Simulations of relativistic collisionless shocks: shock structure and particle acceleration
Abstract
We discuss 3D simulations of relativistic collisionless shocks in electronpositron pair plasmas using the particleincell (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 gammaray bursts and pulsar wind outflows.
 Authors:
 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), 2024 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 electronpositron pair plasmas using the particleincell (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 gammaray 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 electronpositron pair plasmas using the particleincell (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 »

PARTICLE ACCELERATION IN RELATIVISTIC MAGNETIZED COLLISIONLESS ELECTRONION SHOCKS
We investigate shock structure and particle acceleration in relativistic magnetized collisionless electronion shocks by means of 2.5dimensional particleincell simulations with iontoelectron mass ratios (m{sub i} /m{sub e} ) ranging from 16 to 1000. We explore a range of inclination angles between the preshock 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 firstorder Fermi process. The downstream ion spectrum consists of a relativistic Maxwellian and a highenergy powerlaw tail, which contains {approx}5% of ions and {approx}30% of ion energy. Itsmore » 
SYNTHETIC SPECTRA FROM PARTICLEINCELL SIMULATIONS OF RELATIVISTIC COLLISIONLESS SHOCKS
We extract synthetic photon spectra from firstprinciples particleincell simulations of relativistic shocks propagating in unmagnetized pair plasmas. The two basic ingredients for the radiation, namely accelerated particles and magnetic fields, are produced selfconsistently 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 socalled 'jitter'more » 
PARTICLEINCELL SIMULATIONS OF PARTICLE ENERGIZATION VIA SHOCK DRIFT ACCELERATION FROM LOW MACH NUMBER QUASIPERPENDICULAR SHOCKS IN SOLAR FLARES
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 Xrays and contemporaneous radio emission. Here we present new twodimensional particleincell simulations of low Mach number/high beta quasiperpendicular shocks. The simulations show that electrons above a certain energy threshold experience shockdriftacceleration. The transition energy between the thermal and nonthermal spectrum and the spectral index from the simulations are consistent with some of the Xray spectra from RHESSI in the energy regime ofmore » 
The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. I. Particleincell simulations
A 2D 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 firehosesense (P{sub }>P{sub perpendicular}) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a noncoplanar rotational mode. In the downstream region the firehose stability parameter {epsilon}=1{mu}{sub 0}(P{sub }P{submore »