arXiv subjects

M. A. Malkov

Publications and source records attributed to M. A. Malkov.

29 records · Page 2Linked to original sources

arXiv2005

Generation of Mesoscale Magnetic Fields and the Dynamics of Cosmic Ray Acceleration

The problem of the cosmic ray origin is discussed in connection with their acceleration in supernova remnant shocks. The diffusive shock acceleration mechanism is reviewed and its potential to accelerate particles to the maximum energy of (presumably) galactic cosmic rays ($10^{18}eV $) is considered. It is argued that to reach such energies, a strong magnetic field at scales larger than the particle gyroradius must be created as a result of the acceleration process, itself. One specific mechanism suggested here is based on the generation of Alfven wave at the gyroradius scale with a subsequent transfer to longer scales via interaction with strong acoustic turbulence in the shock precursor. The acoustic turbulence in turn, may be generated by Drury instability or by parametric instability of the Alfven waves.

astro-ph↗

arXiv2005

On the gamma-ray spectra radiated by protons accelerated in SNR shocks near molecular clouds: The case of SNR RX J1713.7-3946

Cosmic rays (CRs) are thought to be accelerated in SNRs. The most favorable situation for proving that the main, hadronic CR component is accelerated there is when CRs interact with dense gases, such as molecular clouds (MC) which surround the SN shock. Here, a new mechanism of spectrum formation in partially ionized gases near SNRs is proposed. Using an analytic model of nonlinear diffusive shock acceleration, we calculate the spectra of protons and estimate the resulting $γ$-ray emission occurring when the SNR shock approaches a MC. We show that the spectrum develops a break in the TeV range and that its GeV component is suppressed. These modifications to the standard theory occur because of the proximity of the partially ionized MC-gas and because of the physics of particle and Alfven wave propagation inside the gas. Possible applications of the new spectra to the recent CANGAROO and HESS observations of the SNR RX J1713.7-3946 are discussed.

astro-ph↗

arXiv2001

On the possible reason for non-detection of TeV-protons in SNRs

The theory of shock acceleration predicts the maximum particle energy to be limited only by the acceleration time and the size (geometry) of the shock. This led to optimistic estimates for the galactic cosmic ray energy achievable in the SNR shocks. The estimates imply that the accelerated particles, while making no strong impact on the shock structure (test particle approach) are nevertheless scattered by strong self-generated Alfven waves (turbulent boost) needed to accelerate them quickly. We demonstrate that these two assumptions are in conflict when applied to SNRs of the age required for cosmic ray acceleration to the ``knee'' energy. We study the combined effect of acceleration nonlinearity (shock modification by accelerated particles) and wave generation on the acceleration process. We show that the refraction of self-generated waves resulting from the deceleration of the plasma flow by the pressure of energetic particles causes enhanced losses of these particles. This effect slows down the acceleration and changes the shape of particle spectrum near the cut-off. The implications for observations of TeV emission from SNR remnants are also discussed.

astro-ph↗

arXiv2001

Intrinsic energy cut-off in diffusive shock acceleration: possible reason for non-detection of TeV-protons in SNRs

The linear theory of shock acceleration predicts the maximum particle energy to be limited only by the acceleration time and the size of the shock. We study the combined effect of acceleration nonlinearity (shock modification by accelerated particles, that must be present in strong astrophysical shocks) and propagation of Alfven waves that are responsible for particle confinement to the shock front. We show that wave refraction to larger wave numbers in the nonlinearly modified flow causes enhanced losses of particles in the momentum range p_max/R < p < p_max, where R>1 is the nonlinear pre-compression of the flow and p_max is a conventional maximum momentum, that could be reached if there was no refraction.

astro-ph↗

arXiv2001

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.

astro-ph↗

arXiv2000

Critical self-organization of astrophysical shocks

There are two distinct regimes of the first order Fermi acceleration at shocks. The first is a linear (test particle) regime in which most of the shock energy goes into thermal and bulk motion of the plasma. The second is an efficient regime when it goes into accelerated particles. Although the transition region between them is narrow, we identify the factors that drive the system to a {\it self-organized critical state} between those two. Using an analytic solution, we determine this critical state and calculate the spectra and maximum energy of accelerated particles.

astro-ph↗

arXiv1998

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.

