The stability of astrophysical jets

Abstract

Abstract. Jets are produced by young stellar objects (YSOs), by black hole binary star system "microquasars" (µQSOs), by active galactic nuclei (AGN), are associated with neutron stars and pulsar wind nebulae (PWNe), and are thought responsible for the gamma-ray bursts (GRBs). An understanding of these outflows must include how they are launched and collimated into jets, and how they propagate to large distances. Jets be they Poynting flux and/or kinetic flux dominated are current driven (CD) and/or Kelvin-Helmholtz (KH) velocity shear driven unstable. Here I present some of the work that is leading to a better understanding of the properties required for the observed relative stability of astrophysical jets.

Jet Stability, Dynamics and Energy Transport

“…CDI in magnetized flows has been studied from an analytical perspective in the non-relativistic regime by a number of authors. 7 Results show that this instability becomes dominant at large values of helicity (ratio between the toroidal and poloidal field components) and it is stabilised by a strong poloidal magnetic field. Under the physical conditions typical of jets close to the forming regions, CDI is expected to be dominant, though with small growth rates.…”

“…Under the physical conditions typical of jets close to the forming regions, CDI is expected to be dominant, though with small growth rates. 7 In the relativistic regime, numerical simulations of static columns have shown that growth rates decrease with decreasing Alfvén speed and, regarding the non-linear regime, an increasing helicity of the magnetic field with increasing radius in the jet, stabilises the flow with respect to kink CDI. 9 The introduction of a sub-Alfvénic velocity shear generates important differences in the growth of this instability: In the case that the shear is inside the characteristic radius of the static column, the plasma flows through a temporally growing kink, whereas if the shear is outside that radius, the kink is advected with the flow and grows in distance.…”

“…1,2 Thus, it is reasonable to expect that jets are magnetically dominated close to these regions. 7 In this regime, CDI is triggered by differences in the magnetic forces when a toroidal field is present. In the case that a pinch is produced, the larger force produced by the compressed lines allows the pinch to grow.…”

Probing the Internal Structure of Magnetized, Relativistic Jets with Numerical Simulations

“…In some cases-remarkably, those corresponding to cold, Poynting-flux-dominated jets PK02 and PK03-the pinch exerted at some points of the jet axis during this transient phase (due to the coupling of the sideways oscillation caused by the jet overpressure with current driven instabilities, CDI; see, e.g., [18]) decelerates the flow to subsonic speeds and breaks the flow collimation beyond some axial distance (note, however, that since our simulations are axisymmetric, they are free of three-dimensional CDI, as the kink instability).…”

“…Numerical experiments have shown that the CDI growth rates are reduced in the case of magnetized flows with parallel magnetic fields or flows shrouded by (magnetized) winds [18][19][20]. Kim et al [21] focussed on the stability of (non-relativistic) magnetized jets that carry no net electric current and do not have current sheets.…”