Interplay between Kelvin‐Helmholtz and Current‐driven Instabilities in Jets

Abstract

We investigate, by means of three-dimensional compressiblemagnetohydrodynamic numerical simulations, the interaction ofKelvin-Helmholtz (KH) and current-driven (CD) instabilities in amagnetized cylindrical jet configuration. The jet has a supersonic axialflow, sheared in the radial direction, and is embedded in a helicalmagnetic field. The strength of the axial magnetic field component ischosen to be weak, in accord with the ``weak field regime'' previouslydefined by Ryu, Jones, & Frank for uniformly magnetizedconfigurations. We follow the time evolution of a periodic section wherethe jet surface is perturbed at m=+/-1 azimuthal mode numbers. A m=-1 KHsurface mode linearly develops dominating the m=+1 KH one, in agreementwith results obtained using an independent ideal stability code. Thislifted degeneracy, because of the presence of the helical field, leadsnonlinearly to clear morphological differences in the jet deformation ascompared to uniformly magnetized configurations. As predicted bystability results, a m=-1 CD instability also develops linearly insidethe jet core for configurations having a small enough magnetic pitchlength. As time proceeds, this magnetic mode interacts with the KHvortical structures and significantly affects the further nonlinearevolution. The magnetic field deformation induced by the CD instabilityprovides a stabilizing effect through its azimuthal componentBtheta. This helps to saturate the KH vortices in thevicinity of the jet surface. Beyond saturation, the subsequentdisruptive effect on the flow is weaker than in cases having similaruniform and helical magnetic field configurations without the CD mode.We discuss the implications of this stabilizing mechanism for thestability of astrophysical jets.