Generation of zonal flows by interchange modes in a plasma

The generation of zonal flows by flute-like interchange modes in a nonuniform magnetoplasma is considered. The guiding center particle drifts are then used to derive a system of coupled mode equations. The latter are Fourier analyzed to obtain a nonlinear dispersion relation, which exhibits the excitation of zonal flows by the ponderomotive force of the interchange modes. The growth rate of the parametrically driven zonal flows is obtained. Received 26 July 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: ps@tp4.ruhr-uni-bochum.de     

Dissipation in turbulent plasma due to reconnection in thin current sheets

We present in situ measurements in a space plasma showing that thin current sheets the size of an ion inertial length exist and are abundant in strong and intermittent plasma turbulence. Many of these current sheets exhibit the microphysical signatures of reconnection. The spatial scale where intermittency occurs corresponds to the observed structures. The reconnecting current sheets represent a type of dissipation mechanism, with observed dissipation rates comparable to or even dominating over collisionless damping rates of waves at ion inertial length scales (x100), and can have far reaching implications for small-scale dissipation in all turbulent plasmas.     

Saturation of relativistic Weibel instability and the formation of stationary current sheets in collisionless plasma

We have studied the features of formation and the possible stationary structures of a self-consistent magnetic field in a relativistic collisionless plasma, which are characteristic of a simple geometry of the Weibel instability that is well known in the nonrelativistic case. The universal condition is established, the growth rate is determined, and the criteria of saturation of the Weibel instability are analyzed for a broad class of anisotropic particle distribution functions (for definiteness, in application to an electron-positron plasma). A nonlinear equation of the Grad-Shafranov type describing the potential current structures is derived and its solutions are analytically studied. Special attention is paid to spatially harmonic, nonlinear current configurations with parameters determined by the properties of the initial homogeneous plasma subject to the Weibel instability. It is demonstrated that the magnetic field energy density in the obtained solutions (both harmonic and nonharmonic) can be comparable with the kinetic energy density of plasma particles.     

Particle acceleration in relativistic current sheets

Relativistic current sheets have been proposed as the sites of dissipation in pulsar winds, jets in active galaxies, and other Poynting flux dominated flows. It is shown that the steady versions of these structures differ from their nonrelativistic counterparts because they do not permit transformation to a de Hofmann-Teller frame with zero electric field. Instead, their generic form is that of a true neutral sheet with no linking magnetic field component normal to the sheet. The maximum energy to which such structures can accelerate particles is derived, and used to compute the maximum frequency of the subsequent synchrotron radiation. This can be substantially in excess of standard estimates. In the magnetically driven gamma-ray burst scenario, acceleration of electrons is possible to energies sufficient to enable photon-photon pair production after an inverse Compton scattering event.     

Collisional relaxation of superthermal electrons generated by relativistic laser pulses in dense plasma

Energy relaxation of the hot electron population generated by relativistic laser pulses in overdense plasma is analyzed for densities ranging from below to 1000 times solid density. It is predicted that longitudinal beam-plasma instabilities, which dominate energy transfer between hot electrons and plasma at lower densities, are suppressed by collisions beyond solid density. The respective roles of collisional energy transfer modes, i.e., direct collisions, diffusion, and resistive return current heating, are identified with respect to plasma density. The transition between the kinetic and the collisional regimes and scalings of collisional process are demonstrated by a fully integrated one-dimensional collisional particle simulation.     

Rogue wave triplets in an ion-beam dusty plasma with superthermal electrons and negative ions

A new dust ion-acoustic wave structure called ‘Rogue wave triplets’ is investigated in an unmagnetized plasma consisting of stationary negatively charged dust grains, charged positive and negative ions, and electrons obeying kappa distribution, which is penetrated by an ion beam. The reductive perturbation theory is used to derive the nonlinear Schrödinger equation governing the dynamics as well as the modulation of wave packets. The rogue wave triplets which are composed of three separate Peregrine breathers can be generated in the modulation instability region. It has been suggested that a laboratory experiment be performed to test the theory presented here.     

Dust ion-acoustic shock waves due to dust charge fluctuation in a superthermal dusty plasma

The nonlinear propagation of dust ion-acoustic (DIA) shock waves is studied in a charge varying dusty plasma with electrons having kappa velocity distribution. We use hot ions with equilibrium streaming speed and a fast superthermal electron charging current derived from orbit limited motion (OLM) theory. It is found that the presence of superthermal electrons does not only significantly modify the basic properties of shock waves, but also causes the existence of shock profile with only positive potential in such plasma with parameter ranges corresponding to Saturn?s rings. It is also shown that the strength and steepness of the shock waves decrease with increase of the size of dust grains and ion temperature.     

Resistive instabilities of current sheets in the solar wind

Summary We present some numerical results on resistive magnetohydrodynamic instabilities and discuss their relevance to current sheets of the type indicated by spacecraft observations in the interplanetary plasma.

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Riassunto Si presentano dei risultati numerici sulle instabilità magnetoidrodinamiche resistive. Si discute inoltre la loro rilevanza per l'origine di strati di corrente che sono osservati da satelliti nel mezzo interplanetario.

