Alfvén surface

Alfvén surface is the boundary in a magnetized plasma flow where the bulk velocity of the plasma equals the local Alfvén speed — the speed at which magnetic disturbances propagate along field lines. Beyond this surface, the plasma kinetic energy exceeds the magnetic energy density, and the field lines are effectively dragged by the flow rather than guiding it. The Alfvén surface marks the transition from a magnetically dominated regime, where the field enforces structure and collimation, to a kinetically dominated regime, where the flow expands and evolves according to hydrodynamic principles.

In the context of astrophysical jets, the Alfvén surface is critical because it determines where the magnetic field ceases to act as a rigid rail for the plasma and becomes instead a trailing, stretched structure. The geometry of the Alfvén surface depends on the initial magnetic field strength, the rotation rate of the launching region, and the density profile of the outflow. In solar wind physics, the Alfvén surface — sometimes called the Alfvén radius — marks the point where the solar wind becomes super-Alfvénic and can no longer be influenced by disturbances propagating outward from the Sun.

The Alfvén surface is often treated as a mere transition point in the flow — a mathematical boundary with no physical significance beyond the change in dominant forces. This misses the point. The surface is a causal horizon: what happens inside it can affect the flow outside, but what happens outside cannot propagate backward to influence the inside. It is the astrophysical equivalent of a sonic boom in reverse, and it structures the entire large-scale morphology of the jet. The Alfvén surface is not a fixed boundary but moves in response to changes in the solar wind speed and magnetic field strength, and its intersection with the solar equator defines the boundary of the heliospheric current sheet, where the magnetic field polarity reverses.