astro-ph↗

arXiv1998

Ion leakage from quasiparallel collisionless shocks: implications for injection and shock dissipation

A simplified model of particle transport at a quasiparallel one-dimensional collisionless shock is suggested. In this model the MHD-turbulence behind the shock is dominated by a circularly polarized, large amplitude Alfvén wave originated upstream from the turbulence excited by particles leaking from the downstream medium. It is argued that such a wave having significantly increased its magnetic field during the transmission through the shock interface can effectively trap thermal ions, regulating their leakage upstream. Together with a background turbulence this wave also plays a fundamental role in thermalization of the incoming ion flow. The spectrum of leaking particles and the amplitude of the wave excited by these particles are selfconsistently calculated. The injection rate into the first order Fermi acceleration based on this leakage mechanism is obtained and compared with computer simulations. The related problem of shock energy distribution between thermal and nonthermal components of the shocked plasma is discussed. The chemical composition of the leaking particles is studied.

astro-ph↗

arXiv1997

Bifurcation, efficiency, and the role of injection in shock acceleration with the Bohm diffusion

The efficiency and uniqueness of the diffusive shock acceleration is studied on the basis of the novel kinetic solutions. These solutions obtained earlier (paper I, astro-ph/9707152) selfconsistently describe a strong coupling of cosmic rays with the gas flow. They show that the dependence of the acceleration efficiency upon physical parameters is critical in nature. In this paper we investigate a steady acceleration in the parameter space formed by the injection rate (ν), the upper cut-off momentum p_1 and the Mach number M while the flow compression R serves as an order parameter. We determine a manifold of all possible solutions in this parameter space.

astro-ph↗

arXiv1997

Analytic solution for nonlinear shock acceleration in the Bohm limit

The selfconsistent steady state solution for a strong shock, significantly modified by accelerated particles is obtained on the level of a kinetic description, assuming Bohm-type diffusion. The original problem that is commonly formulated in terms of the diffusion-convection equation for the distribution function of energetic particles, coupled with the thermal plasma through the momentum flux continuity equation, is reduced to a nonlinear integral equation in one variable. Its solution provides selfconsistently both the particle spectrum and the structure of the hydrodynamic flow. A critical system parameter governing the acceleration process is found to be $Λ= M^{-3/4}Λ_1 $, where $ Λ_1 =ηp_1/mc $, with a suitably normalized injection rate $ η$, the Mach number M >> 1, and the cut-off momentum $ p_1 $. We particularly focus on an efficient solution, in which almost all the energy of the flow is converted into a few energetic particles. It was found that (i) for this efficient solution (or, equivalently, for multiple solutions) to exist, the parameter $ ζ=η\sqrt{p_0 p_1}/mc $ must exceed a critical value $ζ_{cr} \sim 1$ ($p_0$ is the injection momentum), (ii) the total shock compression ratio r increases with M and saturates at a level that scales as $ r \propto Λ_1 (iii) the downstream power-law spectrum has the universal index q=3.5 over a broad momentum range. (iv) completely smooth shock transitions do not appear in the steady state kinetic description.

astro-ph↗

arXiv1996

Spatial chaos in weakly dispersive and viscous media: a nonperturbative theory of the driven KdV-Burgers equation

The asymptotic travelling wave solution of the KdV-Burgers equation driven by the long scale periodic driver is constructed. The solution represents a shock-train in which the quasi-periodic sequence of dispersive shocks or soliton chains is interspersed by smoothly varying regions. It is shown that the periodic solution which has the spatial driver period undergoes period doublings as the governing parameter changes. Two types of chaotic behavior are considered. The first type is a weak chaos, where only a small chaotic deviation from the periodic solution occurs. The second type corresponds to the developed chaos where the solution ``ignores'' the driver period and represents a random sequence of uncorrelated shocks. In the case of weak chaos the shock coordinate being repeatedly mapped over the driver period moves on a chaotic attractor, while in the case of developed chaos it moves on a repellor. Both solutions depend on a parameter indicating the reference shock position in the shock-train. The structure of a one dimensional set to which this parameter belongs is investigated. This set contains measure one intervals around the fixed points in the case of periodic or weakly chaotic solutions and it becomes a fractal in the case of strong chaos. The capacity dimension of this set is calculated.

chao-dyn↗