253 research outputs found
Spatio-temporal evolution of the nonresonant instability in shock precursors of young supernova remnants
A nonresonant cosmic-ray-current-driven instability may operate in the shock
precursors of young supernova remnants and be responsible for magnetic-field
amplification, plasma heating and turbulence. Earlier simulations demonstrated
magnetic-field amplification, and in kinetic studies a reduction of the
relative drift between cosmic rays and thermal plasma was observed as
backreaction. However, all published simulations used periodic boundary
conditions, which do not account for mass conservation in decelerating flows
and only allow the temporal development to be studied. Here we report results
of fully kinetic Particle-In-Cell simulations with open boundaries that permit
inflow of plasma on one side of the simulation box and outflow at the other
end, hence allowing an investigation of both the temporal and the spatial
development of the instability. Magnetic-field amplification proceeds as in
studies with periodic boundaries and, observed here for the first time, the
reduction of relative drifts causes the formation of a shock-like compression
structure at which a fraction of the plasma ions are reflected. Turbulent
electric field generated by the nonresonant instability inelastically scatters
cosmic rays, modifying and anisotropizing their energy distribution. Spatial CR
scattering is compatible with Bohm diffusion. Electromagnetic turbulence leads
to significant nonadiabatic heating of the background plasma maintaining bulk
equipartition between ions and electrons. The highest temperatures are reached
at sites of large-amplitude electrostatic fields. Ion spectra show
supra-thermal tails resulting from stochastic scattering in the turbulent
electric field. Together, these modifications in the plasma flow will affect
the properties of the shock and particle acceleration there.Comment: Accepted for publication in MNRAS. 16 pages, 15 figure
Electron Pre-Acceleration at Nonrelativistic High-Mach-Number Perpendicular Shocks
We perform particle-in-cell simulations of perpendicular nonrelativistic
collisionless shocks to study electron heating and pre-acceleration for
parameters that permit extrapolation to the conditions at young supernova
remnants. Our high-resolution large-scale numerical experiments sample a
representative portion of the shock surface and demonstrate that the efficiency
of electron injection is strongly modulated with the phase of the shock
reformation. For plasmas with low and moderate temperature (plasma beta
and ), we explore the
nonlinear shock structure and electron pre-acceleration for various
orientations of the large-scale magnetic field with respect to the simulation
plane while keeping it at to the shock normal. Ion reflection off
the shock leads to the formation of magnetic filaments in the shock ramp,
resulting from Weibel-type instabilities, and electrostatic Buneman modes in
the shock foot. In all cases under study, the latter provides first-stage
electron energization through the shock-surfing acceleration (SSA) mechanism.
The subsequent energization strongly depends on the field orientation and
proceeds through adiabatic or second-order Fermi acceleration processes for
configurations with the out-of-plane and in-plane field components,
respectively. For strictly out-of-plane field the fraction of supra-thermal
electrons is much higher than for other configurations, because only in this
case the Buneman modes are fully captured by the 2D simulation grid. Shocks in
plasma with moderate provide more efficient pre-acceleration.
The relevance of our results to the physics of fully three-dimensional systems
is discussed
Plasma effects on relativistic pair beams from TeV blazars: PIC simulations and analytical predictions
Pair beams produced by very high-energy radiation from TeV blazars emit gamma
rays in the GeV band by inverse-Compton scattering of soft photons. The
observed GeV-band signal is smaller than that expected from the full
electromagnetic cascade. This means that the pair beams must be affected by
other physical processes reducing their energy flux. One possible loss
mechanism involves beam-plasma instabilities that we consider in the present
work. For realistic parameters the pair beams can not be simulated by modern
computers. Instead, we use a simple analytical model to find a range of the
beam parameters that (i) provides a physical picture similar to that of
realistic pair beams and (ii) at the same time can be handled by available
computational resources. Afterwards, we performed corresponding 2D PIC
simulations. We confirm that the beams experience only small changes in the
relevant parameter regime, and other processes such as deflection in magnetic
field must be at play.Comment: 11 pages, 19 figures, 1table, accepted for publication in A&
Kinetic simulations of turbulent magnetic-field growth by streaming cosmic rays
Efficient acceleration of cosmic rays (via the mechanism of diffusive shock
acceleration) requires turbulent, amplified magnetic fields in the shock's
upstream region. We present results of multidimensional particle-in-cell
simulations aimed at observing the magnetic field amplification that is
expected to arise from the cosmic-ray current ahead of the shock, and the
impact on the properties of the upstream interstellar medium. We find that the
initial structure and peak strength of the amplified field is somewhat
sensitive to the choice of parameters, but that the field growth saturates in a
similar manner in all cases: the back-reaction on the cosmic rays leads to
modification of their rest-frame distribution and also a net transfer of
momentum to the interstellar medium, substantially weakening their relative
drift while also implying the development of a modified shock. The upstream
medium becomes turbulent, with significant spatial fluctuations in density and
velocity, the latter in particular leading to moderate upstream heating; such
fluctuations will also have a strong influence on the shock structure.Comment: 8 pages, 6 figures, accepted by Ap
Non-relativistic perpendicular shocks modeling young supernova remnants: nonstationary dynamics and particle acceleration at forward and reverse shocks
For parameters that are applicable to the conditions at young supernova
remnants, we present results of 2D3V particle-in-cell simulations of a
non-relativistic plasma shock with a large-scale perpendicular magnetic field
inclined at 45-deg angle to the simulation plane to approximate 3D physics. We
developed an improved clean setup that uses the collision of two plasma slabs
with different density and velocity, leading to the development of two
distinctive shocks and a contact discontinuity. The shock formation is mediated
by Weibel-type filamentation instabilities that generate magnetic turbulence.
