Colloquium: Magnetohydrodynamic turbulence and time scales in astrophysical and space plasmas

Magnetohydrodynamic (MHD) turbulence has been employed as a physical model for a wide range of applications in astrophysical and space plasma physics. This Colloquium reviews fundamental aspects of MHD turbulence, including spectral energy transfer, nonlocality, and anisotropy, each of which is related to the multiplicity of dynamical time scales that may be present. These basic issues are discussed based on the concepts of sweeping of the small scales by a large-scale field, which in MHD occurs due to effects of counterpropagating waves, as well as the local straining processes that occur due to nonlinear couplings. These considerations give rise to various expected energy spectra, which are compared to both simulation results and relevant observations from space and astrophysical plasmas.