Magnetohydrodynamic instabilities and turbulence in accretion disks

In this paper we review the possibilities for magnetohydrodynamic processes to handle the angular momentum transport in accretion disks. Traditionally the angular momentum transport has been considered to be the result of turbulent viscosity in the disk, although the Keplerian flow in accretion disks is linearly stable towards hydrodynamic perturbations. It is on the other hand linearly unstable to some magnetohydrodynamic (MHD) instabilities. The most important instabilities are the Parker and Balbus-Hawley instabilities that are related to the magnetic buoyancy and the shear flow, respectively. We discuss these instabilities not only in the traditional MHD framework, but also in the context of slender flux tubes, that reduce the complexity of the problem while keeping most of the stability properties of the complete problem. In the non-linear regime the instabilities produce turbulence. Recent numerical simulations describe the generation of magnetic fields by a dynamo in the resulting turbulent flow. Eventually such a dynamo may generate a global magnetic field in the disk. The relation of the MHD-turbulence to observations of accretion disks is still obscure. It is commonly believed that magnetic fields can be highly efficient in transporting the angular momentum, but emission lines, short-time scale variability and non-thermal radiation, which a stellar astronomer would take as signs of magnetic variability, are more commonly observed during periods of low accretion rates. Received October 12, 1995 / Accepted November 16, 1995     

Electron surfing acceleration in oblique magnetic fields

Initially, inhomogeneous plasma jets, ejected by active galactic nuclei and associated with gamma-ray bursts, are thermalized by the formation of internal shocks. Jet subpopulations can hereby collide at Lorentz factors of a few. As the resulting relativistic shock expands into the upstream plasma, a significant fraction of the upstream ions is reflected. These ions, together with downstream ions that leak through the shock, form relativistic beams of ions that outrun the shock. The thermalization of these beams via the two-stream instability is thought to contribute significantly to plasma heating and particle acceleration by the shock. Here, the capability of a two-stream instability to generate relativistic field-aligned and cross-field electron flow, is examined for a magnetized plasma by means of a particle-in-cell (PIC) simulation. The electrons interact with the developing quasi-electrostatic waves and oblique magnetic fields. The simulation results bring forward evidence that such waves, by their non-linear interactions with the plasma, produce a highly relativistic field-aligned electron flow and electron energies, which could contribute to the radio synchrotron emissions from astrophysical jets, to ultrarelativistic leptonic subpopulations propagating with the jet and to the halo particles surrounding the accretion disc of the black hole.     

GAINS中重力传感器信息的扰动改正

重力辅助惯性导航系统GAINS(Gravity Aided Inertial Navigation System)是利用地球物理特征信息重力来完成水下运动载体的辅助惯性导航与定位.为实现重力匹配以校正惯性导航随时间累积的误差,首先必须对重力传感器输出信息进行扰动改正.分析了水下运动状态下重力传感器受到的各种重力扰动,如垂直扰动加速度、水平扰动加速度以及厄特弗斯效应影响所产生的原因,研究了扰动误差模型与INS导航精度之间的关系,并通过计算,提出了可直接以INS输出数据而无需其它外部有源导航信息进行扰动分离的方法.     

TeV γ-ray fluxes from the long campaigns on Mrk 421 as constraints on the emission of TeV–PeV neutrinos and UHECRs

Long TeV γ-ray campaigns have been carried out to study the spectrum, variability and duty cycle of the BL Lac object Markarian 421. These campaigns have given some evidence of the presence of protons in the jet: (i) Its spectral energy distribution which shows two main peaks; one at low energies (∼1 keV) and the other at high energies (hundreds of GeV), has been described by using synchrotron proton blazar model. (ii) The study of the variability at GeV γ-rays and X-rays has indicated no significant correlation. (iii) TeV γ-ray detections without activity in X-rays, called “orphan flares” have been observed in this object.Recently, The Telescope Array Collaboration reported the arrival of 72 ultra-high-energy cosmic rays with some of them possibly related to the direction of Markarian 421. The IceCube Collaboration reported the detection of 37 extraterrestrial neutrinos in the TeV–PeV energy range collected during three consecutive years. In particular, no neutrino track events were associated with this source. In this paper, we consider the proton–photon interactions to correlate the TeV γ-ray fluxes reported by long campaigns with the neutrino and ultra-high-energy cosmic ray observations around this blazar. Considering the results reported by The IceCube and Telescope Array Collaborations, we found that only from ∼25% to 70% of TeV fluxes described with a power law function with exponential cutoff can come from the proton–photon interactions.     

The interaction of cosmic rays and magnetized plasma

A large flux of cosmic rays streaming through a magnetized plasma creates cavities of low plasma density and low magnetic field. The magnetic field focuses the cosmic ray trajectories into the cavities with the possible formation of filaments or beams of high-energy cosmic rays.     

Interstellar scintillation and clouds of the interstellar turbulent plasma

Data on interstellar diffraction and refraction scintillation of pulsars are analyzed. Comparison between theory and the observational data shows that two types of spectra for electron density fluctuations are realized in the interstellar medium: pure power law and piecewise with a break. The distribution of turbulent plasma in the Galaxy has a three component structure. Component A is diffuse and it is distributed outside of the spiral arms of the Galaxy. Component BI is cloudy and associated with Galactic arms. Component BII is extremely nonuniform and associated with HII regions and supernova remnants. The origin of the interstellar plasma turbulence is considered, and possible sources of turbulent energy are discussed. The contribution of supernova bursts in the interstellar gas ionization and generation of turbulence are analyzed among other factors. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cosmic Ray Acceleration Inside Molecular Clouds

Acceleration of charged particles by neutral gas turbulence in giant molecular clouds is considered. The gamma-ray emission from these clouds is estimated. It is shown that molecular clouds can be the counterparts of some of unidentified sources.     

Numerical evaluation of the dilogarithm of complex argument

A numerical algorithm to evaluate the dilogarithmic function of a complex argument is proposed. The use of the dilogarithm in celestial mechanics appears in the exact Delaunay normalization of some functions involving the equation of the centre.     

Diffusion in momentum space as a picture of second-order Fermi acceleration

Energetic particles in a turbulent medium can be subject to second-order Fermi acceleration due to scattering on moving plasma waves. This mechanism leads to growing particle momentum dispersion and, at the same time, increases the mean particle energy. In the most frequently met situations both processes can be represented by a single momentum diffusion term in the particle kinetic equation. In the present paper we discuss the conditions allowing the additional term for regular acceleration to arise. For forward-backward asymmetric scattering centres, besides the diffusive term one should explicitly consider the regular acceleration term in momentum space, which can consist of the first-order (∝ V), as well as the second-order (∝ V²) part in the wave velocity V. We derive the condition for the scattering probability in the wave rest frame requied for vanishing the regular acceleration term and provide a simple mechanical example illustrating the theoretical concepts. Finally, we address its possible role in cosmic ray acceleration processes.