STABILITY OF RELATIVISTIC FORCE-FREE JETS

We consider a two-parameter family of cylindrical force-free equilibria, modeled to match numerical simulations of relativistic force-free jets. We study the linear stability of these equilibria, assuming a rigid impenetrable wall at the outer cylindrical radius R(j). Equilibria in which the Lorentz factor gamma(R) increases monotonically with increasing radius R are found to be stable. On the other hand, equilibria in which gamma(R) reaches a maximum value at an intermediate radius and then declines to a smaller value gamma(j) at R(j) are unstable. A feature of these unstable equilibria is that poloidal field line curvature plays a prominent role in maintaining transverse force balance. The most rapidly growing mode is an m = 1 kink instability which has a growth rate similar to(0.4/gamma(j))(c/R(j)). The e-folding length of the equivalent convected instability is similar to 2.5 gamma(j)R(j). For a typical jet with an opening angle theta(j) similar to few/gamma(j), the mode amplitude grows only weakly with increasing distance from the base of the jet. The growth is much slower than one might expect from a naive application of the Kruskal-Shafranov stability criterion.