Astronomy and Astrophysics – Astronomy
Scientific paper
Feb 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010head...11.3204n&link_type=abstract
American Astronomical Society, HEAD meeting #11, #32.04; Bulletin of the American Astronomical Society, Vol. 41, p.704
Astronomy and Astrophysics
Astronomy
Scientific paper
Magnetic-field generation by cosmic rays (CRs) streaming through an electron-ion plasma along a homogeneous magnetic field is studied with particle-in-cell simulations. We present results for the case, in which CRs represent a relativistic hot population drifting with nonrelativistic velocity relative to the ambient plasma, and also for relativistic CR ion beams. 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, AGN jets, and GRBs. Representing CRs as a constant external current, i.e. excluding a backreaction of the magnetic turbulence on CR particles, we observe non-resonant 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, in agreement with recent MHD and PIC simulations. However, if all particles are fully modeled, the backreaction on CRs leads to a moderate field amplification only. Furthermore, if CRs constitute the relativistic beam, the backreaction also leads to filamentation of the ambient plasma and the beam, which in turn influences the properties of the magnetic turbulence. We find good agreement in properties of the turbulence observed in our simulations compared with the dispersion relation calculated for linear waves with arbitrary orientation of a wavevector. In both cases studied, the precursor becomes turbulent in the nonlinear stage, with significant spatial fluctuations in density and velocity which should have a strong influence on the shock structure. We also demonstrate that the aperiodic turbulence generated by the CR beam can provide efficient particle scattering with a rate compatible with Bohm diffusion.
Stroman, Pohl, Niemiec, 2009, ApJ accepted
Niemiec, Pohl, Bret, Stroman, 2009, ApJ accepted
Bret Antoine
Niemiec Jacek
Pohl Mario
Stroman Thomas
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