Observation and simulation of MHD turbulence in the solar wind

We present evidence from observations and simulations to show that the fluctuations in the interplanetary magnetic and velocity fields are better described as dynamically evolving turbulene than as a superposition of noninteracting MHD waves. Spectral evolution, fluctuation anisotropies, and crosshelicity evolution are all better understood from the turbulence viewpount. The picture that emerges from this work is that the interplanetary fluctuations are initially produced near the Sun as outward propagating Alfven waves, but are then mixed with inward propagating waves by a cascade from non-Alfvenic low wave numbers and dissipated at high wave numbers. In the outer heliosphere the cascade and dissipation rates decrease and become more nearly equal, producting a quasi-steady spectrum with a Kolmogoroff-like spectral index. The cascade from low wave numbers seems to be related to shear between and within interplanetary steams. The compressive fluctuations may be understood as largely consisting of density variations induced by the incompressive pressure fluctuations, with an additional component due to fast mode fluctuations. (author). 23 refs.; 1 fig