Kink instability of force-free jets: A parameter space study

E. Sobacchi, Y. E. Lyubarsky, M. C. Sormani

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


In the paradigm of magnetic acceleration of relativistic jets, one of the key points is identifying a viable mechanism to convert the Poynting flux into the kinetic energy of the plasma beyond equipartition. A promising candidate is the kink instability, which deforms the body of the jet through helical perturbations. Since the detailed structure of real jets is unknown, we explore a large family of cylindrical, force-free equilibria to get robust conclusions. We find that the growth rate of the instability depends primarily on two parameters: (i) the gradient of the poloidal magnetic field and (ii) the Lorentz factor of the perturbation, which is closely related to the velocity of the plasma. We provide a simple fitting formula for the growth rate of the instability. As a tentative application, we use our results to interpret the dynamics of the jet in the nearby active galaxy M87. We show that the kink instability becomes non-linear at a distance from the central black hole comparable to where the jet stops accelerating. Hence (at least for this object), the kink instability of the jet is a good candidate to drive the transition from a Poynting-dominated to a kinetic-energy-dominated flow.

Original languageEnglish
Pages (from-to)4635-4641
Number of pages7
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - 1 Jul 2017


  • Galaxies
  • Instabilities
  • Jets
  • MHD


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