Properties of solar wind turbulence near the Sun as deduced from coronal radio sounding experiments

The characteristics of plasma turbulence in the inner solar wind, as deduced from radio frequency fluctuation measurements recorded during solar conjunction, are reviewed. Special emphasis is placed on results from the radio occultation experiments performed with the Galileo and Ulysses spacecraft in the interval 1991-2000. Estimates of the power spectral index are obtained over the range of heliocentric distances 5R⊙ < R < 80R⊙ and the radial evolution of this parameter is discussed. Low-heliolatitude data from Galileo are compared with high-heliolatitude data from Ulysses during the period of solar activity minimum. Unusual power spectra with a sharp break at high fluctuation frequencies are detected for the first time in the solar wind acceleration region. The observations are interpreted within the framework of a theoretical model based on local generation of density fluctuations via nonlinear interactions of Alfvén waves propagating away from the Sun.