Probing the Large‐Scale Plasma Structure of the Solar Corona with Faraday Rotation Measurements

Faraday rotation measurements of the solar corona made with the Very LargeArray (VLA) at frequencies of 1465 and 1665 MHz are reported. The measurementswere made along 20 lines of sight to 19 extragalactic radio sources in Marchand April, 2005. The closest heliocentric distances of the lines of sightranged from 9.7 to 5.6 $R_{\odot}$. Measured rotation measures range from -25to +61 rad/m$^2$. The purpose of these observations is to probe the threedimensional structure of the coronal plasma in the heliocentric distance range$5-10 R_{\odot}$, and particularly the strength and structure of the coronalmagnetic field. The measured rotation measures are compared with models for thecoronal plasma structure. For the majority of the lines of sight, the observedrotation measures are reasonably well represented by the predictions from themodels. However, 4 of the 20 lines of sight have large observed-modelresiduals, which do not seem associated with coronal mass ejections. Themagnitude of the field necessary to reproduce the majority of the observationsis in the range 46-120 milliGauss at $5 R_{\odot}$, with a smaller, preferredrange of 46-52 mG at $5 R_{\odot}$.Comment: 32 pages, 7 figure