Anisotropy and Energy Decay in Magneto-Hydrodynamic Turbulence: Theory and Solar Wind Observations

Anisotropies in the spectra of magnetohydrodynamic (MHD) scale fluctuations are observed or inferred in many plasmas, e.g.,laboratory fusion machines, the solar wind and corona, and the interstellar medium (see [1, 2] for references). Anisotropy is expected since a mean magnetic field B 0 (unlike a mean flow), cannot be transformed away and thus provides a preferred direction. Understanding the evolution and development of this spectral anisotropy is a major theoretical challenge as nonlinear effects play a crucial role. Nonetheless, some progress has been made. Using numerical simulations and a reduced MHD (RMHD) approach, we have developed a relatively simple model of the process [1, 2], which we present below.