153 research outputs found
Diffusive radiation in Langmuir turbulence produced by jet shocks
Anisotropic distributions of charged particles including two-stream
distributions give rise to generation of either stochastic electric fields (in
the form of Langmuir waves, Buneman instability) or random quasi-static
magnetic fields (Weibel and filamentation instabilities) or both. These
two-stream instabilities are known to play a key role in collisionless shock
formation, shock-shock interactions, and shock-induced electromagnetic
emission. This paper applies the general non-perturbative stochastic theory of
radiation to study electromagnetic emission produced by relativistic particles,
which random walk in the stochastic electric fields of the Langmuir waves. This
analysis takes into account the cumulative effect of uncorrelated Langmuir
waves on the radiating particle trajectory giving rise to angular diffusion of
the particle, which eventually modifies the corresponding radiation spectra. We
demonstrate that the radiative process considered is probably relevant for
emission produced in various kinds of astrophysical jets, in particular, prompt
gamma-ray burst spectra, including X-ray excesses and prompt optical flashes.Comment: 9 pages, 5 figures, MNRAS, accepte
Radio emission from acceleration sites of solar flares
The Letter takes up a question of what radio emission is produced by
electrons at the very acceleration site of a solar flare. Specifically, we
calculate incoherent radio emission produced within two competing acceleration
models--stochastic acceleration by cascading MHD turbulence and regular
acceleration in collapsing magnetic traps. Our analysis clearly demonstrates
that the radio emission from the acceleration sites: (i) has sufficiently
strong intensity to be observed by currently available radio instruments and
(ii) has spectra and light curves which are distinctly different in these two
competing models, which makes them observationally distinguishable. In
particular, we suggest that some of the narrowband microwave and decimeter
continuum bursts may be a signature of the stochastic acceleration in solar
flares.Comment: ApJL, in pres
Cosmic ray diffusive acceleration at shock waves with finite upstream and downstream escape boundaries
In the present paper we discuss the modifications introduced into the
first-order Fermi shock acceleration process due to a finite extent of
diffusive regions near the shock or due to boundary conditions leading to an
increased particle escape upstream and/or downstream the shock. In the
considered simple example of the planar shock wave we idealize the escape
phenomenon by imposing a particle escape boundary at some distance from the
shock. Presence of such a boundary (or boundaries) leads to coupled steepening
of the accelerated particle spectrum and decreasing of the acceleration time
scale. It allows for a semi-quantitative evaluation and, in some specific
cases, also for modelling of the observed steep particle spectra as a result of
the first-order Fermi shock acceleration. We also note that the particles close
to the upper energy cut-off are younger than the estimate based on the
respective acceleration time scale. In Appendix A we present a new
time-dependent solution for infinite diffusive regions near the shock allowing
for different constant diffusion coefficients upstream and downstream the
shock.Comment: LaTeX, 14 pages, 4 postscript figures; Solar Physics (accepted
Diffusive Radiation in One-dimensional Langmuir Turbulence
We calculate spectra of radiation produced by a relativistic particle in the
presence of one-dimensional Langmuir turbulence which might be generated by a
streaming instability in the plasma, in particular, in the shock front or at
the shock-shock interactions. The shape of the radiation spectra is shown to
depend sensitively on the angle between the particle velocity and electric
field direction. The radiation spectrum in the case of exactly transverse
particle motion is degenerate and similar to that of spatially uniform Langmuir
oscillations. In case of oblique propagation, the spectrum is more complex, it
consists of a number of power-law regions and may contain a distinct
high-frequency spectral peak. %at \omega=2\omega\pe \gamma^2. The emission
process considered is relevant to various laboratory plasma settings and for
astrophysical objects as gamma-ray bursts and collimated jets.Comment: 4 pages, 1 figure, accepted for Phys. Rev.
