Magnetic Field Diagnostics in the Low Corona from Microwave Circular Polarization Inversion

The characteristics of the observed polarization of radio waves are determined by the emission mech-anism and by the propagation conditions in the corona. In the case of weak coupling between the two electromagnetic wave modes, the polarization changes along the ray path, reflecting the local physical con-ditions; this results in inversion of the sense of circular polarization when a transverse field region (TFR) is crossed. On the contrary, if the wave coupling is strong, the polarization is fixed and its sense does not change when a TFR is crossed. As a result, Stokes V maps of active regions do not always correspond to the magnetic polarities shown in photospheric magnetograms. The differences depend on the wavelength, the heliographic position of the region, as well as on the density and the magnetic field of the corona, at about 0.1 R above the photosphere. In this short review I present older and recent observation of polarization inversion and will discuss the diagnostics they provide on the magnetic field