots, if gyroresonance emission is assumed. In the observation of Jul y 4 an inversion and strong suppression of the circular polarization w as clearly present above different portions of the AR, which indicates that particular relationships exist between the electron density and the magnetic field in the region where the corresponding lines of sigh t cross the field quasi-perpendicularly. The extrapolated magnetic fie ld at a much higher level (similar to 10(10) cm), satisfies the constr aints required by the wave propagation theory all over the AR. However , a rather low electron density is derived.