On The Ultrarelativistic Two-stream Instability, Electrostatic Turbulence And Brownian Motion
2006 (English)In: IAU XXVIth General Assembly,2006, Prague: International Astronomical Union , 2006Conference paper (Other academic)
The observation of ultra-relativistic plasma flow in the form of the collimated jets of active galactic nuclei and gamma ray bursts requires a better understanding of their relaxation. A description of the plasma thermalization requires, in principle, a kinetic model, e.g. in form of relativistic particle-in-cell simulations. The computational cost of such simulations imposes, however, strong limitations on the system size and geometry, which restricts the physical accuracy of the simulation, e.g. by reducing the proton-to-electron mass ratio or the plasma flow speed. Alternatively, one may attempt to subdivide the overall system into well-defined components, which can then be resolved at an appropriate resolution. This may provide qualitative insight into the plasma relaxation, which could be compared to experimental data. An important observation is the similarity of gamma ray bursts in terms of the emitted radiation. The gamma factor associated with the flow speed of gamma ray bursts is of the order 100-1000. A similar emission of gamma ray bursts, despite the large variations in their flow speeds, suggests plasma processes that do not strongly depend on the flow speed. We present one-dimensional particle-in-cell simulation studies of the ultrarelativistic two-stream instability. The instability is driven by two spatially homogeneous inter-penetrating plasma beams, which consist of electrons and protons. The simulation box is aligned with the plasma flow velocity vector. This system thus excludes the important Weibel and mixed mode instabilities, but it allows us to model the two-stream instability over a wide spatial interval. We find a universal behaviour of the instability that does not depend on the beam speed. We observe for flow gamma factors between 100-1000 the development of broadband electrostatic turbulence, which yields the relativistic Brownian motion of the electrons. This Brownian motion results in the development of a Jüttner-Synge electron momentum distribution that shows a linear scaling of the momentum spread with the initial beam gamma factor. The reaction of the proton component to the wave fields locally accelerates electrons to ultra-relativistic speeds, yielding a thin high energy tail in addition to the thermal bulk population.
Place, publisher, year, edition, pages
Prague: International Astronomical Union , 2006.
Engineering and Technology
IdentifiersURN: urn:nbn:se:liu:diva-35712Local ID: 28247OAI: oai:DiVA.org:liu-35712DiVA: diva2:256560