Statistical properties of MHD fluctuations associated with high-speed streams from Helios-2 observations

A variance analysis of Helios-2 magnetic data has been used to derive several statistical properties of MHD fluctuations associated with the trailing edge of a given stream observed in different solar rotations. Such properties are derived both as a function of distance from the Sun and as a function of the frequency range included in the sample. The most noticeable result is that the radial gradients of various parameters, such as anisotropy and normalized power of the fluctuations, depend from frequency range. In particular the variation with distance of the normalized power does not correspond, for periods 1 hr, to what is expected from WKB propagation effects.Istituto di Fisica, Università di Roma, Rome, Italy.Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Md 20771, U.S.A.