MHD turbulence in the solar wind: self-similarity, intermittency and coherent structures

High-resolution numerical simulations on the one hand and solar wind data a nalysis on the other hand have allowed for much progress in our understandi ng of magnetohydrodynamic (MHD) turbulence. In particular the high resoluti on of solar wind measurements has allowed us to characterize the intermitte ncy observed on small scales. In this paper we will give a schematic view of the main properties of solar wind MHD turbulence and discuss some recent results obtained from the anal ysis of velocity and magnetic field data measured during the space experime nts Hellos and ISEE. In particular, we will show that applying Haar wavelet s technique to about one year of data taken every minute during the ISEE sp ace experiment, it is possible to calculate spectra and structure functions of the turbulence; moreover the definition of conditioned structure functi on allows: (a) for the elimination of intermittency effects in spectra and thus for a clear identification of which kind of phenomenology of nonlinear cascade be tween Kolmogorov and Kraichnan is taking place in the solar wind turbulence ; (b) for the identification of the mast intermittent;structures which rum ou t to be either shock waves or one-dimensional current sheets, at variance w ith ordinary fluid intermittency, where the most intermittent structures ar e found to be two-dimensional vortices.