Asymptotic structure of Poynting dominated jets

In relativistic, Poynting dominated outflows, acceleration and collimation are intimately connected. An important point is that the Lorentz force is nearly compensated by the electric force therefore the acceleration zone spans a large range of scales. We derived the asymptotic equations describing relativistic, axisymmetric MHD flows far beyond the light cylinder. These equations do not contain either intrinsic small scales (like the light cylinder radius) or terms that nearly cancel each other (like the electric and magnetic forces) therefore they could be easily solved numerically. They also suit well for qualitative analysis of the flow and in many cases, they could even be solved analytically or semianalytically. We show that there are generally two collimation modes. In the first mode, the residual of the hoop stress and the electric force is counterbalanced by the pressure of the poloidal magnetic field so that at any distance from the source, the structure of the flow is the same as the structure of an appropriate cylindrical equilibrium configuration. In the second mode, the pressure of the poloidal magnetic field is negligible small so that the flow could be conceived as composed from coaxial shrinking magnetic loops. In the two collimation modes, the flow is accelerated in different ways. We study in detail the structure of jets confined by the external pressure with a power law profile. In particular, we obtained simple scalings for the extent of the acceleration zone, for the terminal Lorentz factor and for the collimation angle. Subject headings: (magnetohydrodynamics:) MHD – relativity – galaxies:jets – gamma rays: bursts