10,690 research outputs found
Dynamics of a 1-D model for the emergence of the plasma edge shear flow layer with momentum conserving Reynolds stress
A one-dimensional version of the second-order transition model based on the
sheared flow amplification by Reynolds stress and turbulence supression by
shearing is presented. The model discussed in this paper includes a form of the
Reynolds stress which explicitly conserves momentum. A linear stability
analysis of the critical point is performed. Then, it is shown that the
dynamics of weakly unstable states is determined by a reduced equation for the
shear flow. In the case in which the flow damping term is diffusive, the
stationary solutions are those of the real Ginzburg-Landau equation.Comment: 21 pages, 8 figure
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
Reader-Writer Exclusion Supporting Upgrade and Downgrade with Reader-Priority
The Reader-Writer Exclusion problem seeks to provide a lock that protects some critical section of code for two classes of processes, readers and writers, where multiple readers are permitted to hold the lock at a time, but only one writer can hold the lock to the exclusion of all other processes. The difficulties in solving this problem lie not only in developing a good algorithm, but in rigorously formulating desirable properties for such an algorithm to have. Recently, Bhatt and Jayanti accomplished both of these tasks for several variants of the Reader-Writer Exclusion problem. We seek to extend their work by augmenting one of their algorithms (the one giving readers priority over writers) with the notions of upgrading and downgrading. We augment the algorithm by allowing processes in the critical section that are only permitted to read to attempt to acquire permission to write by upgrading, and by allowing processes that are permitted to write to relinquish their permission to write--but still remain in the critical section as readers--by downgrading
Fluid Models for Kinetic Effects on Coherent Nonlinear Alfven Waves. II. Numerical Solutions
The influence of various kinetic effects (e.g. Landau damping, diffusive and
collisional dissipation, and finite Larmor radius terms) on the nonlinear
evolution of finite amplitude Alfvenic wave trains in a finite-beta environment
is systematically investigated using a novel, kinetic nonlinear Schrodinger
(KNLS) equation. The dynamics of Alfven waves is sensitive to the sense of
polarization as well as the angle of propagation with respect to the ambient
magnetic field. Numerical solution for the case with Landau damping reveals the
formation of dissipative structures, which are quasi-stationary, S-polarized
directional (and rotational) discontinuities which self-organize from parallel
propagating, linearly polarized waves. Parallel propagating circularly
polarized packets evolve to a few circularly polarized Alfven harmonics on
large scales. Stationary arc-polarized rotational discontinuities form from
obliquely propagating waves. Collisional dissipation, even if weak, introduces
enhanced wave damping when beta is very close to unity. Cyclotron motion
effects on resonant particle interactions introduce cyclotron resonance into
the nonlinear Alfven wave dynamics.Comment: 38 pages (including 23 figures and 1 table
Hadronic Gamma Rays from Supernova Remnants
A gas cloud near a supernova remnant (SNR) provides a target for
pp-collisions leading to subsequent gamma-ray emission through neutral pion
decay. The assumption of a power-law ambient spectrum of accelerated particles
with index near -2 is usually built into models predicting the spectra of
very-high energy (VHE) gamma-ray emission from SNRs. However, if the gas cloud
is located at some distance from the SNR shock, this assumption is not
necessarily correct. In this case, the particles which interact with the cloud
are those leaking from the shock and their spectrum is approximately
monoenergetic with the injection energy gradually decreasing as the SNR ages.
In the GLAST energy range the gamma-ray spectrum resulting from particle
interactions with the gas cloud will be flatter than expected, with the cutoff
defined by the pion momentum distribution in the laboratory frame. We evaluate
the flux of particles escaping from a SNR shock and apply the results to the
VHE diffuse emission detected by the HESS at the Galactic centre.Comment: 4 pages, 3 figures. Contribution to the 30th ICRC, Merida, Mexico,
2007 (final version
Fast Zonal Field Dynamo in Collisionless Kinetic Alfven Wave Turbulence
The possibility of fast dynamo action by collisionless kinetic Alfven Wave
turbulence is demonstrated. The irreversibility necessary to lock in the
generated field is provided by electron Landau damping, so the induced electric
field does not vanish with resistivity. Mechanisms for self-regulation of the
system and the relation of these results to the theory of alpha quenching are
discussed. The dynamo-generated fields have symmetry like to that of zonal
flows, and thus are termed zonal fields
Influence of zonal flows on unstable drift modes in ETG turbulence
The linear instability of the electron temperature gradient (ETG) driven
modes in the presence of zonal flows is investigated. Random and deterministic
- like profiles of the zonal flow are considered. It is shown that the
presence of shearing by zonal flows can stabilize the linear instability of ETG
drift modes
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