Alex Lukyanov

Parametric decay of upper hybrid plasma waves trapped inside density irregularities in the ionosphere

Abstract The parametric decay of an upper hybrid (UH) wave into a lower hybrid (LH) wave and a downshifted UH wave inside inhomogeneous ionospheric striations, when both pump and decay UH modes are trapped, is investigated. The threshold and the growth rate of the instability are determined. The possibility of the existence of both main features in the usually observed spectrum of stimulated electromagnetic emission (SEE), downshifted maximum and long continuum tail, is demonstrated. The conditions of the excitation of the second and third decay modes are established.

Energetic neutral atom imaging of Mercury's magnetosphere 3. Simulated images and instrument requirements

Abstract The paper presents simulations of the energetic neutral atom (ENA) production in the Mercury magnetosphere and the obtained ENA images for the equatorial and polar vantage points. The ENA fluxes are found to be 102– 10 3 ( cm 2 sr s keV ) −1 and up to 104– 10 5 ( cm 2 sr s keV ) −1 in the energy range 10– 50 keV . Due to the small size of the magnetosphere, the particles injected in the tail can fill up the entire dayside magnetosphere making possible ENA imaging of the magnetospheric shape. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”) which can be used to study the global dynamics. The ENA instrument requirements, 10°×10° angular resolution and 20 s accumulation time, can be easily met by modern ENA instrumentation. Therefore, ENA imaging of the Mercury magnetosphere is feasible.

Energetic neutral atom imaging of Mercury's magnetosphere 2. Distribution of energetic charged particles in a compact magnetosphere

Abstract In order to investigate the feasibility of energetic neutral atom (ENA) imaging of Mercurys magnetosphere interacting with the tenuous exosphere, we developed a simple model simulating dynamics of hot proton plasma near Mercury. We did not consider specific acceleration mechanisms but parameterised the proton source by its location, size and the distribution function of the injected protons. The characteristics of the source are free parameters of the model and can be easily varied. To model the magnetic field in the magnetosphere we used the Tsyganenkos model scaled for Mercurys conditions. The dynamics of energetic protons 1 keV keV are calculated in the drift approximation taking into account the convective electric field produced by the solar wind interaction with the magnetosphere. This enabled to establish the particle steady-state distribution. As for more energetic protons, E>50 keV , the large Larmour radius results in their almost immediate loss via intercepting the Hermean magnetopause. The model was tested for 10, 30, and 50 keV protons injected from a source of the size 1Rm×1Rm located at 3R m , 2.5R m , 2R m , and 1.5Rm in the tail (Rm is the Hermean radius). This simple model turned out to be rather useful and flexible, and can be used to study proton dynamics in Mercurys magnetosphere.

Electron temperature measurements by incoherent scattering in the presence of strong small scale temperature irregularities

Abstract The results of electron temperature measurements by incoherent scattering have been considered under the conditions of existence of strong small scale temperature irregularities in the F-region of the ionosphere. A method to determine such local temperature enhancement from the analysis of the mean square error in the ACF fitting and the total scattered power of the ISR signal is pointed out.

Instability of MHD-modified interfacial gravity waves revisited

We reveal the basic mechanism of instability of the two-layer conductive fluid system carrying a normal current and exposed to a uniform external magnetic field. This process is a reflection of a MHD-modified interfacial gravity wave from the boundary. Due to special boundary conditions, the reflection coefficient turns out to be greater than 1 for some directions of the wave propagation. We consider two cases: reflection of a monochromatic plane wave from the plane boundary and reflection of rotating waves in a circular geometry. We believe that the proposed mechanism gives a new understanding of the instability formation in the system ‘liquid metal–electrolyte’ type.  2001 Elsevier Science B.V. All rights reserved.

