SIII 23Fine structure of the Solar Wind turbulence inferred from simultaneous radio occultation observations at widely-spaced ground stations

M. K. Bird , P. Janardhan , A. Efimov  , L. Samoznaev , V. Andreev , P. Edenhofer , D. Plettemeier , R. Wohlmuth 
Radio coronal sounding experiments with the Ulysses and Galileo spacecraft at superior conjunction provided numerous opportunities for simultaneous observations of the downlink signals at two widely-spaced ground stations. In some instances the duration of these observations extended for up to four hours, thereby providing a unique opportunity to track solar wind dynamical behavior at spatial scales comparable with the corona-projected distance between ground stations (a few thousand km). The frequency and phase fluctuations produced by electron density inhomogeneities are normally quite well correlated on these scales. Whereas the mean frequency fluctuation intensity sigma_f was found to change only slightly over the duration of the observations, the spectral index of the temporal frequency fluctuation spectra varied over a wide range. The cross-correlation coefficient reached maximal values ( 0.5) when the spectral index was high ( 1), but no correlation could be detected when the spectral index became (< 0.4). This dependence is in good agreement with theoretical cross-correlation calculations using model solar wind spectra with various spectral indices. It is remarkable that the solar wind seems to changes its spatial spectrum over intervals of the order of 30 minutes between states of well-developed turbulence on the one and nearly structureless laminar flow on the other. Similar behavior in many of the data sets implies that this is a common, if not permanent, feature of the solar wind.