Asymptotic structure of Poynting dominated jets

TL;DR

This paper derives asymptotic equations for relativistic Poynting dominated jets, revealing two collimation regimes and providing insights into jet acceleration, structure, and key scaling laws for terminal Lorentz factor and collimation angle.

Contribution

The paper introduces simplified asymptotic equations for relativistic MHD jets that facilitate analysis and reveal two distinct collimation regimes with different acceleration mechanisms.

Findings

Identified two collimation regimes with different force balances.

Derived simple scalings for jet acceleration and collimation.

Provided analytical and semi-analytical solutions for jet structure.

Abstract

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 semi-analytically. We show that there are generally two collimation regimes. In the first regime, 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 regime, 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 regimes, 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.