Vortex triplets in dusty plasma with sheared flow and magnetic fields

Localized quasi-stationary rotational structures on the dust time scale in low-β dusty plasmas involving sheared flow and magnetic fields are investigated. For self-consistent equilibrium density, flow, and magnetic field profiles, solution in the form of a localized vortex triplet is obtained and its properties investigated. The magnitude of the electrostatic potential of the vortex structure increases with the velocity and magnetic shear.     

脉冲星的射电发射

脉冲星的射电辐射与其他天体物理辐射源有很大的不同 ,因为它们有着极高的亮温度和高度的线或圆偏振。极高的亮温度意味着起作用的发射机制一定是相干的。至今尚无对这种辐射普遍接受的模型。本文讨论了关于脉冲星的射电辐射产生和传播研究中的新进展。     

PAMPRE: A dusty plasma experiment for Titan's tholins production and study

Organic aerosols play a significant role in the properties and evolution of Titan's atmosphere. But our knowledge of them and their physico-chemical mechanisms of formation and evolution are currently limited to a few data obtained by Titan observations from the Earth or from space probes. For this reason, laboratory experiments are developed to simulate the atmospheric chemistry and produce analogues of these aerosols in order to understand better their properties and how they are formed. The plasma discharges are the most efficient devices for the production of such analogues. However, the existing plasmas simulations introduce experimental biases compared with the conditions of aerosols production in Titan's atmosphere: chemistry is induced by electrons instead of photons; the solid analogues are produced and deposited on solid surfaces; direct analysis of the particles inside the reactive chamber is not easy. In order to avoid some of these experimental problems, we have developed another method of production of Titan's aerosols analogues. It is based on a capacitively coupled radio-frequency (RF) cold plasma system at low pressure in a N₂-CH₄ gaseous mixture. In this plasma, solid particles produced from the gas phase are in levitation, thus preventing any wall effect on their production, and allowing the study of the formation and growth of the particles directly in the plasma. Moreover, the electron energy distribution of this plasma can be compared with the solar spectrum. This article describes the RF plasma experiment and presents the first results obtained with an initial N₂-CH₄ (90:10) gaseous mixture which produced our first studied analogues of Titan's aerosols.     

Power Fluctuations in Meteor Head Echoes Observed with the EISCAT VHF Radar

We present observations and preliminary results from a meteor experiment carried out with the 224 MHz EISCAT VHF radar in Tromsø, Norway, which was run for 6 h on November 26, 2003. The data set contains echoes with peculiar pulsations in received power in the frequency range 20–200 Hz, limited by instrumental parameters. The process causing the echo power pulsations has not yet been identified. Plasma effects are the most likely cause, a possible mechanism is for instance asymmetrical dust grains in rotation causing a modulation of the ionization rate.     

Analytical treatment of the force acting on a relativistic electron beam spreading in dense gas plasma by the ohmic plasma channel

A method is described for the analytical evaluation of the force acting on a relativistic electron beam spreading in dense gas plasma by the ohmic plasma channel. This is useful for the solution of the general definitions for the force of the beam plasma interactions in the case of an arbitrary displacement of the symmetry axis of the plasma channel relative to the corresponding axis of the beam. The accuracy of these procedures is tested and their efficiency illustrated with practical applications, including the computation of the tracking force exerting on a relativistic electron beam by the ohmic plasma channel.     

Two-photon 2s ↔ 1s transitions during hydrogen recombination in the universe

Based on the standard cosmological model, we calculate the correction to the rate of two-photon 2s ? 1s transitions in the hydrogen atom under primordial hydrogen plasma recombination conditions that arises when the induced transitions under equilibrium background radiation with a blackbody spectrum and plasma recombination radiation are taken into account.     

Analytical model of magnetic reconnection in the presence of shock waves attached to a current sheet

We consider a stationary two-dimensional model of magnetic reconnection in plasma. The model includes a current sheet and four MHD shock waves attached to its endpoints. The solution to the problem has been found in an analytical form that admits of efficient numerical implementation. We analyze in detail the structure of the magnetic field in the reconnection region and its variation with model parameters.     

Compressible turbulence in Hall Magnetohydrodynamics

By direct numerical simulations we investigate the nonlinear dynamics of a compressible Hall Magnetohydrodynamic (MHD) plasma. At small scales, where the Hall effect dominates, we found an increase of the compressibility of the system and the breakdown of the strong link between velocity and magnetic fields, typical of usual MHD. Moreover, we find that small-scale fluctuations are characterized by an anti-correlation between density and magnetic field intensity. These features characterize the excitation of a quasi-perpendicular magnetosonic turbulence that can be interpreted as the small-scale signature of the break-down of the MHD nonlinear energy cascade due to Hall effect. Fluctuations with the same properties, based on measurements by Cluster spacecraft in space plasma turbulence during different magnetopause crossings, have been recently observed.     

Excitation of Electromagnetic Flute Modes in the Process of Interaction of Plasma Flow with Inhomogeneous Magnetic Field

Laboratory experiments on the interaction of a plasma flow, produced by laser ablation of a solid target with the inhomogeneous magnetic field from the Zebra pulsed power generator demonstrated the presence of strong wave activity in the region of the flow deceleration. The deceleration of the plasma flow can be interpreted as the appearance of a gravity-like force. The drift due to this force can lead to the excitation of flute modes. In this paper a linear dispersion equation for the excitation of electromagnetic flute-type modes with plasma and magnetic field parameters, corresponding to the ongoing experiments is examined. Results indicate that the wavelength of the excited flute modes strongly depends on the strength of the external magnetic field. For magnetic field strengths ∼0.1 MG the excited wavelengths are larger than the width of the laser ablated plasma plume and cannot be observed during the experiment. But for magnetic field strengths ∼1 MG the excited wavelengths are much smaller and can then be detected.