Magnetohydrodynamic turbulence

Magnetohydrodynamic (MHD) turbulence is the chaotic, multi-scale cascade of energy in a conducting fluid threaded by a magnetic field, where the nonlinear interaction of velocity and magnetic fluctuations produces a spectrum of structures spanning from the driving scale to the resistive and viscous dissipation scales. Unlike hydrodynamic turbulence, MHD turbulence is anisotropic: the magnetic field imposes a preferred direction, and energy cascades preferentially perpendicular to the local field lines, producing sheet-like structures rather than the vortical filaments of neutral fluids. This anisotropy is not a perturbation but a structural feature of the dynamics, fundamentally altering the rate of energy dissipation and the transport of magnetic flux in environments from the interstellar medium to accretion disks. The Alfvén speed sets the nonlinear turnover time at each scale, and in strong turbulence, the energy cascade rate is determined by the velocity of turbulent eddies relative to the local Alfvén speed — a ratio that defines the turbulence regime and its capacity to drive magnetic reconnection or amplify magnetic helicity.

MHD turbulence is not a complication of magnetized flow; it is the default state of virtually all astrophysical plasmas, and any model that ignores it is modeling a vacuum rather than a system.