Abstract
Relativistic jets, be they Poynting flux or kinetic flux dominated, are current driven ( CD) and/or Kelvin-Helmholtz (KH) velocity shear driven unstable. These macroscopic MHD instabilities may be respon- sible for some of the observed larger scale twisted jet structures and typically do not disrupt jets on less than kiloparsec scales. Here I review our understanding of the jet properties that will lead to the observed relative stability of astrophysical jets. In addition, I review the progress made on the microscopic scale plasma instabil- ities in shocks and velocity shears that may lead to magnetic field generation and that does lead to the particle acceleration required to produce the observed emission from radio wavelengths to TeV energies. Finally, I discuss these instabilities in the context of the jet in M 87.
Highlights
Jets are subject to current driven instability (CDI) where toroidal magnetic fields are dominant, and when superAlfvénic are subject to the Kelvin-Helmholtz instability (KHI) driven by velocity shear between the jet and the surrounding medium
If the kink moves more slowly, present results suggest a temporal development scaling like τ ∼ γkατst with 3 ≥ α ≥ 1 for a moving kink with R j a. In this case the condition for the instability to affect the jet structure might be written as z > 100γkα(vk/c)a, where τst ≡ 100(a/c) and 0 < vk ≤ v j is a function of R j/a that is sensitively dependent on the location of the velocity shear surface provided R j/a
When the flow is super-Alfvénic or supermagnetosonic particles can be accelerated via CDI driven reconnection, and in shocks via the filamentation instability, Fermi processes and reconnection, and in velocity shears via the kinetic Kelvin-Helmholtz instability (KKHI)
Summary
Jets are subject to current driven instability (CDI) where toroidal magnetic fields are dominant, and when superAlfvénic are subject to the Kelvin-Helmholtz instability (KHI) driven by velocity shear between the jet and the surrounding medium. Twisted structures and the sense of twist can be induced by precession of the jet base [16], by rotation of the jet fluid, or by the helicity of the magnetic field It is still not clear whether CDI, KHI or precession is responsible for the observed structures, or whether these different processes operate on different spatial scales. Ghisellini et al (2005) [30] proposed a fast jet spine slow jet sheath configuration These scenarios allow the reduced Doppler factor values indicated by VLBI studies and suggest significant internal longitudinal and/or transverse velocity structure, e.g., shocks and velocity shears, that provide sites for particle acceleration. X-ray/TeV flares imply small fast moving “needles-in-a-jet" or “jet-in-a-jet" [39,40,41, 168] scenarios which provide CDI/reconnection, shock, and velocity shear rapid particle acceleration sites
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