Magnetized centrifugal winds

It is argued that for steady, axisymmetric, non‐relativistic magneto‐centrifugal winds, not only the boundary and criticality conditions but also the current‐closure condition are of crucial significance as global conditions in resolving the acceleration‐collimation problem. In Sakurai's numerical models, the split‐monopole field adopted at the surface of the source provided the most favourable condition for global collimation of the flow, by making the domain of anti ‐collimating flow with outgoing electric current degenerate into an infinitely thin boundary layer at the equator, and hence suppressing the explicit appearance of the current‐closure condition. For more general or realistic boundary conditions at the source, it is shown that the current‐closure condition yields a two‐component structure (with the return current at least in part in a volume current, not totally a sheet current) as a natural consequence of the transfield equation in the asymptotic domain. This equation, combined with the Bernoulli (and other) integrals, requires the wind to tend asymptotically to a ‘quasi‐conical’ structure, as a natural consequence of the flow particles’ becoming more and more ballistic as a result of the magnetohydrodynamic (MHD) acceleration. This is a result that the Poynting energy flux diminishes to zero along each field line. The criticality problem is solved for magneto‐centrifugal winds, to give the eigenvalues of the Alfvénic distance and other quantities at the fast magnetosonic surface, situated somewhere between the subasymptotic and asymptotic domains.