628 research outputs found
Is the Weibel instability enhanced by the suprathermal populations, or not?
The kinetic instabilities of the Weibel-type are presently invoked in a large
variety of astrophysical scenarios because anisotropic plasma structures are
ubiquitous in space. The Weibel instability is driven by a temperature
anisotropy which is commonly modeled by a bi-axis distribution function, such
as a bi-Maxwellian or a generalized bi-Kappa. Previous studies have been
limited to a bi-Kappa distribution and found a suppression of this instability
in the presence of suprathermal tails. In the present paper it is shown that
the Weibel growth rate is rather more sensitive to the shape of the anisotropic
distribution function. In order to illustrate the distinguishing properties of
this instability a \emph{product-bi-Kappa distribution} is introduced, with the
advantage that this distribution function enables the use of different values
of the spectral index in the two directions, . The growth rates and the instability threshold are derived and
contrasted with those for a simple bi-Kappa and a bi-Maxwellian. Thus, while
the maximum growth rates reached at the saturation are found to be higher, the
threshold is drastically reduced making the anisotropic product-bi-Kappa (with
small kappas) highly susceptible to the Weibel instability. This effect could
also rise questions on the temperature or the temperature anisotropy that seems
to be not an exclusive source of free energy for this instability, and
definition of these notions for such Kappa distributions must probably be
reconsidered
Scattering of Cosmic Rays by Magnetohydrodynamic Interstellar Turbulence
Recent advances in understanding of magnetohydrodynamic (MHD) turbulence call
for substantial revisions in our understanding of cosmic ray transport. In this
paper we use gyroresonance recently obtained scaling laws for MHD modes to
calculate the scattering frequency for cosmic rays in the ISM. We consider
gyroresonance with MHD modes (Alfvenic, slow and fast) and transit-time damping
(TTD) by fast modes. We conclude that the gyroresonance with fast modes is the
dominant contribution to cosmic ray scattering for the typical interstellar
conditions. In contrast to earlier studies, we find that Alfvenic and slow
modes are inefficient because they are far from isotropy usually assumed.Comment: 4 pages, 2 figures, Phys. Rev. Lett. in press, minor change
Interplay of Kinetic Plasma Instabilities
<a href="http://www.intechopen.com/books/wave-propagation-in-materials-for-modern-applications/interplay-of-kinetic-plasma-instabilities" title="interplay-of-kinetic-plasma-instabilities">Interplay of Kinetic Plasma Instabilities</a>status: publishe
Limits on the AGN activities in X-ray underluminous galaxy groups
We have observed four X-ray underluminous groups of galaxies using the Giant
Meterwave RadioTelescope. The groups NGC 524, 720, 3607, and 4697 are
underluminous in relation to the extrapolation of the Lx - T relation from rich
clusters and do not show any evidence of current AGN activities that can
account for such a departure. The GMRT observations carried out at low
frequencies (235 and 610 MHz) were aimed at detecting low surface brightness,
steep-spectrum sources indicative of past AGN activities in these groups. No
such radio emissions were detected in any of these four groups. The
corresponding upper limits on the total energy in relativistic particles is
about 3 X 10 erg. This value is more than a factor of 100 less than that
required to account for the decreased X-ray luminosities (or, enhanced
entropies) of these four groups in the AGN-heating scenario. Alternatively, the
AGN activity must have ceased about 4 Gyr ago, allowing the relativistic
particles to diffuse out to such a large extent (about 250 kpc) that their
radio emission could have been undetected by the current observations. If the
latter scenario is correct, the ICM was pre-heated before the assembly of
galaxy clusters.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter
Non-linear Weibel-type Soliton Modes
Discussion is given of non-linear soliton behavior including coupling between
electrostatic and electromagnetic potentials for non-relativistic, weakly
relativistic, and fully relativistic plasmas. For plasma distribution functions
that are independent of the canonical momenta perpendicular to the soliton
spatial structure direction there are, in fact, no soliton behaviors allowed
because transverse currents are zero. Dependence on the transverse canonical
momenta is necessary. When such is the case, it is shown that the presence or
absence of a soliton is intimately connected to the functional form assumed for
the particle distribution functions. Except for simple situations, the coupled
non-linear equations for the electrostatic and electromagnetic potentials would
seem to require numerical solution procedures. Examples are given to illustrate
all of these points for non-relativistic, weakly relativistic, and fully
relativistic plasmas.Comment: Accepted for publication at Journal of Physics A: Mathematical and
Theoretica
Cosmic magnetization: from spontaneously emitted aperiodic turbulent to ordered equipartition fields
It is shown that an unmagnetized nonrelativistic thermal electron-proton
plasma spontaneously emits aperiodic turbulent magnetic field fluctuations of
