MHD turbulence in the heliosphere: Evolution and Intermittency

The solar wind is an excellent laboratory to study the behaviour of MHD turbulence in a collisionless plasma. This is a fundamental topic in both plasma physics and astrophysics. The impressive amount of observations at different solar distances and latitudes collected in last three decades has allowed us to reach a good understanding on many as- pects of the complex phenomenon of solar wind turbulence. The present review focuses on two major topics. One is the type of evolution fol- lowed by the turbulence as the solar wind expands into the interplan- etary space. A comparison is performed between variations observed in low- and high-latitude solar wind, as derived from old measurements in the ecliptic and from recent data by Ulysses. Implications about processes of local generation of turbulence are discussed. The second topic is related to the fact that we are dealing with fluctuating fields which are neither isotropic nor single scale–invariant. These features cause intermittency in the solar wind MHD turbulence. In this review we will address both interplanetary observations and theoretical models which allow us to shed some light on the nature of the intermittency as observed in interplanetary space.