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. from the center of the Sun. Nearly continuous p olarimetric 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 obs ervations were made near solar minimum conditions. Observations of radio ga laxies have two advantages with respect to spacecraft transmitter signals. (1) The lambda(2) dependence of the polarization position angle expected of Faraday rotation can be verified. (2) Observations of spatially extended r adio galaxies have the potential of directly measuring the propagation spee d 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 ser ies was obtained for the most polarized component of the source. This rotat ion-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 1 5 minutes to 1 hour, which may be due to coronal Alfven waves. This fluctua ting component of the coronal rotation measure has an rms value less than o r equal to 0.40 rad m(-2), comparable to previously reported detections. Th is measurement:is then used to place model-dependent upper limits to the Al fven wave flux at the coronal base. Depending on the precise geometry of th e solar wind flow from the coronal base to 8.6 R., the inferred wave flux a t the coronal base ranges from 2.4 x 10(4) to 2.3 x 10(5) 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 ob served 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.