Abstract
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.