VLA Measurements of Faraday Rotation through Coronal Mass Ejections

Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the Sun, which play an important role in space weather. Faraday rotation is the rotation of the plane of polarization that results when a linearly polarized signal passes through a magnetized plasma such as a CME. Faraday rotation is proportional to the path integral through the plasma of the electron density and the line-of-sight component of the magnetic field. Faraday-rotation observations of a source near the Sun can provide information on the plasma structure of a CME shortly after launch. We report on simultaneous white-light and radio observations made of three CMEs in August 2012. We made sensitive Very Large Array (VLA) full-polarization observations using 1 – 2 GHz frequencies of a constellation of radio sources through the solar corona at heliocentric distances that ranged from 6 – 15 R ⊙ . Two sources (0842+1835 and 0900+1832) were occulted by a single CME, and one source (0843+1547) was occulted by two CMEs. In addition to our radioastronomical observations, which represent one of the first active hunts for CME Faraday rotation since Bird et al. ( Solar Phys. , 98 , 341, 1985 ) and the first active hunt using the VLA, we obtained white-light coronagraph images from the Large Angle and Spectrometric Coronagraph (LASCO) C3 instrument to determine the Thomson-scattering brightness [ B T ], providing a means to independently estimate the plasma density and determine its contribution to the observed Faraday rotation. A constant-density force-free flux rope embedded in the background corona was used to model the effects of the CMEs on B T and Faraday rotation. The plasma densities ( 6 – 22 × 10 3 cm − 3 ) and axial magnetic-field strengths (2 – 12 mG) inferred from our models are consistent with the modeling work of Liu et al. ( Astrophys. J. , 665 , 1439, 2007 ) and Jensen and Russell ( Geophys. Res. Lett. , 35 , L02103, 2008 ), as well as previous CME Faraday-rotation observations by Bird et al. ( 1985 ).