Kinetic Energy Flux Vs Poynting Flux in Mhd Winds and Jets: the Intermediate Regime

We show that the formal asymptotic limit for all rotating polytropic axisymmetric perfect MHD flows is a kinetic energy dominated wind which collimates to paraboloids around the symmetry axis. We reach this result by showing that another, a priori possible, solution with finite Poynting flux can be excluded on the following physical grounds: (1) the wind velocity does not exceed the fast mode speed everywhere and (2) the circumpolar current increases with distance from the source. We show that asymptotic hoop stress collimation is mathematically robust and we give strong arguments why recent ’anti-collimation’ claims are not correct. However, in practice, due to the very slow logarithmic decline of the circumpolar current with increasing distance from the source, there is a broad intermediate regime with significant Poynting flux. This intermediate regime, rather Observatoire, Université Louis Pasteur, 11 Rue de l’Université, 67000 Strasbourg, France Department of Physics and Astronomy, The Johns Hopkins University, Homewood Campus, 3400 North Charles Street, Baltimore, MD 21218 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 Visiting Scientist at Space Telescope Science Institute and Department of Physics and Astronomy, Johns Hopkins University