An Alfvén surface is where the poloidal flow speed equals the poloidal Alfvén speed. Smooth passage through it imposes a regularity condition relating field-line angular velocity and angular momentum.
The Alfvén radius characterizes where a magnetized stellar wind crosses its Alfvén surface. For a roughly radial wind, and . The wind's angular-momentum lever arm is of order : its torque is , up to geometry factors. The precise wind invariant uses the cylindrical radius of the surface crossing, rather than necessarily its spherical radius.
If is the cylindrical radius at which , finite azimuthal velocity and magnetic field requireThis cancels the simultaneous numerator and denominator zeros in the wind's azimuthal integrals.
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The Alfvén surface is a concept in plasma physics, particularly in the context of magnetohydrodynamics (MHD) and astrophysical plasmas. It represents a boundary within a plasma where the influence of magnetic fields on plasma dynamics undergoes a significant transition. In more detail, the Alfvén surface is defined as the locus of points in space where the Alfvén speed equals the local speed of the plasma.