The direction, but not the magnitude, of the solar coronal magnetic field is encoded in the linear polarization of certain coronal emission lines. The High Altitude Observatory, in collaboration with the Sacramento Peak Observatory, has constructed a polarimeter which measures the plane polarization of the 10747A and 10798A Fe XIII emission lines. The instrument simultaneously provides similar measurements of the continuum corona and the sky at 10690A to give the net emission line intensity and polarization. Polarization brightness maps made with the 10747A line clearly show the large scale organization of the coronal magnetic field and signatures characteristic of diverging fields over sunspots, horizontal fields above filament channels, and the Van Vleck turnover in helmet streamers.
C. W. Querfeld, C. W. Querfeld,
"A Near-Infrared Coronal Emission-Line Polarimeter", Proc. SPIE 0112, Optical Polarimetry: Instrumentation and Applications, (19 October 1977); doi: 10.1117/12.955564; https://doi.org/10.1117/12.955564