Dynamic Contact Angle at the Nanoscale: A Unified View

Generation of a dynamic contact angle in the course of wetting is a fundamental phenomenon of nature. Dynamic wetting processes have a direct impact on flows at the nanoscale, and therefore, understanding them is exceptionally important to emerging technologies. Here, we reveal the microscopic mechanism of dynamic contact angle generation. It has been demonstrated using large-scale molecular dynamics simulations of bead-spring model fluids that the main cause of local contact angle variations is the distribution of microscopic force acting at the contact line region. We were able to retrieve this elusive force with high accuracy. It has been directly established that the force distribution can be solely predicted on the basis of a general friction law for liquid flow at solid surfaces by Thompson and Troian. The relationship with the friction law provides both an explanation of the phenomenon of dynamic contact angle and a methodology for future predictions. The mechanism is intrinsically microscopic, universal, and irreducible and is applicable to a wide range of problems associated with wetting phenomena.

Flexural vibrations induced in thin metal wires carrying high currents

Exploding wires are widely used in many experimental set-ups and pulsed power systems such as Z-pinch, high-current switches, copper-vapour lasers and high-brightness x-ray lithography. However, many aspects of the process of wire explosion still remain unclear. If the current density is not too high, the wire may break up in the solid state. The experiments have shown that the wires break in tension due to longitudinal forces of unknown nature. Previous theoretical and numerical investigations served to provide a search for these forces and have identified the pinch effect and thermal expansion as a source of strong longitudinal vibrations. But the mechanism does not give a satisfactory explanation for the phenomenon in the wires with clamped ends. In this investigation, we use a simplified magneto-thermo-elastic model to study flexural vibrations induced by high pulsed currents in wires with clamped ends on account of their role in the disintegration process. Several aspects are studied, namely (i) the buckling instability due to simultaneous action of the thermal expansion and the magnetic force, and (ii) the flexural vibrations induced in initially bent wires. It is shown that the induced flexural vibrations are strong enough to lead to the breaking of the wire in a wide range of parameters.

Relaxation of surface tension in the liquid-solid interfaces of Lennard-Jones liquids.

We have established the surface tension relaxation time in the liquid-solid interfaces of Lennard-Jones (LJ) liquids by means of direct measurements in molecular dynamics (MD) simulations. The main result is that the relaxation time is found to be almost independent of the molecular structures and viscosity of the liquids (at 70-fold change) used in our study and lies in such a range that in slow hydrodynamic motion the interfaces are expected to be at equilibrium. The implications of our results for the modeling of dynamic wetting processes and interpretation of dynamic contact angle data are discussed.

Experimental model of the interfacial instability in aluminium reduction cells

A solution has been found to the long-standing problem of experimental modelling of the interfacial instability in aluminium reduction cells. The idea is to replace the electrolyte overlaying molten aluminium with a mesh of thin rods supplying current down directly into the liquid metal layer. This eliminates electrolysis altogether and all the problems associated with it, such as high temperature, chemical aggressiveness of media, products of electrolysis, the necessity for electrolyte renewal, high power demands, etc. The result is a room temperature, versatile laboratory model which simulates Sele-type, rolling pad interfacial instability. Our new, safe laboratory model enables detailed experimental investigations to test the existing theoretical models for the first time.

Superfast nonlinear diffusion: capillary transport in particulate porous media.

The migration of liquids in porous media, such as sand, has been commonly considered at high saturation levels with liquid pathways at pore dimensions. In this Letter, we reveal a low saturation regime observed in our experiments with droplets of extremely low volatility liquids deposited on sand. In this regime, the liquid is mostly found within the grain surface roughness and in the capillary bridges formed at the contacts between the grains. The bridges act as variable-volume reservoirs and the flow is driven by the capillary pressure arising at the wetting front according to the roughness length scales. We propose that this migration (spreading) is the result of interplay between the bridge volume adjustment to this pressure distribution and viscous losses of a creeping flow within the roughness. The net macroscopic result is a special case of nonlinear diffusion described by a superfast diffusion equation for saturation with distinctive mathematical character. We obtain solutions to a moving boundary problem defined by superfast diffusion equation that robustly convey a time power law of spreading as seen in our experiments.