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Measurements of Coronal Faraday Rotation at 4.6 Solar Radii

Many competing models for the coronal heating and acceleration mechanisms of the high-speed solar wind depend on the solar magnetic field and plasma structure in the corona within heliocentric distances of $5R_\odot$. We report on sensitive VLA full-polarization observations made in August, 2011, at 5.0 and 6.1 GHz (each with a bandwidth of 128 MHz) of the radio galaxy 3C228 through the solar corona at heliocentric distances of $4.6-5.0R_\odot$. Observations at 5.0 GHz permit measurements deeper in the corona than previous VLA observations at 1.4 and 1.7 GHz. These Faraday rotation observations provide unique information on the magnetic field in this region of the corona. The measured Faraday rotation on this day was lower than our a priori expectations, but we have successfully modeled the measurement in terms of observed properties of the corona on the day of observation. Our data on 3C228 provide two lines of sight (separated by 46'', 33,000 km in the corona). We detected three periods during which there appeared to be a difference in the Faraday rotation measure between these two closely spaced lines of sight. These measurements (termed differential Faraday rotation) yield an estimate of $2.6$ to $4.1$ GA for coronal currents. Our data also allow us to impose upper limits on rotation measure fluctuations caused by coronal waves; the observed upper limits were $3.3$ and $6.4$ rad/m$^2$ along the two lines of sight. The implications of these results for Joule heating and wave heating are briefly discussed.