MAGNETOHYDRODYNAMIC TURBULENCE REVISITED

In 1965, Kraichnan proposed that MHD turbulence occurs as a result of collisions between oppositely directed Alfven wave packets, Recent wor k has generated some controversy over the nature of nonlinear coupling s between colliding Alfven waves, We find that the resolution to much of the confusion lies in the existence of a new type of turbulence, in termediate turbulence, in which the cascade of energy in the inertial range exhibits properties intermediate between those of weak and stron g turbulent cascades. Some properties of intermediate MWD turbulence a re the following: (1) in common with weak turbulent cascades, wave pac kets belonging to the inertial range are long-lived; (2) however, comp onents of the strain tensor are so large that, similar to the situatio n in strong turbulence, perturbation theory is not applicable; (3) the breakdown of perturbation theory results from the divergence of neigh boring field lines due to wave packets whose perturbations in velocity and magnetic fields are localized, but whose perturbations in displac ement are not; (4) three-wave interactions dominate individual collisi ons between wave packets, but interactions of all orders n greater tha n or equal to 3 make comparable contributions to the intermediate turb ulent energy cascade; (5) successive collisions are correlated since w ave packets are distorted as they follow diverging field lines; (6) in common with the weak MHD cascade, there is no parallel cascade of ene rgy, and the cascade to small perpendicular scales strengthens as it r eaches higher wavenumbers; (7) for an appropriate weak excitation, the re is a natural progression from a weak, through an intermediate, to a strong cascade.