Observations of magnetohydrodynamic turbulence in the 3D heliosphere

The solar wind is a high Reynolds' number plasma flow of solar origin that permeates the whole heliosphere. It is also the only accessible medium in which to study collisionless magnetohydrodynamic turbulence performing direct measurements. This represents a topic of fundamental importance to both plasma physics and astrophysics. During the past decades, in situ observations on the ecliptic and at high heliographic latitudes have been very valuable to shed some light on the intricate nature of space plasma turbulence. In this brief review, we will mainly describe the evolution experienced by the turbulence as the solar wind expands into the interplanetary space. We will also address implications due to different processes of local generation of turbulence which might be at work on the ecliptic and at high latitude. Moreover, the fact that solar wind fluctuations are not isotropic and poorly single scale-invariant, two of the fundamental hypotheses at the basis of Kolmogorov's theory (K41), will give us the possibility to discuss also the relevance of intermittency in the study of space plasma turbulence. (c) 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.