Y. Zhou, W. H. Matthaeus, and P. Dmitruk, Colloquium: Magnetohydrodynamic turbulence and time scales in astrophysical and space plasmas, Rev. Mod. Phys. 76, 1015 (2004). [PDF]
Magnetohydrodynamic (MHD) turbulence is characterized by nonlinear interactions among fluctuations of the magnetic field and flow velocity over a range of spatial and temporal scales. It plays an important role in plasma heating, the transport of energetic particles, and radiative transfer and is ubiquitous in space and astrophysical plasmas. The solar wind and the diffuse interstellar medium (ISM) are both examples of plasmas that exhibit turbulent behavior, as evidenced by the power spectra determined from radio propagation observations (ISM, solar wind) and in situ data (solar wind).
These observations have stimulated an ongoing effort to develop theoretical treatments of MHD turbulence appropriate to the two similar systems. Although this effort relies heavily on studies of the solar wind as the more fully characterized of the two plasmas, it is strongly interdisciplinary in character, drawing on insights from theoretical plasma physics and laboratory plasma physics as well as on observational and theoretical studies.
The last 20 years have seen considerable progress in this area. However, a number of important theoretical issues remain to be resolved:
- scaling in MHD turbulence and compressibility effects
- the mechanisms responsible for ISM turbulence are poorly understood, and heliospheric analogies are expected to provide useful insights
- supersonic turbulence is studied in the star-forming regions in dense molecular clouds, and it is noteworthy that turbulent energy decay rates are found to scale much the same way as in the heliosphere, suggesting a commonality of MHD turbulence principles
- random electric fields associated with turbulence can also play a role in the stochastic acceleration of charged particles, and this is a possible mechanism for reacceleration of cosmic rays in the Galaxy.
Magnetohydrodynamic (MHD) turbulence has been employed as a physical model for a wide range of applications in astrophysical and space plasma physics. The fundamental aspects of MHD turbulence include 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 can be 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.