210 research outputs found
Full particle simulation of a perpendicular collisionless shock: A shock-rest-frame model
The full kinetic dynamics of a perpendicular collisionless shock is studied
by means of a one-dimensional electromagnetic full particle simulation. The
present simulation domain is taken in the shock rest frame in contrast to the
previous full particle simulations of shocks. Preliminary results show that the
downstream state falls into a unique cyclic reformation state for a given set
of upstream parameters through the self-consistent kinetic processes.Comment: 4 pages, 2 figures, published in "Earth, Planets and Space" (EPS),
the paper with full resolution images is
http://theo.phys.sci.hiroshima-u.ac.jp/~ryo/papers/shock_rest.pd
Efficiency of Nonlinear Particle Acceleration at Cosmic Structure Shocks
We have calculated the evolution of cosmic ray (CR) modified astrophysical
shocks for a wide range of shock Mach numbers and shock speeds through
numerical simulations of diffusive shock acceleration (DSA) in 1D quasi-
parallel plane shocks. The simulations include thermal leakage injection of
seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion
is assumed. We model shocks similar to those expected around cosmic structure
pancakes as well as other accretion shocks driven by flows with upstream gas
temperatures in the range K and shock Mach numbers spanning
. We show that CR modified shocks evolve to time-asymptotic states
by the time injected particles are accelerated to moderately relativistic
energies (p/mc \gsim 1), and that two shocks with the same Mach number, but
with different shock speeds, evolve qualitatively similarly when the results
are presented in terms of a characteristic diffusion length and diffusion time.
For these models the time asymptotic value for the CR acceleration efficiency
is controlled mainly by shock Mach number. The modeled high Mach number shocks
all evolve towards efficiencies %, regardless of the upstream CR
pressure. On the other hand, the upstream CR pressure increases the overall CR
energy in moderate strength shocks (). (abridged)Comment: 23 pages, 12 ps figures, accepted for Astrophysical Journal (Feb. 10,
2005
Ion acceleration processes at reforming collisionless shocks
The identification of pre-acceleration mechanisms for cosmic ray ions in
supernova remnant shocks is an important problem in astrophysics. Recent
particle-in-cell (PIC) shock simulations have shown that inclusion of the full
electron kinetics yields non-time-stationary solutions, in contrast to previous
hybrid (kinetic ions, fluid electrons) simulations. Here, by running a PIC code
at high phase space resolution, ion acceleration mechanisms associated with the
time dependence of a supercritical collisionless perpendicular shock are
examined. In particular the components of
are analysed along trajectories for ions that reach both high and low energies.
Selection mechanisms for the ions that reach high energies are also examined.
In contrast to quasi-stationary shock solutions, the suprathermal protons are
selected from the background population on the basis of the time at which they
arrive at the shock, and thus are generated in bursts.Comment: 12 Pages, 7 Figures, To be published in Phys. Plasma
Analytic solution for nonlinear shock acceleration in the Bohm limit
The selfconsistent steady state solution for a strong shock, significantly
modified by accelerated particles is obtained on the level of a kinetic
description, assuming Bohm-type diffusion. The original problem that is
commonly formulated in terms of the diffusion-convection equation for the
distribution function of energetic particles, coupled with the thermal plasma
through the momentum flux continuity equation, is reduced to a nonlinear
integral equation in one variable. Its solution provides selfconsistently both
the particle spectrum and the structure of the hydrodynamic flow. A critical
system parameter governing the acceleration process is found to be , where , with a suitably
normalized injection rate , the Mach number M >> 1, and the cut-off
momentum . We particularly focus on an efficient solution, in which
almost all the energy of the flow is converted into a few energetic particles.
It was found that (i) for this efficient solution (or, equivalently, for
multiple solutions) to exist, the parameter
must exceed a critical value ( is the injection
momentum), (ii) the total shock compression ratio r increases with M and
saturates at a level that scales as $ r \propto \Lambda_1 (iii) the downstream
power-law spectrum has the universal index q=3.5 over a broad momentum range.
(iv) completely smooth shock transitions do not appear in the steady state
kinetic description.Comment: 39 pages, 3 PostScript figures, uses aasms4.sty, to appear in Aug.
20, 1997 issue ApJ, vol. 48
Cosmic Ray Electrons in Groups and Clusters of Galaxies: Primary and Secondary Populations from a Numerical Cosmological Simulation
We study the generation and distribution of high energy electrons in cosmic
environment and their observational consequences by carrying out the first
cosmological simulation that includes directly cosmic ray (CR) particles.
