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Abstract
Properties of plasma turbulence in the solar wind can be determined from radio frequency fluctuation measurements recorded during solar conjunctions. Noteworthy recent results were obtained from radio occultation experiments performed with the spacecraft Galileo in the interval 1994–2002 and Ulysses in 1991–1997. The power spectral index was calculated for the range of heliocentric distances 5 RS<R<80 RS(RS = solar radius) and the radial evolution of this parameter is discussed. Galileo data (at low solar latitudes) are compared with those recorded with Ulysses (high heliographic latitudes) during the solar activity minimum. Estimates of the turbulence outer scale are obtained using long Galileo frequency fluctuation records. It is found that the turbulence outer scale increases approximately linearly with increasing solar distance in the inner region of the developed solar wind. The observations are interpreted under the assumption that the density fluctuations are generated locally via nonlinear interactions of outwardly propagating Alfvén waves.
Acknowledgements
The present work was supported by the Program “Plasma processes in the solar system” of the Russian Academy of Sciences. This paper presents results of research partly funded by the Deutsche Forschungsgemeinschaft (DFG) under a cooperative program between the DFG and the Russian Foundation of Basic Research (RFBR). The authors wish to express their gratitude for the continued support of the Galileo Project, the Multi-Mission Radio Science Team and the NASA Deep Space Network.