Measurements of Coronal and Chromospheric Magnetic Fields Using Polarization Observations by the Nobeyama Radioheliograph

Abstract

Abstract Coronal and chromospheric magnetic fields are derived from polarization and spectral observations of the thermal free-free emission using the Nobeyama Radioheliograph (NoRH). In magnetized plasma, the ordinary and extraordinary modes of free-free emission have different optical depths. This creates a circularly polarized component in an atmosphere with a temperature gradient. We observed an active region on 2012 April 13 to derive its coronal and chromospheric magnetic fields. The observed degree of circular polarization was between 0.5% and 1.7%. The radio circular polarization images were compared with ultraviolet images observed by the Atmospheric Imaging Assembly and the photospheric magnetic field observed by the Helioseismic and Magnetic Imager, both on board the Solar Dynamic Observatory. At the edge of the active region, the radio circular polarization was emitted mainly from coronal loops, and the coronal magnetic field was derived to be about 70 G. At the center of the active region, the chromospheric and coronal components cannot be separated. The derived magnetic field is about 20% to 50% of the corresponding photospheric magnetic field, which is an emission-measure-weighted average of the coronal and chromospheric magnetic fields.