Faraday rotation and models for the plasma structure of the solar corona

Faraday rotation observations of polarized radiation from natural radio sou rces are unique among remote diagnostics of the solar corona in that they p rovide information on the coronal magnetic held. Dual frequency radio polar ization measurements yield the rotation measure, a quantity that is proport ional to the integral along the line of sight of the product of the electro n density and the line-of-sight component of the magnetic held. We made lin ear polarization observations with the NRAO Very Large Array of 13 polarize d radio sources occulted by the solar corona. The observations were made at frequencies of 1465 and 1665 MHz on four days in 1997 May and cover a 20 d ay period, sampling elongations ranging from about 5 to 14 R.. The magnitud es of the rotation measures observed range from about 11 to 0 rad m(-2). Th e relatively low values for the rotation measures are due to the solar mini mum configuration of the corona at the time of the observations, with the l ines of sight to the sources generally not crossing sector boundaries. The largest rotation measure was observed for the extended radio source 3C 79 o n 1997 May II and corresponds to a case in which the line of sight passed n ext to the streamer belt at small solar elongations. We have developed a th ree-dimensional model of the solar corona that is in excellent agreement wi th the observed rotation measures, as well as being completely consistent w ith other coronal diagnostics such as coronagraph images. In particular, ou r observations support the coronal magnetic held model of Patzold et al. (1 987); they would be inconsistent with coronal magnetic fields significantly weaker or stronger than this model. The plasma density distribution in the corona is successfully modeled by a dense streamer belt component and a mo re tenuous coronal hole component. Details of these models are given in ? 3 of this paper. The principal disagreement between the model and observatio ns occurs for three lines of sight for which the model predicts nearly zero rotation measure but for which we measure small but significant values of -1 to -2 rad m(-2). These lines of sight passed over the solar polar region s. We discuss the possibility that these residual rotation measures are due to static coronal plasma structures, not described by global coronal model s, or to very long wavelength coronal Alfven waves. Fluctuations in the rot ation measure on timescales of a few bouts were observed for some sources a nd not others. When detected, they were of order 1-2 rad m(-2) and occurred on timescales of several hours.