Control of solar wind formation by the coronal magnetic field

The circumsolar plasma modifies the phase and amplitude of radio waves passing through the medium. These parameters are monitored during radio sounding experiments in order to determine the level and spectral shape of electron density irregularities and flow speed of the solar wind. Coronal Faraday rotation (FR) of the polarization plane of a linearly polarized signal is dependent on both electron density and magnetic field. Many years of radio occultation experiments have shown that the temporal spectra of phase fluctuations can be approximated by power-law functions with a power (spectral index) increasing with heliocentric distance (e.g., Woo & Armstrong, 1979). Whereas the (2D) spectral index is about 2 for solar distances less than 10 R., it seems to approach an asymptotic value near 8/3 at large distances. Temporal spectra of the FR fluctuations are also power-law functions, but the behavior of the spectral index with solar distance is opposite to that of the phase fluctuations. Typical values of the FR spectral index decrease from values near 2 at 3 R. to about half that at 10 R.. The spectral indices of phase and FR fluctuations are thus approximately the same for coronal offset distances less than about 6 R.. It is speculated that solar wind formation is controlled by the coronal magnetic field below this heliocentric distance. Outward from this distance, the shapes of phase and FR fluctuation spectra differ quite strongly. Phase fluctuations are then evidently governed by electron density while FR fluctuations are dominated by the coronal magnetic field.