Authors: G.P. Zank (Center for Space Plasma Research and Aeronomic Research and Department of Space Science, University of Alabama in Huntsville)

The nature of magnetohydrodynamic (MHD) turbulence has been debated for several decades with a particular focus on anisotropy. This in part reflects the possible contribution of the two incompressible magnetic ingredients to turbulence, viz., Alfven waves and magnetic flux ropes, also known as magnetic islands. Both are comprised of transverse (to the mean magnetic field) fluctuating magnetic field components with one a propagating component and the other being an advected structure. These two elements have been identified as important components of solar wind turbulence but how do they fit into the panoply of fluctuations and waves in the solar wind? We will present an introductory discussion of possible fluctuation modes in the context of MHD i.e., not considering frequencies/wavenumbers that are higher than admitted by the MHD description, that will include compressible and entropy modes. Brief discussion about a new method for extracting the modal composition of fluctuations in a solar wind parcel will be given. Recent observational results that address the connection between anisotropy on MHD scales and the identification of fluctuating components will be discussed. We will discuss at an elementary level some possible MHD turbulence models and the corresponding expectations for anisotropy.