On the magnetohydrodynamic stability of current-carrying jets

Abstract

In the framework of ideal magnetohydrodynamics, we analyse the stability of jets carrying an electrical current. We focus on $m = \pm 1$ helical modes of hot flows (with approximate equipartition between thermal and magnetic energies) embedded in different helical magnetic field configurations. Two types of instability are simultaneously present, with magnetic current-driven modes dominating the Kelvin-Helmholtz (KH) modes when the magnetic pitch length parameter is smaller than approximately $0.1 R_{\rm j}$ ( R j being the jet radius). In the supermagnetosonic regime, the enhanced stability of KH modes due to the presence of the electrical current is much weaker than previously obtained for cold flows embedded in a linear force-free equilibrium configuration (Appl et al. 1992). On the other hand, KH modes of transmagnetosonic current-carrying jets appear to be more unstable than their uniformly magnetized current-free counterparts, leading to a mixed current-driven/KH instability branch in the limit of small fast Mach number for a magnetic field configuration dominated by the azimuthal component at the jet radius. Finally, we discuss the relevance of our results for interpreting numerical simulations aiming to explain the remarkable stability and flow coherence observed in many astrophysical jets.