107 research outputs found
Particle Acceleration at High- Shock Waves
First-order Fermi acceleration processes at ultrarelativistic shocks are
studied with Monte Carlo simulations. The accelerated particle spectra are
obtained by integrating the exact particle trajectories in a turbulent magnetic
field near the shock, with a few ``realistic'' features of the field structure
included. We show that the main acceleration process at oblique shocks is the
particle compression at the shock. Formation of energetic spectral tails is
possible in a limited energy range for highly perturbed magnetic fields.
Cut-offs in the spectra occur at low energies in the resonance range
considered. We relate this feature to the structure of the magnetic field
downstream of the shock, where field compression produces effectively 2D
turbulence in which cross-field diffusion is very small. Because of the field
compression downstream, the acceleration process is inefficient also in
parallel high- shocks for larger turbulence amplitudes, and features
observed in oblique shocks are recovered. For small-amplitude perturbations,
particle spectra are formed in a wide energy range and modifications of the
acceleration process due to the existence of long-wave perturbations are
observed. The critical turbulence amplitude for efficient acceleration at
parallel shocks decreases with shock Lorentz factor. We also study the
influence of strong short-wave perturbations downstream of the shock on the
particle acceleration processes. The spectral indices obtained do not converge
to the ``universal'' value . Our results indicate inefficiency of the
first-order Fermi process to generate high-energy cosmic rays at
ultrarelativistic shocks with the considered perturbed magnetic field
structures.Comment: 4 pages, 2 figures, proceedings of the conference "Astrophysical
Sources of High Energy Particles and Radiation" held in Torun, Poland (June
20-24, 2005), to appear in the AIP Proceedings Serie
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
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