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Title: Simulations of relativistic collisionless shocks: shock structure and particle acceleration

Abstract

We discuss 3D simulations of relativistic collisionless shocks in electron-positron pair plasmas using the particle-in-cell (PIC) method. The shock structure is mainly controlled by the shock's magnetization ('sigma' parameter). We demonstrate how the structure of the shock varies as a function of sigma for perpendicular shocks. At low magnetizations the shock is mediated mainly by the Weibel instability which generates transient magnetic fields that can exceed the initial field. At larger magnetizations the shock is dominated by magnetic reflections. We demonstrate where the transition occurs and argue that it is impossible to have very low magnetization collisionless shocks in nature (in more than one spatial dimension). We further discuss the acceleration properties of these shocks, and show that higher magnetization perpendicular shocks do not efficiently accelerate nonthermal particles in 3D. Among other astrophysical applications, this may pose a restriction on the structure and composition of gamma-ray bursts and pulsar wind outflows.

Authors:
 [1]
  1. Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, PO Box 20450, MS 29, Stanford, CA 94309 (United States)
Publication Date:
OSTI Identifier:
20719686
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 801; Journal Issue: 1; Conference: Conference on astrophysical sources of high energy particles and radiation, Torun (Poland), 20-24 Jun 2005; Other Information: DOI: 10.1063/1.2141897; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ACCELERATION; ASTROPHYSICS; COLLISIONLESS PLASMA; COSMIC GAMMA BURSTS; ELECTRONS; INTERSTELLAR MAGNETIC FIELDS; MAGNETIZATION; PLASMA SIMULATION; PLASMA WAVES; POSITRONS; RELATIVISTIC PLASMA; RELATIVISTIC RANGE; SHOCK WAVES

