Observations of MHD turbulence in the solar wind

Within the past decade there has been a remarkable production of papers, both theoretical and observational, that have shed some light on the intriguing and unsolved problem of the MHD turbulence in the solar wind. In situ observations by spacecraft between 0.3 and several AUs, remote sensing of plasma conditions at the basis of the corona and numerical simulations have shown that solar-wind turbulence undergoes a dramatic evolution as the wind expands into the interplanetary space, depending on the region of the solar wind where it is observed. Within high-velocity streams we observe a high level of turbulent energy, relevant presence of outward Alfven waves with a rather flat energy spectrum, near equipartition between kinetic and magnetic energy, weak density fluctuations and high proton temperature. Within low-velocity regions we record a low level of turbulent energy, almost total absence of Alfvenic correlation, predominance of magnetic over kinetic energy, near-Kolmogorov spectrum of the fluctuations, strong density fluctuations and lower proton temperature. After all, these features identify two types of turbulence: a well developed turbulence in slow wind and an Alfvenic turbulence in fast wind. Although the scientific community has agreed upon many points which are at the basis of the temporal and radial evolution of the turbulence, there are still many open issues which are of fundamental importance for a complete understanding of the phenomenon. In this review we will report about the most important observations performed in the solar wind in the past few years and we will try to interpret them in the light of the latest theories and computer simulations