Low frequency turbulence and energy dissipation in the Solar Wind

The possibility to perform in-situ measurements of velocity, magnetic field, density and temperature fluctuations in the Solar Wind has greatly improved our knoweledge of MHD turbulence not only from the point of view of space physics but also from the more general point of view of plasma physics.These fluctuations on the one hand extend over a wide range of frequencies (about 5 decades), a fact which seems to be the signature of turbulent non-linear energy cascade, on the other hand display, mainly in the trailing edge of high speed streams, a number of striking features: (i) a high degree of correlation between magnetic and velocity field fluctuations, (ii) a very low level of fluctuations in mass density and magnetic field intensity, (iii) a considerable anisotropy revealed by minimum variance analysis of the magnetic field correlation tensor. More recently it has been stressed that MHD turbulence in the Solar Wind displays a clear intermittent character.The picture which emerges from the most recent analytical theories and numerical simulations is presented. In particular the observations which give us informations about the dissipation mechanism, which remains yet largely unknown, are discussed.