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S. Mancuso, S.R. Spangler (University of Iowa)
Faraday rotation observations of polarized radiation from natural radio sources yield a unique diagnostic of the coronal magnetic field and electron density at heliocentric distances not reached by spacecraft. Dual frequency polarization measurements yield the {\em rotation measure}, which is proportional to \int ne \vec{B} \cdot ds, where ne is the electron density, \vec{B} is the magnetic field, and the integral is along the line of sight. We made linear polarization observations with the NRAO Very Large Array of several polarized radio sources occulted by the solar corona. The observations were made at frequencies of 1465 and 1665 MHz on four days in May, 1997. The observations cover a full solar rotation and sample solar elongations ranging from about 5 to 14 solar radii. The magnitudes of the rotation measures observed range from 11 to less than 1 radians/m2. We attribute the relatively low values for the rotation measures to the magnetohydrodynamic state of the corona at the time of the observations. The coronal magnetic field was quasi-dipolar with the lines of sight to the sources generally not crossing sector boundaries. The highest plasma density was at the streamer belt at low latitudes, which was missed by many of the lines of sight. The largest rotation measure was observed for the source 3C79 on May 11, 1997, and corresponds to a case in which the line of sight passed through the streamer belt at small solar elongation. This research was supported by grant ATM96-16721 from the National Science Foundation.
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