612 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
On the Structure and Scale of Cosmic Ray Modified Shocks
Strong astrophysical shocks, diffusively accelerating cosmic rays (CR) ought
to develop CR precursors. The length of such precursor is believed to
be set by the ratio of the CR mean free path to the shock speed,
i.e., , which is formally
independent of the CR pressure . However, the X-ray observations of
supernova remnant shocks suggest that the precursor scale may be significantly
shorter than which would question the above estimate unless the
magnetic field is strongly amplified and the gyroradius is strongly
reduced over a short (unresolved) spatial scale. We argue that while the CR
pressure builds up ahead of the shock, the acceleration enters into a strongly
nonlinear phase in which an acoustic instability, driven by the CR pressure
gradient, dominates other instabilities (at least in the case of low
plasma). In this regime the precursor steepens into a strongly nonlinear front
whose size scales with \emph{the CR pressure}as , where is the scale of
the developed acoustic turbulence, and is the ratio of CR to gas
pressure. Since , the precursor scale reduction may be strong
in the case of even a moderate gas heating by the CRs through the acoustic and
(possibly also) the other instabilities driven by the CRs.Comment: EPS 2010 paper, to appear in PPC
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
A critical Mach number for electron injection in collisionless shocks
Electron acceleration in collisionless shocks with arbitrary magnetic field
orientations is discussed. It is shown that the injection of thermal electrons
into diffusive shock acceleration process is achieved by an electron beam with
a loss-cone in velocity space that is reflected back upstream from the shock
through shock drift acceleration mechanism. The electron beam is able to excite
whistler waves which can scatter the energetic electrons themselves when the
Alfven Mach number of the shock is sufficiently high. A critical Mach number
for the electron injection is obtained as a function of upstream parameters.
The application to supernova remnant shocks is discussed.Comment: 4 pages, 2 figure, accepted for publication in Physical Review
Letter
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