903 research outputs found
Spectral universality of strong shocks accelerating charged particles
As a rule, the shock compression controls the spectrum of diffusively
accelerated particles. We argue that this is not so if the backreaction of
these particles on the shock structure is significant. We present a
self-similar solution in which the accelerated particles change the flow
structure near the shock so strongly that the total shock compression may
become arbitrarily large. Despite this, the energy spectrum behind the shock is
close to E^{-3/2} independently of anything at all.Comment: Submitted to ApJL, 4 pages, 1 figure, uses revtex and boxedep
Probing Nearby CR Accelerators and ISM Turbulence with Milagro Hot Spots
Both the acceleration of cosmic rays (CR) in supernova remnant shocks and
their subsequent propagation through the random magnetic field of the Galaxy
deem to result in an almost isotropic CR spectrum. Yet the MILAGRO TeV
observatory discovered a sharp ( arrival anisotropy of CR
nuclei. We suggest a mechanism for producing a weak and narrow CR beam which
operates en route to the observer. The key assumption is that CRs are scattered
by a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade
across the local field direction. The strongest pitch-angle scattering occurs
for particles moving almost precisely along the field line. Partly because this
direction is also the direction of minimum of the large scale CR angular
distribution, the enhanced scattering results in a weak but narrow particle
excess. The width, the fractional excess and the maximum momentum of the beam
are calculated from a systematic transport theory depending on a single scale
which can be associated with the longest Alfven wave, efficiently
scattering the beam. The best match to all the three characteristics of the
beam is achieved at pc. The distance to a possible source of the beam
is estimated to be within a few 100pc. Possible approaches to determination of
the scale from the characteristics of the source are discussed. Alternative
scenarios of drawing the beam from the galactic CR background are considered.
The beam related large scale anisotropic CR component is found to be energy
independent which is also consistent with the observations.Comment: 2 figures, ApJ accepted version2 minor changes and correction
Ion-acoustic shocks with reflected ions: modeling and PIC simulations
Non-relativistic collisionless shock waves are widespread in space and
astrophysical plasmas and are known as efficient particle accelerators.
However, our understanding of collisionless shocks, including their structure
and the mechanisms whereby they accelerate particles remains incomplete. We
present here the results of numerical modeling of an ion-acoustic collisionless
shock based on one-dimensional (1D) kinetic approximation both for electrons
and ions with a real mass ratio. Special emphasis is made on the
shock-reflected ions as the main driver of shock dissipation. The reflection
efficiency, velocity distribution of reflected particles and the shock
electrostatic structure are studied in terms of the shock parameters.
Applications to particle acceleration in geophysical and astrophysical shocks
are discussed.Comment: 6 pages, 7 figures, International Workshop "Complex Plasma Phenomena
in the Laboratory and in the Universe", January 19-20, 2015, Rome, Ital
Modern theory of Fermi acceleration: a new challenge to plasma physics
One of the main features of astrophysical shocks is their ability to
accelerate particles to extremely high energies. The leading acceleration
mechanism, the diffusive shock acceleration is reviewed. It is demonstrated
that its efficiency critically depends on the injection of thermal plasma into
acceleration which takes place at the subshock of the collisionless shock
structure that, in turn, can be significantly smoothed by energetic particles.
Furthermore, their inhomogeneous distribution provides free energy for MHD
turbulence regulating the subshock strength and injection rate. Moreover, the
MHD turbulence confines particles to the shock front controlling their maximum
energy and bootstrapping acceleration. Therefore, the study of the MHD
turbulence in a compressive plasma flow near a shock is a key to understanding
of the entire process. The calculation of the injection rate became part of the
collisionless shock theory. It is argued that the further progress in diffusive
shock acceleration theory is impossible without a significant advance in these
two areas of plasma physics.Comment: 12 pages, 4 figures, invited talk at APS/ICPP, Quebec 2000, to appear
in Phys. of Plasma
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