Coronal Faraday Rotation Observations: Measurements and Limits on Plasma Inhomogeneities

We report Faraday rotation measurements of the extended radio galaxy J0039+0319 (4C+03.01) seen through the solar corona when the source was at an average distance of 8.6 R_solar from the center of the Sun. Nearly continuous polarimetric observations were made over an 11 hour period on 1997 March 28 with the NRAO Very Large Array at frequencies of 1465 and 1635 MHz. The observations were made near solar minimum conditions. Observations of radio galaxies have two advantages with respect to spacecraft transmitter signals. (1) The λ² dependence of the polarization position angle expected of Faraday rotation can be verified. (2) Observations of spatially extended radio galaxies have the potential of directly measuring the propagation speed of coronal MHD irregularities. With the use of observations made when the source was far from the Sun, we measure an average rotation measure of +6.2+/-1.0 rad m^-2 attributable to the corona. A rotation-measure time series was obtained for the most polarized component of the source. This rotation-measure time series showed slow variations during the observing session, with a total change of about 3 rad m^-2. This variation is attributed to large-scale gradients and static plasma structures in the corona. We also obtain a weak detection of rotation-measure fluctuations on timescales of 15 minutes to 1 hour, which may be due to coronal Alfvén waves. This fluctuating component of the coronal rotation measure has an rms value <=0.40 rad m^-2, comparable to previously reported detections. This measurement is then used to place model-dependent upper limits to the Alfvén wave flux at the coronal base. Depending on the precise geometry of the solar wind flow from the coronal base to 8.6 R_solar, the inferred wave flux at the coronal base ranges from 2.4×10⁴ to 2.3×10⁵ ergs s^-1 cm^-2. These values range from slightly below to more than an order of magnitude below the wave flux needed to heat and accelerate the solar wind to its observed values. Our results corroborate an increasing body of observational evidence indicating that long-wavelength MHD waves are not responsible for the heating and acceleration of the solar wind.