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Резюме Мы приводим некоторые численные результаты по магнитноги-дродинамическим неустойчивостям, связанным с активным сопротивлением. Обсуждается их причастность к токовым слоям обнаруженным при наблюдениян на космическин аппаратах в межпланетной плазме.

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To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Electromagnetic radiation and instability of superthermal particles in a magnetoactive plasma with strong small-scale irregularities

The electromagnetic radiation of superthermal electrons in a magnetoactive plasma with strong small-scale irregularities is calculated. The emissive power of such a medium is found. The flux density of electromagnetic radiation I_f is determined for an optically thin medium. The values of I_f are estimated for different ionospheric conditions. The induced radiation of a directed electron beam of charged particles in a magnetoactive plasma with such irregularities is studied. The gain (absorption) factor is derived. The possibility of electromagnetic instability in different ionospheric conditions is analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 3, pp. 286–300, March, 1996.     

Head-on collision of ion acoustic solitary waves in electron-positron-ion plasma with superthermal electrons and positrons

The head-on collision of ion acoustic solitary waves are studied in an electron-positron-ion plasma composed of superthermal electrons, superthermal positrons, and cold ions using the extended Poincaré-Lighthill-Kuo (PLK) method. The effects of the ratio of electron to positron temperature, the spectral index of electron and positron, and the concentration of positron component on the phase shift are studied. It is found that the presence of superthermal electrons and superthermal positrons play a significant role on the collision of ion acoustic solitary waves. It is also been observed that the temperature ratio plays a significant role on the collision of ion acoustic solitary waves.     

Generation of mean flows by thermal convection

Various mechanisms of the generation of mean flows by fluctuating convection motions in fluid layers are reviewed. Reynolds stresses causing mean flows are either an intrinsic nonlinear property of convection, or they are produced when a secondary bifurcation to a more complex form of convection occurs, or they enter in the form of an instability, such that the sign of the mean flow depends on initial conditions. The basic mechanisms are elucidated by simple analytical models. Geophysical and astrophysical applications which have motivated most of the research on the topic of this review are mentioned only in passing. Finally the problem of mean flow generation owing to a temperature dependent viscosity is considered and the analogy to the Reynolds stress mechanism is pointed out.     

Anomalous rotational eddy current loss in electrical steel sheets

The rotational eddy current loss of different qualities of electrical steel sheets has been determined with a dynamical torque magnetometer. The measured values of the losses are higher than the losses calculated with a classical formula. The difference grows with increasing ratio of grain size to sheet thickness. In alternating fields this phenomenon has been explained theoretically by the Pry and Bean model.     

Influence of the current density longitudinal distribution in current sheets on their magnetic field structure and formation dynamics. II: Formation dynamics of variously configured current sheets

The structure of Ampere forces responsible for the formation dynamics of classical and double-humped current sheets in magnetic fields with a null line is calculated numerically. Calculation data are compared with experimental data for the structure and dynamics of current sheets obtained using magnetic measurements and holographic interferometry.     

Universal properties of the nonadiabatic acceleration of ions in current sheets

The electric-field-induced acceleration of ions in current sheets in a collisionless plasma is investigated. The analysis of nonadiabatic ion dynamics provides a universal property of the ion acceleration mechanism, which is independent of the magnetic-field model and the initial particle distribution function. The width of the resonance region is estimated. The theoretical results are compared with the experimental and numerical simulation data.     

ADBD plasma surface treatment of PES fabric sheets

Plasma treatment of textile fabrics is investigated as an alternative to the environmentally hazardous wet chemical fabric treatment and pretreatment processes. Plasma treatment usually results in modification of the uppermost atomic layers of a material surface and leaves the bulk characteristics unaffected. It may result in desirable surface modifications, e.g. surface etching, surface activation, cross-linking, chain scission and oxidation. Presented paper contains results of the applicability study of the atmospheric pressure dielectric discharge (ADBD), i.e. dielectric barrier discharge sustaining in air at atmospheric pressure and ambient temperature for synchronous treatment of several sheets of fabric. For tests sheets of polyester fabric were used. Effectivity of the modification process was determined with hydrophilicity measurements evaluated by means of the drop test. Hydrophilicity of individual sheets of fabric has distinctly increased after plasma treatment. Plasma induced surface changes of textiles were also proven by identification of new functional groups at the modified polyester fabric surface. Existence of new functional groups was detected by ESCA scans. For verification of surface changes we also applied high-resolution microphotography. It has shown distinct variation of the textile surface after plasma treatment. Important aspect for practical application of the plasma treatment is the modification effect time-stability, i.e. time stability of acquired surface changes of the fabric. The recovery of hydrophobicity was fastest in first days after treatment, later gradually diminished until reached almost original untreated state.     

Visualization by means of coherent light sheets: application to different flows

Light-sheet generator systems using a sweeping, focused laser beam and spherocylindrical optical components are described and their respective performances discussed. They allow the visualization of supersonic air flows by means of light scattering. Tomographic recordings give a three-dimensional reconstruction of the flow (especially the stationary structures) and can be realized in real-time.