Coronal Magnetography of an Active Region From Microwave Polarization Inversion

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 of θ=10′′ from the Nobeyama radioheliograph and one-dimensional scans at 9 wavelengths in the range of 1.81–3.43 cm and θ=16.3′′–31.1′′ 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=20–65 G at a height of h= (5.7–8.7)×109 cm above the photosphere on 22 and 30 August and 125 G at the lower height of (3.7–6.4)×109 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)×109 cm on 23 August, adopting a constant value of N Lα (where N is the electron density and Lα is the scale of the coronal field divergence) of 2.5×1018 cm−2. We compare our results with force-free extrapolations of the photospheric magnetic field from a MSFC magnetogram obtained on 20 August.