= Alfvén radius
{c}
{title2=$R_A$}
The Alfvén radius characterizes where a magnetized <stellar wind> crosses its <Alfvén surface>. For a roughly radial wind, $v_{\rm wind}\simeq B/\sqrt{\mu_0\rho}$ and $\dot M_w\simeq4\pi R_A^2\rho v_{\rm wind}$. The wind's angular-momentum lever arm is of order $R_A$: its torque is $\dot J\sim-\dot M_w\Omega R_A^2$, up to geometry factors. The precise wind invariant uses the cylindrical radius of the surface crossing, rather than necessarily its spherical radius.
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