Kinetic Energy Flux versus Poynting Flux in Magnetohydrodynamic Winds and Jets: The Intermediate Regime

Abstract

We show that the formal asymptotic limit for all rotating polytropic axisymmetric perfect MHD flows is a kinetic energy-dominated wind that 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 anticollimation claims are not correct. However, in practice, because of 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 than the mathematically exact asymptotic regime, may well apply along the finite length of the jet. We briefly discuss peculiarities that would be associated with Poynting jets in the intermediate regime. Force-free initial conditions in the near field are most likely to produce such jets, in which most of the energy flux is electromagnetic.