Citation Formats

Spitkovsky, Anatoly. Simulations of relativistic collisionless shocks: shock structure and particle acceleration. United States: N. p., 2005. Web. doi:10.1063/1.2141897.
Spitkovsky, Anatoly. Simulations of relativistic collisionless shocks: shock structure and particle acceleration. United States. doi:10.1063/1.2141897.
Spitkovsky, Anatoly. Tue . "Simulations of relativistic collisionless shocks: shock structure and particle acceleration". United States. doi:10.1063/1.2141897.
@article{osti_20719686,
title = {Simulations of relativistic collisionless shocks: shock structure and particle acceleration},
author = {Spitkovsky, Anatoly},
abstractNote = {We discuss 3D simulations of relativistic collisionless shocks in electron-positron pair plasmas using the particle-in-cell (PIC) method. The shock structure is mainly controlled by the shock's magnetization ('sigma' parameter). We demonstrate how the structure of the shock varies as a function of sigma for perpendicular shocks. At low magnetizations the shock is mediated mainly by the Weibel instability which generates transient magnetic fields that can exceed the initial field. At larger magnetizations the shock is dominated by magnetic reflections. We demonstrate where the transition occurs and argue that it is impossible to have very low magnetization collisionless shocks in nature (in more than one spatial dimension). We further discuss the acceleration properties of these shocks, and show that higher magnetization perpendicular shocks do not efficiently accelerate nonthermal particles in 3D. Among other astrophysical applications, this may pose a restriction on the structure and composition of gamma-ray bursts and pulsar wind outflows.},
doi = {10.1063/1.2141897},
journal = {AIP Conference Proceedings},
number = 1,
volume = 801,
place = {United States},
year = {Tue Nov 22 00:00:00 EST 2005},
month = {Tue Nov 22 00:00:00 EST 2005}
}
  • We discuss 3D simulations of relativistic collisionless shocks in electron-positron pair plasmas using the particle-in-cell (PIC) method. The shock structure is mainly controlled by the shock's magnetization (''sigma'' parameter). We demonstrate how the structure of the shock varies as a function of sigma for perpendicular shocks. At low magnetizations the shock is mediated mainly by the Weibel instability which generates transient magnetic fields that can exceed the initial field. At larger magnetizations the shock is dominated by magnetic reflections. We demonstrate where the transition occurs and argue that it is impossible to have very low magnetization collisionless shocks in naturemore » (in more than one spatial dimension). We further discuss the acceleration properties of these shocks, and show that higher magnetization perpendicular shocks do not efficiently accelerate nonthermal particles in 3D. Among other astrophysical applications, this may pose a restriction on the structure and composition of gamma-ray bursts and pulsar wind outflows.« less
  • We investigate shock structure and particle acceleration in relativistic magnetized collisionless electron-ion shocks by means of 2.5-dimensional particle-in-cell simulations with ion-to-electron mass ratios (m{sub i} /m{sub e} ) ranging from 16 to 1000. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. In 'subluminal' shocks, where relativistic particles can escape ahead of the shock along the magnetic field lines, ions are efficiently accelerated via the first-order Fermi process. The downstream ion spectrum consists of a relativistic Maxwellian and a high-energy power-law tail, which contains {approx}5% of ions and {approx}30% of ion energy. Itsmore » slope is -2.1 {+-} 0.1. The scattering is provided by short-wavelength non-resonant modes produced by Bell's instability, whose growth is seeded by the current of shock-accelerated ions that propagate ahead of the shock. Upstream electrons enter the shock with lower energy than ions (albeit by only a factor of {approx}5 << m{sub i} /m{sub e} ), so they are more strongly tied to the field. As a result, only {approx}1% of the incoming electrons are accelerated at the shock before being advected downstream, where they populate a steep power-law tail (with slope -3.5 {+-} 0.1). For 'superluminal' shocks, where relativistic particles cannot outrun the shock along the field, the self-generated turbulence is not strong enough to permit efficient Fermi acceleration, and the ion and electron downstream spectra are consistent with thermal distributions. The incoming electrons are heated up to equipartition with ions, due to strong electromagnetic waves emitted by the shock into the upstream. Thus, efficient electron heating ({approx}>15% of the upstream ion energy) is the universal property of relativistic electron-ion shocks, but significant nonthermal acceleration of electrons ({approx}>2% by number, {approx}>10% by energy, with slope flatter than -2.5) is hard to achieve in magnetized flows and requires weakly magnetized shocks (magnetization {sigma} {approx}< 10{sup -3}), where magnetic fields self-generated via the Weibel instability are stronger than the background field. These findings place important constraints on the models of gamma-ray bursts and jets from active galactic nuclei that invoke particle acceleration in relativistic magnetized electron-ion shocks.« less
  • We extract synthetic photon spectra from first-principles particle-in-cell simulations of relativistic shocks propagating in unmagnetized pair plasmas. The two basic ingredients for the radiation, namely accelerated particles and magnetic fields, are produced self-consistently as part of the shock evolution. We use the method of Hededal and Nordlund and compute the photon spectrum via Fourier transform of the electric far field from a large number of particles, sampled directly from the simulation. We find that the spectrum from relativistic collisionless shocks is entirely consistent with synchrotron radiation in the magnetic fields generated by Weibel instability. We can recover the so-called 'jitter'more » regime only if we artificially reduce the strength of the electromagnetic fields, such that the wiggler parameter K ident to qBlambda/mc {sup 2} becomes much smaller than unity (B and lambda are the strength and scale of the magnetic turbulence, respectively). These findings may place constraints on the origin of non-thermal emission in astrophysics, especially for the interpretation of the hard (harder than synchrotron) low-frequency spectrum of gamma-ray bursts.« less
  • Low Mach number, high beta fast mode shocks can occur in the magnetic reconnection outflows of solar flares. These shocks, which occur above flare loop tops, may provide the electron energization responsible for some of the observed hard X-rays and contemporaneous radio emission. Here we present new two-dimensional particle-in-cell simulations of low Mach number/high beta quasi-perpendicular shocks. The simulations show that electrons above a certain energy threshold experience shock-drift-acceleration. The transition energy between the thermal and non-thermal spectrum and the spectral index from the simulations are consistent with some of the X-ray spectra from RHESSI in the energy regime ofmore » E {approx}< 40 {approx} 100 keV. Plasma instabilities associated with the shock structure such as the modified-two-stream and the electron whistler instabilities are identified using numerical solutions of the kinetic dispersion relations. We also show that the results from PIC simulations with reduced ion/electron mass ratio can be scaled to those with the realistic mass ratio.« less
  • A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Simulations are carried out for varying ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehose-sense (P{sub ||}>P{sub perpendicular}) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the downstream region the firehose stability parameter {epsilon}=1-{mu}{sub 0}(P{sub ||}-P{submore » perpendicular})/B{sup 2} tends to plateau at 0.25. This balance arises from the competition between counterstreaming ions, which drive {epsilon} down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2-D turbulence also develops in the downstream region.« less