Niemiec 2009 相对论喷流和磁场的数值模拟

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Title:		Aperiodic Magnetic Turbulence Produced by Relativistic Ion Beams
Authors:		Niemiec, Jacek; Pohl, Martin; Bret, Antoine; Stroman, Thomas
Affiliation:		AA(Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland Jacek.Niemiec@ifj.edu.pl), AB(Institut für Physik und Astronomie, Universität Potsdam, 14476 Potsdam-Golm, Germany ; DESY, 15738 Zeuthen, Germany ), AC(ETSI Industriales, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain ), AD(Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA)
Publication:		The Astrophysical Journal, Volume 709, Issue 2, pp. 1148-1156 (2010). (ApJ Homepage)
Publication Date:		02/2010
Origin:		IOP
ApJ Keywords:		acceleration of particles, cosmic rays, gamma-ray burst: general, methods: numerical, shock waves, turbulence
DOI:		10.1088/0004-637X/709/2/1148
Bibliographic Code:		2010ApJ...709.1148N

Abstract

Magnetic-field generation by a relativistic ion beam propagating through an electron-ion plasma along a homogeneous magnetic field is investigated with 2.5D high-resolution particle-in-cell (PIC) simulations. The studies test predictions of a strong amplification of short-wavelength modes of magnetic turbulence upstream of nonrelativistic and relativistic parallel shocks associated with supernova remnants (SNRs), jets of active galactic nuclei, and gamma-ray bursts. We find a good agreement in the properties of the turbulence observed in our simulations compared with the dispersion relation calculated for linear waves with arbitrary orientation of {\vec{k}}. Depending on the parameters, the back-reaction on the ion beam leads to filamentation of the ambient plasma and the beam, which in turn influences the properties of the magnetic turbulence. For mildly and ultrarelativistic beams, the instability saturates at field amplitudes a few times larger than the homogeneous magnetic field strength. This result matches our recent studies of nonrelativistically drifting, hot cosmic-ray particles upstream of SNR shocks which indicated only a moderate magnetic-field amplification by nonresonant instabilities. We also demonstrate that the aperiodic turbulence generated by the beam can provide efficient particle scattering with a rate compatible with Bohm diffusion. Representing the ion beam as a constant external current, i.e., excluding a back-reaction of the magnetic turbulence on the beam, we observe nonresonant parallel modes with wavelength and growth rate as predicted by analytic calculations. In this unrealistic setup, the magnetic field is amplified to amplitudes far exceeding the homogeneous field, as observed in recent magnetohydrodynamic and PIC simulations.

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