Starting from cosmological initial conditions we follow the evolution of
primary and secondary electrons (CRE), CR ions (CRI) and a passive magnetic
field. CRIs and primary CREs are injected and accelerated at large scale
structure shocks. Secondary CREs are continuously generated through inelastic
p-p collisions. We include spatial transport, adiabatic expansion/compression,
Coulomb collisions, bremsstrahlung, synchrotron (SE)and inverse Compton (IC)
emission. We find that, from the perspective of cosmic shock energy and
acceleration efficiency, the few detections of hard X-ray radiation excess
could be explained in the framework of IC emission of primary CREs in clusters
undergoing high accretion/merger phase. Instead, IC emission from both primary
and secondary CREs accounts at most for a small fraction of the radiation
excesses detected in the extreme-UV (except for the Coma cluster as reported by
Bowyer et al.1999). Next, we calculate the SE after normalizing the magnetic
field so that for a Coma-like cluster ^1/2~3 \muG. Our results indicate
that the SE from secondary CREs reproduces several general properties of radio
halos, including the recently found P_1.4GHz vs T relation, the morphology and
polarization of the emitting region and, to some extent, the spectral index.
Moreover, SE from primary CREs turns out sufficient to power extended regions
resembling radio relics observed at the outskirts of clusters. Again we find
striking resemblance between morphology, polarization and spectral index of our
synthetic maps and those reported in the literature.Comment: emulateapj, 27 pages, 10 figures, 5 tables; ApJ in pres
Numerical Studies of Cosmic Ray Injection and Acceleration
A numerical scheme that incorporates a thermal leakage injection model into a
combined gas dynamics and cosmic ray (CR, hereafter) diffusion-convection code
has been developed. The particle injection is followed numerically by filtering
the diffusive flux of suprathermal particles across the shock to the upstream
region according to a velocity-dependent transparency function that controls
the fraction of leaking particles. We have studied CR injection and
acceleration efficiencies during the evolution of CR modified planar shocks for
a wide range of initial shock Mach numbers, , assuming a Bohm-like
diffusion coefficient. The injection process is very efficient when the
subshock is strong, leading to fast and significant modification of the shock
structure. As the CR pressure increases, the subshock weakens and the injection
rate decreases accordingly, so that the subshock does not disappear. Although
some fraction of the particles injected early in the evolution continue to be
accelerated to ever higher energies, the postshock CR pressure reaches an
approximate time-asymptotic value due to a balance between fresh
injection/acceleration and advection/diffusion of the CR particles away from
the shock. We conclude that the injection rates in strong parallel shocks are
sufficient to lead to rapid nonlinear modifications to the shock structures and
that self-consistent injection and time-dependent simulations are crucial to
understanding the non-linear evolution of CR modified shocks.Comment: 28 pages, To appear in ApJ November 1, 2002 issu
Nonthermal Electrons at High Mach Number Shocks: Electron Shock Surfing Acceleration
We study the suprathermal electron acceleration mechanism in a perpendicular
magnetosonic shock wave in a high Mach number regime by using a
particle-in-cell simulation. We find that shock surfing/surftron acceleration
producing the suprathermal electrons occurs in the shock transition region
where a series of large amplitude electrostatic solitary waves (ESWs) are
excited by Buneman instability under the interaction between the reflected ions
and the incoming electrons. It is shown that the electrons are likely to be
trapped by ESWs, and during the trapping phase they can be effectively
accelerated by the shock motional/convection electric field. We discuss that
suprathermal electrons can be accelerated up to , where is the ion rest mass energy and is the shock upstream flow velocity.
Furthermore, some of these suprathermal electrons may be effectively trapped
for infinitely long time when Alfv\'en Mach number exceeds several 10,
and they are accelerated up to the shock potential energy determined by the
global shock size.Comment: 21 pages, 6 figure
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Investigation of laminar shocks
Studies of laminar shocks based on ISEE observations of terrestrial bow shocks, simulation, and theory are presented
Cosmic Ray Protons Accelerated at Cosmological Shocks and Their Impact on Groups and Clusters of Galaxies
We investigate the production of cosmic ray (CR) protons at cosmological
shocks by performing, for the first time, numerical simulations of large scale
structure formation that include directly the acceleration, transport and
energy losses of the high energy particles. CRs are injected at shocks
according to the thermal leakage model and, thereafter, accelerated to a
power-law distribution as indicated by the test particle limit of the diffusive
shock acceleration theory. The evolution of the CR protons accounts for losses
due to adiabatic expansion/compression, Coulomb collisions and inelastic p-p
scattering. Our results suggest that CR protons produced at shocks formed in
association with the process of large scale structure formation could amount to
a substantial fraction of the total pressure in the intra-cluster medium. Their
presence should be easily revealed by GLAST through detection of gamma-ray flux
from the decay of neutral pions produced in inelastic p-p collisions of such CR
protons with nuclei of the intra-cluster gas. This measurement will allow a
direct determination of the CR pressure contribution in the intra-cluster
medium. We also find that the spatial distribution of CR is typically more
irregular than that of the thermal gas because it is more influenced by the
underlying distribution of shocks. This feature is reflected in the appearance
of our gamma-ray synthetic images. Finally, the average CR pressure
distribution appears statistically slightly more extended than the thermal
pressure.Comment: 16 pages, 7 figures, ApJ in pres
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