CORONAL MAGNETOGRAPHY OF AN ACTIVE REGION FROM MICROWAVE POLARIZATION INVERSION

Abstract

The microwave circular polarization of the active region (AR) NOAA 7260 on 21 - 23 August 1992 is analyzed. Two-dimensional images at 1.76 cm with spatial resolution ofD 10 00 from the Nobeyama radioheliograph and one-dimensional scans at 9 wavelengths in the range of 1.81 - 3.43 cm andD 16:3 00 -3 1:1 00 from the radio telescope RATAN-600 were used. An inversion of the sense of circular polarization through the wavelength range was recorded on 22 August. It is shown that both the wavelength and the time dependence of the inversion are consistent with quasi-transverse (QT) propagation of the radiation in the solar corona. From this, the strength of the coronal magnetic field in the active region was found to be H D 20 - 65 G at a height of hD (5.7 -8 :7/ 10 9 cm above the photosphere on 22 and 30 August and 125 G at the lower height of (3.7 -6 .4)10 9 cm on 23 August. We present a new technique, based on the radio mapping (in both Stokes I and V ) of an AR undergoing circular polarization inversion; applying this method to the Nobeyama data we obtained, for the first time, a magnetogram of the coronal magnetic field. For AR 7260 we found values in the range of 70 - 100 G at heights of (4 - 6)10 9 cm on 23 August, adopting a constant value ofNL (whereN is the electron density andL is the scale of the coronal field divergence) of 2:5 10 18 cm 2 . We compare our results with force-free extrapolations of the photospheric magnetic field from a MSFC magnetogram obtained on 20 August.