, M.P. Miralles
, S.R. Cranmer, and J.C. Raymond
Department of Physics and Astronomy, University of Iowa
Harvard-Smithsonian Center for Astrophysics
Measurement of Faraday rotation of radio waves which propagate through the solar corona is one of the best ways of measuring the coronal magnetic field. Faraday rotation can provide information on both the large scale, static component of this field as well as the fluctuating, turbulent component, but the technique requires supplementary information on the coronal plasma. On August 16, 2003, the line of sight to the extended, polarized radio source 3C228 passed through the corona, with a closest heliocentric distance of 7 to 8 solar radii. Polarimetric observations with the Very Large Array of the National Radio Astronomy Observatory were made at 21 and 18 cm. These data yielded measurements of the rotation measure (proportional to the path integral of plasma density and line-of-sight component of the magnetic field) along several, closely-spaced lines of sight through the corona. Simultaneous observations of the OVI and HI Lyman alpha emission lines with the Ultraviolet Coronagraph Spectrometer (UVCS) aboard SOHO were used to determine kinetic temperature, average densities, and outflow speeds in the corona. On that day, the line of sight passed close to a coronal streamer. The VLA data show a very large Faraday rotation event during the eight hour observing session, in which the rotation measure changed by 62 rad/m. This large variation seems to be associated with passage of the coronal current sheet through the line of sight. We will present models of the coronal magnetic field consistent with our observations. This work was supported at the University of Iowa by grant ATM03-54782 from the Division of Atmospheric Sciences, National Science Foundation. At Smithsonian Astrophysical Observatory, this work is supported by NASA under grant NNG04GE84G, by the Italian Space Agency, and by PRODEX (Swiss contribution).