Kinetic approaches to particle acceleration at cosmic ray modified shocks
Kinetic approaches provide an effective description of the process of
particle acceleration at shock fronts and allow to take into account the
dynamical reaction of the accelerated particles as well as the amplification of
the turbulent magnetic field as due to streaming instability. The latter does
in turn affect the maximum achievable momentum and thereby the acceleration
process itself, in a chain of causality which is typical of non-linear systems.
Here we provide a technical description of two of these kinetic approaches and
show that they basically lead to the same conclusions. In particular we discuss
the effects of shock modification on the spectral shape of the accelerated
particles, on the maximum momentum, on the thermodynamic properties of the
background fluid and on the escaping and advected fluxes of accelerated
particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA
GRB spectral parameters within the fireball model
Fireball model of the GRBs predicts generation of numerous internal shocks,
which then efficiently accelerate charged particles and generate magnetic and
electric fields. These fields are produced in the form of relatively
small-scale stochastic ensembles of waves, thus, the accelerated particles
diffuse in space due to interaction with the random waves and so emit so called
Diffusive Synchrotron Radiation (DSR) in contrast to standard synchrotron
radiation they would produce in a large-scale regular magnetic fields. In this
paper we present first results of comprehensive modeling of the GRB spectral
parameters within the fireball/internal shock concept. We have found that the
non-perturbative DSR emission mechanism in a strong random magnetic field is
consistent with observed distributions of the Band parameters and also with
cross-correlations between them; this analysis allowed to restrict GRB physical
parameters from the requirement of consistency between the model and observed
distributions.Comment: 14 pages, 17 figures, MNRAS in pres
Diffusive Synchrotron Radiation from Pulsar Wind Nebulae
Diffusive Synchrotron Radiation (DSR) is produced by charged particles as
they random walk in a stochastic magnetic field. The spectrum of the radiation
produced by particles in such fields differs substantially from those of
standard synchrotron emission because the corresponding particle trajectories
deviate significantly from gyration in a regular field. The Larmor radius,
therefore, is no longer a good measure of the particle trajectory. In this
paper we analyze a special DSR regime which arises as highly relativistic
electrons move through magnetic fields which have only random structure on a
wide range of spatial scales. Such stochastic fields arise in turbulent
processes, and are likely present in pulsar wind nebulae (PWNe). We show that
DSR generated by a single population of electrons can reproduce the observed
broad-band spectra of PWNe from the radio to the X-ray, in particular producing
relatively flat spectrum radio emission as is usually observed in PWNe. DSR can
explain the existence of several break frequencies in the broad-band emission
spectrum without recourse to breaks in the energy spectrum of the relativistic
particles. The shape of the radiation spectrum depends on the spatial spectrum
of the stochastic magnetic field. The implications of the presented DSR regime
for PWN physics are discussed.Comment: 15 pages, 5 figures, accepted to MNRA
The generation of low-energy cosmic rays in molecular clouds
It is argued that if cosmic rays penetrate into molecular clouds, the total
energy they lose can exceed the energy from galactic supernovae shocks. It is
shown that most likely galactic cosmic rays interacting with the surface layers
of molecular clouds are efficiently reflected and do not penetrate into the
cloud interior. Low-energy cosmic rays ( GeV) that provide the primary
ionization of the molecular cloud gas can be generated inside such clouds by
multiple shocks arising due to supersonic turbulence.Comment: 11 pages, no figure
On the Conductivity of a Magnetoactive Turbulent Plasma
The problem of determining the effective conductivity tensor of a
magnetoactive turbulent plasma is considered in the approximation of isolated
particles. Additional gyrotropicterms are shown to appear in the conductivity
tensor in the presence of mean, nonzero magnetic helicity. The dispersion of
propagating electro- magnetic waves changes, additional modes and additional
rotation of the polarization plane appear, and the waves can be amplified. The
properties acquired by plasma with helicity are similar those observed in
chiral and bianisotropic electrodynamic media.Comment: 15 page
- …