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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 . Measured rotation measures range from À25 to +6 rad m À2 , and perhaps as high as +61 rad m À2 . The purpose of these observations is to probe the three-dimensional structure of the coronal plasma in the heliocentric distance range 5Y10 R , 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 46Y120 mG at a fiducial heliocentric distance of 5 R , with a smaller, preferred range of 46Y52 mG.

I discuss a method for determining the strength and spatial structure of the coronal magnetic field by observations of the Faraday rotation of a radio galaxy which is in conjunction with the Sun. Given a knowledge of the plasma density in the outer corona, and the magnetic field sector structure (both independently available), the strength of the coronal field can be determined, as well as the magnitude of spatial variations on scales of 1000 km to several solar radii. Such knowledge is crucial for testing computational models of the solar corona, which are prominently featured in this meeting. Results are presented from observations with the Very Large Array radio telescope of the radio galaxy 3C228 on August 16, 2003, when the line of sight to the source was at heliocentic distances of 7.1 − 6.2R . The observations are consistent with a coronal magnetic field which is proportional to the inverse square of the distance in the range 6 ≤ r ≤ 10R , and has a value of 39 mG at 6.2R . The Faraday rotation is uniform across the source, indicating an absence of strong plasma inhomogeneity on spatial scales up to 35,000 km.

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