Using Thomson-Scattering Brightness to Enhance Coronal Magnetic Field Measurements

The Sun's strong magnetic field plays a significant role in space weather events such as coronal mass ejections and observation of Faraday rotation (FR) is one of the best remote-sensing techniques for determining the structure of the coronal magnetic field. FR is the rotation of the plane of polarization of linearly polarized light due to the presence of a magnetized plasma. FR measurements can be used to determine the magnetic field strength of the Sun's corona along a given line of sight if (1) a suitable model for plasma density is provided or (2) independent measurements of plasma density are available. We calculated the plasma density by measuring Thomson-Scattering Brightness (TSB) for 11 lines of sight (corresponding to Karl G. Jansky Very Large Array radio observations made on August 2 - 3, 2015) using Large Angle and Spectrometric Coronagraph (LASCO C3) images from the Solar and Heliospheric Observatory (SOHO) satellite. For each line of sight (LOS), we generated TSB model profiles based on three different plasma density models: the LDB model (Leblanc, Dulk, and Bougeret, Sol. Phys., 183, 165, 1998) and the coronal hole and coronal streamer models from Kooi et al. (ApJ, 784, 68, 2014). For LOS located within the distinct dim hole and bright streamer regions of the corona, the coronal hole and coronal streamer density models, respectively, provided the best fit to the TSB data. For LOS, typically at low heliographic latitudes, that did not pierce obvious streamer regions, the LDB model generally provided the best fit. For LOS that did not fit the TSB model profiles generated by these three plasma density models, we assumed a single power law plasma density model and computed the least-squares fit to the observed TSB data. We then used the best-fit plasma density model to calculate the expected Faraday rotation for each radio source's LOS. The predicted Faraday rotation ranged in value from - 33.9 rad m^-2 to - 1.3 9 rad m^-2 at heliocentric distances of 6.5 solar radii to 16.4 solar radii, respectively. These best-fit plasma density models will be used in the future to enhance the coronal magnetic field measurements determined from the simultaneous radio observations.