Probing the large-scale plasma structure of the solar corona with Faraday rotation measurements

Faraday rotation measurements of the solar corona made with the VLA at frequencies of 1465 and 1665 MHz are reported. The measurements were made along 20 lines of sight to 19 extragalactic radio sources in 2005 March and April. The closest heliocentric distances of the lines of sight ranged from 5.6 to 9.7 R-circle dot. Measured rotation measures range from -25 to +6 rad m(-2), and perhaps as high as +61 rad m(-2). The purpose of these observations is to probe the three-dimensional structure of the coronal plasma in the heliocentric distance range 5-10 R-circle dot, and particularly the strength and structure of the coronal magnetic field. The measured rotation measures are compared with two types of models for the coronal plasma structure. For the majority of the lines of sight, the observed rotation measures are reasonably well represented by the predictions from either type of model. However, 3 or 4 of the 20 lines of sight have large observed-model residuals. These anomalous sources do not seem to be affected by coronal mass ejections (which did occur during the program), but there may be an association between anomalous rotation measures and the presence of streamers along the line of sight. The magnitude of the coronal field necessary to reproduce the majority of the observations is in the range of 46-120 mG at a fiducial heliocentric distance of 5 R-circle dot, with a smaller, preferred range of 46-52 mG.