THEORETICAL-MODELS FOR MHD TURBULENCE IN THE SOLAR-WIND

The in situ measurements of velocity, magnetic field, density and temp erature fluctuations performed in the solar wind have greatly improved our knowledge of MHD turbulence not only from the point of view of sp ace physics but also from the more general point of view of plasma phy sics. These fluctuations which extend over a nide range of frequencies (about 5 decades), a fact which seems to be the signature of turbulen t nonlinear energy cascade, display mainly in the trailing edge of hig h-speed streams, a number of features characteristic of a self-organiz ed situation: i) a high degree of correlation between magnetic and vel ocity field fluctuations, ii) a very low level of fluctuations in mass density and magnetic-field intensity iii) a considerable anisotropy r evealed by minimum variance analysis of the magnetic-field correlation tensor. Many fundamental processes in plasma physics, which were larg ely unknown or not understood before their observations in the solar w ind, have been explained, by building up analytical models or performi ng numerical simulations. We discuss the most recent analytical theori es and numerical simulations and outline the limits implicit in any an alysis which consider the low-frequency solar-wind fluctuations as a s uperposition of linear modes. The characterization of low-frequency fl uctuations during Alfvenic periods, which results from the models disc ussed, is finally presented.