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Record 40 of 272
Particle Acceleration and Magnetic Field Generation in Electron-Positron Relativistic Shocks
Author and Affiliation:
Nishikawa, K.-I.(National Space Science and Technology Center, Huntsville, AL, United States)
Hardee, P.(Alabama Univ., Dept. of Physics and Astronomy, Huntsville, AL, United States)
Richardson, G.(Alabama Univ., Dept. of Mechanial and Aerospace Engineering, Huntsville, AL, United States)
Preece, R.(Alabama Univ., Dept. of Physics, Huntsville, AL, United States)
Sol, H.(Observatoire de Paris-Meudon, France)
Fishman, G. J.(NASA Marshall Space Flight Center, Huntsville, AL, United States)
Abstract: Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic electron-positron jet front propagating into an ambient electron-positron plasma with and without initial magnetic fields. We find small differences in the results for no ambient and modest ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The non-linear fluctuation amplitudes of densities, currents, electric, and magnetic fields in the electron-positron shock are larger than those found in the electron-ion shock studied in a previous paper. This comes from the fact that both electrons and positrons contribute to generation of the Weibel instability. Additionally, we have performed simulations with different electron skin depths. We find that growth times scale inversely with the plasma frequency, and the sizes of structures created by the Weibel instability scale proportional to the electron skin depth. This is the expected result and indicates that the simulations have sufficient grid resolution. While some Fermi acceleration may occur at the jet front, the majority of electron and positron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying nonuniform, small-scale magnetic fields which contribute to the electron's (positron's) transverse deflection behind the jet head. This small scale magnetic field structure is appropriate to the generation of "jitter" radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation has different properties than synchrotron radiation calculated assuming a a uniform magnetic field. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.
Publication Date: Jan 01, 2004
Document ID:
(Acquired Nov 19, 2004)
Subject Category: ASTROPHYSICS
Document Type: Preprint
Financial Sponsor: NASA Marshall Space Flight Center; Huntsville, AL, United States
Organization Source: NASA Marshall Space Flight Center; Huntsville, AL, United States
Description: 2p; In English
Distribution Limits: Unclassified; Publicly available; Unlimited
Rights: Copyright
Availability Source: Other Sources
Availability Notes: Abstract Only
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