Cyclic reformation is observed in both shocks with similar period, for which we
note global variations on account of shock rippling and local variations
arising from turbulent current filaments. The shock rippling occurs on spatial
and temporal scales given by gyro-motions of shock-reflected ions. The drift
motion of electrons and ions is not a gradient drift, but commensurates with E
x B drift. We observe a stable suprathermal tail in the ion spectra, but no
electron acceleration because the amplitude of Buneman modes in the shock foot
is insufficient for trapping relativistic electrons. We see no evidence of
turbulent reconnection. A comparison with other 2D simulation results suggests
that the plasma beta and the ion-to-electron mass ratio are not decisive for
efficient electron acceleration, but pre-acceleration efficacy might be reduced
with respect to the 2D results once three-dimensional effects are fully
accounted for. Other microphysical factors may also be at play to limit the
amplitude of Buneman waves or prevent return of electrons to the foot region.Comment: Astrophysical Journal, in press, some figures with low resolutio
Kinetic simulations of nonrelativistic perpendicular shocks of young supernova remnants. I. Electron shock-surfing acceleration
Electron injection at high Mach-number nonrelativistic perpendicular shocks
is studied here for parameters that are applicable to young SNR shocks. Using
high-resolution large-scale two-dimensional fully kinetic particle-in-cell
(PIC) simulations and tracing individual particles we in detail analyze the
shock surfing acceleration (SSA) of electrons at the leading edge of the shock
foot. The central question is to what degree the process can be captured in
2D3V simulations. We find that the energy gain in SSA always arises from the
electrostatic field of a Buneman wave. Electron energization is more efficient
in the out-of-plane orientation of the large-scale magnetic field because both
the phase speed and the amplitude of the waves are higher than for the in-plane
scenario. Also, a larger number of electrons is trapped by the waves compared
to the in-plane configuration. We conclude that significant modifications of
the simulation parameters are needed to reach the same level of SSA efficiency
as in simulations with out-of-plane magnetic field or 3D simulations
Could Cosmic Rays Affect Instabilities in the Transition Layer of Nonrelativistic Collisionless Shocks?
There is an observational correlation between astrophysical shocks and
non-thermal particle distributions extending to high energies. As a first step
toward investigating the possible feedback of these particles on the shock at
the microscopic level, we perform particle-in-cell (PIC) simulations of a
simplified environment consisting of uniform, interpenetrating plasmas, both
with and without an additional population of cosmic rays. We vary the relative
density of the counterstreaming plasmas, the strength of a homogeneous parallel
magnetic field, and the energy density in cosmic rays. We compare the early
development of the unstable spectrum for selected configurations without cosmic
rays to the growth rates predicted from linear theory, for assurance that the
system is well represented by the PIC technique. Within the parameter space
explored, we do not detect an unambiguous signature of any cosmic-ray-induced
effects on the microscopic instabilities that govern the formation of a shock.
We demonstrate that an overly coarse distribution of energetic particles can
artificially alter the statistical noise that produces the perturbative seeds
of instabilities, and that such effects can be mitigated by increasing the
density of computational particles.Comment: 22 pages, 5 figures, published in Ap
Cosmic-Ray Acceleration at Ultrarelativistic Shock Waves: Effects of Downstream Short-Wave Turbulence
The present paper is the last of a series studying the first-order Fermi
acceleration processes at relativistic shock waves with the method of Monte
Carlo simulations applied to shocks propagating in realistically modeled
turbulent magnetic fields. The model of the background magnetic field structure
of Niemiec & Ostrowski (2004, 2006) has been augmented here by a
large-amplitude short-wave downstream component, imitating that generated by
plasma instabilities at the shock front. Following Niemiec & Ostrowski (2006),
we have considered ultrarelativistic shocks with the mean magnetic field
oriented both oblique and parallel to the shock normal. For both cases
simulations have been performed for different choices of magnetic field
perturbations, represented by various wave power spectra within a wide
wavevector range. The results show that the introduction of the short-wave
component downstream of the shock is not sufficient to produce power-law
particle spectra with the "universal" spectral index 4.2. On the contrary,
concave spectra with cutoffs are preferentially formed, the curvature and
cutoff energy being dependent on the properties of turbulence. Our results
suggest that the electromagnetic emission observed from astrophysical sites
with relativistic jets, e.g. AGN and GRBs, is likely generated by particles
accelerated in processes other than the widely invoked first-order Fermi
mechanism.Comment: 9 pages, 8 figures, submitted to Ap
Electron Acceleration at Rippled Low-Mach-number Shocks in High-beta Collisionless Cosmic Plasmas
Using large-scale fully-kinetic two-dimensional particle-in-cell simulations,
we investigate the effects of shock rippling on electron acceleration at
low-Mach-number shocks propagating in high- plasmas, in application to
merger shocks in galaxy clusters. We find that the electron acceleration rate
increases considerably when the rippling modes appear. The main acceleration
mechanism is stochastic shock-drift acceleration, in which electrons are
confined at the shock by pitch-angle scattering off turbulence and gain energy
from the motional electric field. The presence of multi-scale magnetic
turbulence at the shock transition and the region immediately behind the main
shock overshoot is essential for electron energization. Wide-energy non-thermal
electron distributions are formed both upstream and downstream of the shock.
The maximum energy of the electrons is sufficient for their injection into
diffusive shock acceleration. We show for the first time that the downstream
electron spectrum has a~power-law form with index , in agreement
with observations.Comment: 15 pages, 14 figures, to be published in Ap
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