Angular broadening of radio sources by solar wind turbulence

The observed apparent angular broadening of radio sources as they pass close to the sun is discussed in terms of statistical ray theory. Unlike earlier works in which the turbulent interplanetary plasma is assumed to possess a spectrum of Gaussian form, we here treat power spectra for fluctuations in electron density, which are assumed to be of the same form as have been obtained for magnetic field and velocity fluctuations by in situ measurements. Such spectra are distinctly non-Gaussian. We find that the data suggest that the solar wind turbulence is strong between 10 and 100 solar radii distance from the sun. This result disagrees with earlier analyses, which suggest that the fluctuations are of the order of only a few per cent of the mean, but it agrees with the large fluctuations in electron density indicated by direct satellite measurement near the orbit of the earth. The data usually imply that the solar wind electron density varies as r−2, and that the correlation length and inner scale (that wavelength below which there is little power) of the turbulence are constant with distance from the sun. At times, however, the data imply that the electron density varies as r−2.5, and the two length scales are constant; or that the density varies as r−2, and the two length scales increase linearly with distance from the sun.