WAVES AND STREAMS IN THE EXPANDING SOLAR-WIND

The expanding box model (EBM) allows the simulation of the evolution o f compressible MHD turbulence within the expanding solar wind, taking into account the basic properties of expansion. Using the EBM we follo w the evolution of waves within a compressive stream shear and magneti c sector structure in the range of 0.1 to 1 AU from the Sun. We analyz e the physical processes which lead in these simulations to the modula tion and erosion of the wave component, combined with WKB and non-WKB processes due to expansion. A strong erosion by stream shear correspon ds indeed to one of the observed regimes in the solar wind; however, w e are unable to reproduce the regime which holds during solar minimum, in which the correlation between large-scale stream structure and tur bulence remains high independently from distance to the Sun. The main point of disagreement with observations concerns the energy spectrum ( it is difficult to generate and sustain small-scale turbulence with an Alfvenic wave band present, and even more so in an expanding medium); the main point of agreement concerns the statistics of density fluctu ations, which are independent of distance, and matches the observed am plitudes both within slow and fast wind. At the same time, small scale s appear to be dominated in the simulations by compressible effects, w hich contradicts popular ideas on solar wind turbulence.