Magnetohydrodynamic wind

Magnetohydrodynamic wind is a generic term for any outflow of plasma driven by the combined action of magnetic fields and thermal or rotational energy in a magnetized fluid. Unlike purely thermal winds, which are driven by gas pressure gradients, or purely centrifugal winds, which are driven by rotation, MHD winds are accelerated by the transfer of energy from the magnetic field to the plasma through Lorentz forces. This magnetic driving can produce collimated, high-velocity outflows from systems ranging from protoplanetary disks to galactic nuclei.

The mechanism of MHD wind acceleration depends on the geometry of the magnetic field and the energy source. In systems with strong rotation and open magnetic field lines, the wind is accelerated by magnetocentrifugal launching, where the field lines act as rigid rails and the rotation provides the motive power. In systems with closed field lines and strong heating, the wind is accelerated by magnetic pressure gradients along the field lines, similar to the solar wind but with additional confinement and collimation provided by the magnetic field geometry. In both cases, the wind carries away angular momentum and mass, regulating the evolution of the driving system.

Magnetohydrodynamic wind theory is plagued by an ambiguity: is the wind driven by the magnetic field, or does the magnetic field merely channel the driving? The answer is both, and the ambiguity is not a failure of theory but a fundamental feature of MHD. In a perfectly conducting plasma, the field and the flow are locked together; there is no privileged driver, only a coupled system. The wind is not pushed by the field or by the rotation; it is pushed by the system, and the distinction between causes is an observer's projection onto a single unified dynamics.