strength G, where is the
normalized thermal electron temperature, the thermal plasma energy
density and the plasma parameter. Aperiodic modes fluctuate only in space,
but are not propagating. For the unmagnetized intergalactic medium, immediately
after the reionization onset, the field strength from this mechanism is about
G, too weak to affect the dynamics of the plasma. The shear
and/or compression of the intergalactic medium exerted by the first supernova
explosions amplify these seed fields and make them anisotropic, until the
magnetic restoring forces affect the gas dynamics at ordered plasma betas near
unity.Comment: 4 pages, 1 figur
On the Momentum Diffusion of Radiating Ultrarelativistic Electrons in a Turbulent Magnetic Field
Here we investigate some aspects of stochastic acceleration of
ultrarelativistic electrons by magnetic turbulence. In particular, we discuss
the steady-state energy spectra of particles undergoing momentum diffusion due
to resonant interactions with turbulent MHD modes, taking rigorously into
account direct energy losses connected with different radiative cooling
processes. For the magnetic turbulence we assume a given power spectrum of the
type . In contrast to the previous approaches, however, we
assume a finite range of turbulent wavevectors , consider a variety of
turbulence spectral indexes , and concentrate on the case of a
very inefficient particle escape from the acceleration site. We find that for
different cooling and injection conditions, stochastic acceleration processes
tend to establish a modified ultrarelativistic Maxwellian distribution of
radiating particles, with the high-energy exponential cut-off shaped by the
interplay between cooling and acceleration rates. For example, if the timescale
for the dominant radiative process scales with the electron momentum as
, the resulting electron energy distribution is of the form
, where , and
is the equilibrium momentum defined by the balance between stochastic
acceleration and energy losses timescales. We also discuss in more detail the
synchrotron and inverse-Compton emission spectra produced by such an electron
energy distribution, taking into account Klein-Nishina effects. We point out
that the curvature of the high frequency segments of these spectra, even though
being produced by the same population of electrons, may be substantially
different between the synchrotron and inverse-Compton components.Comment: 42 pages, 14 figures included. Slightly modified version, accepted
for publication in Ap
Some properties of synchrotron radio and inverse-Compton gamma-ray images of supernova remnants
The synchrotron radio maps of supernova remnants (SNRs) in uniform
interstellar medium and interstellar magnetic field (ISMF) are analyzed,
allowing different `sensitivity' of injection efficiency to the shock
obliquity. The very-high energy gamma-ray maps due to inverse Compton process
are also synthesized. The properties of images in these different wavelength
bands are compared, with particular emphasis on the location of the bright
limbs in bilateral SNRs. Recent H.E.S.S. observations of SN 1006 show that the
radio and IC gamma-ray limbs coincide, and we found that this may happen if: i)
injection is isotropic but the variation of the maximum energy of electrons is
rather quick to compensate for differences in magnetic field; ii) obliquity
dependence of injection (either quasi-parallel or quasi-perpendicular) and the
electron maximum energy is strong enough to dominate magnetic field variation.
In the latter case, the obliquity dependence of the injection and the maximum
energy should not be opposite. We argue that the position of the limbs alone
and even their coincidence in radio, X-rays and gamma-rays, as it is discovered
by H.E.S.S. in SN 1006, cannot be conclusive about the dependence of the
electron injection efficiency, the compression/amplification of ISMF and the
electron maximum energy on the obliquity angle.Comment: Accepted for publication in MNRA
Nonthermal Emissions from Particles Accelerated by Turbulence in Clusters of Galaxies
We consider nonthermal emission from clusters of galaxies produced by
particle acceleration by resonant scattering of Alfv\'{e}n waves driven by
fluid turbulence through the Lighthill mechanism in the intracluster medium. We
assume that the turbulence is driven by cluster mergers. We find that the
resonant Alfv\'{e}n waves can accelerate electrons up to
\gamma ~10^5 through resonant scattering. We also find that the turbulent
resonant acceleration can give enough energy to electrons to produce the
observed diffuse radio relic emission from clusters if the clusters have a pool
of electrons with \gamma ~10^3.
This mechanism can also explain the observed hard X-ray emission from
clusters if the magnetic field in a cluster is small enough (~<
\mu G) or the fluid turbulence spectrum is flatter than the Kolmogorov law.
The fluid turbulence could be observed with Astro-E2 in the regions where
diffuse radio emission is observed. Although non-gravitational heating before
cluster formation (preheating) steepens a relation between radio luminosity and
X-ray temperature, our predicted relation is still flatter than the observed
one.Comment: 33 pages, accepted for publication on Ap
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