Coronal mass ejections (CMEs) are fast-moving magnetic field structures of enhanced plasma density that play an important role in space weather. The Solar Orbiter and Parker Solar Probe will usher in a new era of in situ measurements, probing CMEs within distances of 60 and 10 solar radii, respectively. At the present, only remote-sensing techniques such as Faraday rotation can probe the plasma structure of CMEs at these distances. Faraday rotation is the change in polarization position angle of linearly polarized radiation as it propagates through a magnetized plasma (e.g. a CME) and is proportional to the path integral of the electron density and line-of-sight magnetic field. In conjunction with white-light coronagraph measurements, Faraday rotation observations have been used in recent years to determine the magnetic field strength of CMEs. We report recent results from simultaneous white-light and radio observations made of a CME in July 2015. We made radio observations using the Karl G. Jansky Very Large Array (VLA) at 1 - 2 GHz frequencies of a set of radio sources through the solar corona at heliocentric distances that ranged between 8 - 23 solar radii. These Faraday rotation observations provide a priori estimates for comparison with future in situ measurements made by the Solar Orbiter and Parker Solar Probe. Similar Faraday rotation observations made simultaneously with observations by the Solar Orbiter and Parker Solar Probe in the future could provide information about the global structure of CMEs sampled by these probes and, therefore, aid in understanding the in situ measurements.

In support of our ongoing program to measure coronal magnetic fields using Faraday rotation (the rotation of the plane of polarization when linearly polarized light propagates through a magnetized plasma), we report on measurements of angular broadening of cosmic radio sources as they were occulted by the corona. Angular broadening results from plasma density irregularities elongated along the coronal magnetic field that cause radio wave scattering. In radioastronomical observations, the measured intensity (power per unit area per unit solid angle along the path to the detector) is the convolution of the true radio source intensity with a point spread function (PSF, the effective response of an imaging system to a point-like source). In most radio interferometric applications, the PSF is simply the synthesized beam; however, when observing through a turbulent plasma like the corona, the PSF is the convolution of the synthesized beam with the power pattern of the angular broadening. The angular broadening acts to reduce the measured intensity of a radio source. This can have important consequences for coronal Faraday rotation studies because the error associated with measuring the polarization angle is inversely proportional to the intensity of the linearly polarized light. We made full-polarization observations at 1 – 2 GHz frequencies using the Karl G. Jansky Very Large Array of 21 linearly polarized cosmic radio sources occulted by the solar corona in July and August, 2015. The radio sources were scattered over a range of heliocentric distances; however, sources within a heliocentric distance of 10 solar radii experienced the most pronounced angular broadening. The observed angular broadening is consistent with a 15% - 35% decrease in intensity. We discuss the implications of these angular broadening measurements on the observed coronal Faraday rotation as well as potential methods to correct for this phenomenon in mapping the radio sources.