The current consensus on the nature of collimated astrophysical jets from young stellar objects (YSO) and active galactic nuclei (AGN) holds that their magnetic field and associated electric current may play a key role in their structure and dynamics. While there is an exhaustive literature concerning jet launching, collimation and propagation, the stability properties of such current carrying magnetized jets have been investigated only recently.
In a preceding paper (Appl et al. 2000, hereafter paper I), we have addressed the stability of magnetized astrophysical jets with respect to modes driven by the electric current density distribution. These current-driven (CD) instabilities have been suspected to disrupt (Eichler 1993, Lucek & Bell 1996, Spruit et al. 1997) or at least to affect the magnetic structure drastically as the jet propagates (Todo et al. 1993, Begelman 1998). In realistic magnetized jet configurations, magnetohydrodynamic instabilities are generally a mixture of Kelvin-Helmholtz (KH), pressure-driven, and CD modes. The non linear development of KH instabilities in jets has been widely studied in the literature (see for example Hardee et al. 1997, Bodo et al. 1998, Micono et al. 1998), unlike the CD modes.
In paper I, we have performed a linear stability analysis of cold supermagnetosonic jets for a large variety of magnetic configurations. It has been shown that the CD instabilities grow rapidly on time scales of order of the Alfvén crossing time in the jet frame, and they are therefore likely to modify the magnetic structure of the jet. However, they are internal modes since the radial displacement becomes very small at the jet surface as shown by the linear eigenfunctions. This led us to conclude that the CD instabilities would not disrupt the jet.
The aim of the present paper is to investigate the non linear development of CD instabilities for magnetized astrophysical jet. Magnetic configurations representative of the general classes defined in paper I are considered. We carry out numerical computations using a 3-dimensional evolution code issued from laboratory plasma physics (Lerbinger & Luciani 1991, Baty et al. 1993) which was adapted to astrophysical jets. For numerical reasons, we mainly focus on early non linear phases that are quasi-ideal (the resistivity effect being negligible), and we only superficially investigate more resistive later stages.
The paper is organized as follows. Magnetic equilibria assumed for the jet configuration and their stability properties are presented in Sect. 2. The next section is devoted to the numerical procedure and is followed by the results of the non linear simulations. Finally, consequences for jet structure are discussed in Sect. 5, and conclusions are drawn in Sect. 6.
© European Southern Observatory (ESO) 2000
Online publication: March 21, 2000