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 $< 20~\mathrm{R}_{\odot }$. Faraday rotation, a radioastronomical propagation measurement, is an extremely valuable diagnostic for studying CMEs. Faraday rotation measurements [RM] contain information on the magnetic field in the medium causing the Faraday rotation. Recent observations of CME-induced Faraday rotation (e.g., Howard et al. in Astrophys. J. 831, 208, 2016; Kooi et al. in Solar Phys. 292, 56, 2017; Bisi et al. in EGU General Assembly Conference Abstracts, 13243, 2017) have all been restricted to a single line of sight (LOS) and, therefore, limited to providing estimates of the magnetic field strength. Modeling by Liu et al. (Astrophys. J. 665, 1439, 2007) and Jensen and Russell (Geophys. Res. Lett. 35, L02103, 2008) demonstrated that multiple LOS are necessary to recover the magnetic field strength and structure of the observed CME. We report the first successful observations of Faraday rotation through a CME using multiple lines of sight: 13 LOS 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 $8.2-19.5~\mathrm{R}_{\odot }$. For LOS within $10~\mathrm{R}_{\odot }$, the CME’s contribution to the measured RM was $\approx 0$ to −20 rad m⁻², a significant enhancement over the coronal contribution. 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 $5.4-6.4\times 10^{3}$ cm⁻³ and $26-35$ mG, respectively. Further, using all 13 LOS, we successfully determined the CME’s orientation and helicity.