Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks
Abstract
We study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic e+e- shock. Our 2D particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are approximately isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially, non-propagating, and decaying. Using linear kinetic theory, we find a simple analytic form for these damping rates. Our theory predicts that the overall magnetic energy decays as (ωp t)-q with q ~ 1, which compares favorably with simulations, but predicts overly rapid damping of short-wavelength modes. The magnetic trapping of particles within the magnetic structures may be the origin of this discrepancy. We conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields. These results put interesting constraints on synchrotron models for the prompt and afterglow emission from GRBs.
- Publication:
-
International Journal of Modern Physics D
- Pub Date:
- 2008
- DOI:
- 10.1142/S021827180801339X
- arXiv:
- arXiv:0801.4583
- Bibcode:
- 2008IJMPD..17.1769C
- Keywords:
-
- Shock waves;
- turbulence;
- gamma-ray bursts;
- plasmas;
- Astrophysics
- E-Print:
- 4 pages, 3 figures, contributed talk at the workshop: High Energy Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 2007