VLA Measurements of Faraday Rotation through a Coronal Mass Ejection Using Multiple Lines of Sight

Abstract:

Coronal mass ejections (CMEs) are large eruptions of magnetized plasma from the Sun that play an important role in space weather. The key to understanding the fundamental physics of a CME is measurement of the plasma properties within heliocentric distances of < 20 solar radii. Faraday rotation, a radio remote-sensing measurement, is an extremely valuable diagnostic for studying CMEs. Faraday rotation measurements contain information on the magnetic field in the medium causing the Faraday rotation. We report the first successful observations of Faraday rotation through a CME using multiple lines of sight: 13 lines of sight across seven target radio fields. We made these radio observations using the Karl G. Jansky Very Large Array (VLA) at 1-2 GHz frequencies in the triggered operation mode on 31 July 2015, using a constellation of cosmic radio sources through the solar corona at heliocentric distances of 8.2-19.5 solar radii. We assumed a force-free flux rope structure for the CME's magnetic field and explored three separate models for the CME's plasma density: constant density, thin shell, and thick shell. The plasma densities and axial magnetic field strengths for the three models ranged over 5.4-6.4 x 10³ cm^-3 and 26-35 mG, respectively. Further, using all 13 lines of sight, we successfully determined the CME's orientation and helicity.