Authors:V. V. Zaitsev; P. V. Kronshtadtov Pages: 169 - 176 Abstract: We study the possible origin of long-lasting soft X-ray flares on the AD Leo star, which were observed onboard the Extreme Ultraviolet Explorer (EUVE) spacecraft for the period of 1993-2000 [1]. These flares have relatively long rise and decay times of the radiation intensity (τ
R ≈ 104 s and τ
d ≈ 5 · 103, respectively), as well as a relatively large emission measure EM ≈ 1051cm−3, which exceeds by 1–3 orders of magnitude the emission measure of soft X-ray flares on the Sun. Assuming that the radiation appears in magnetic loops and basing on the observed values of the emission measure and radiation decay time, the authors of [1] determined the typical length
\( \overline{l}\approx 1.5\cdot {10}^{10} cm \)
, electron number density
\( \overline{n}\approx 3\cdot {10}^{11}c{m}^{-3} \)
, and plasma temperature
\( \overline{T}\approx 2.5\cdot 107 \)
K of the loops. This paper considers plasma heating due to dissipation of the electric currents in the coronal magnetic loops of the star induced by the photospheric convection. The large inductance of the loop as an equivalent electric circuit determines the long time of the current rise in the source and explains the observed time of plasma heating and the rise time of the X-ray radiation intensity. It is shown that the parameters of the X-ray sources in the AD Leo atmosphere agree with the parameters calculated under the assumption of simultaneous emission of a great number of loops (about 50) with electric currents greater than 1013 A, which exceeds the electric currents in the solar coronal magnetic loops by 1–3 orders of magnitude. Such an exceeding can be related to the higher photospheric convection velocities on the late-type stars compared with the Sun. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9687-5 Issue No:Vol. 59, No. 3 (2016)

Authors:V. L. Frolov; V. O. Rapoport; E. A. Schorokhova; A. S. Belov; M. Parrot; J.-L. Rauch Pages: 177 - 198 Abstract: In this paper we systematize the results of studying the characteristics of the plasma-density ducts, which was conducted in 2005–2010 during the DEMETER-satellite operation. The ducts are formed at altitudes of about 700 km as a result of the ionospheric F
2 region modification by high-power high-frequency radio waves radiated by the midlatitude SURA heating facility. All the performed measurements are used as the basis for determining the formation conditions for such ducts, the duct characteristics are studied, and the opportunities for the duct influence on the ionosphere–magnetosphere coupling and propagation of radio waves of various frequency ranges are demonstrated. The results of numerical simulation of the formation of such ducts are presented. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9688-4 Issue No:Vol. 59, No. 3 (2016)

Authors:N. N. Slyunyaev; A. A. Zhidkov Pages: 199 - 216 Abstract: We consider the problem of generator parameterization in the global electric circuit (GEC) models. The relationship between the charge density and external current density distributions inside a thundercloud is studied using a one-dimensional description and a three-dimensional GEC model. It is shown that drastic conductivity variations in the vicinity of the cloud boundaries have a significant impact on the structure of the charge distribution inside the cloud. Certain restrictions on the charge density distribution in a realistic thunderstorm are found. The possibility to allow for conductivity inhomogeneities in the thunderstorm regions by introducing an effective external current density is demonstrated. Replacement of realistic thunderstorms with equivalent current dipoles in the GEC models is substantiated, an equation for the equivalent current is obtained, and the applicability range of this equation is analyzed. Relationships between the main GEC characteristics under variable parameterization of GEC generators are discussed. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9689-3 Issue No:Vol. 59, No. 3 (2016)

Authors:B. V. Kerzhakov; V. V. Kulinich Pages: 217 - 224 Abstract: We use the field matching method to solve the inverse problem of estimating the geoacoustic parameters of a stratified sea bed using the objective function based on the norm of difference between the experimental and simulated vertical angular spectra of the acoustic field and combination of the rapid-annealing method with direct search methods for localization of the global minimum of the objective function. To reduce the influence of the ravine effects of the objective function, we use regularization on the basis of mutual correlations of the experimental and simulated vertical angular spectra of the acoustic field. The numerical experiment has been performed to retrieve the parameters of the model waveguide, e.g., the thickness of the water layer and the layer of sediments, the velocity and attenuation coefficients of longitudinal waves, and the density of the sediment layer and the subjacent half-space in the presence of noise interference of different intensity levels. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9690-x Issue No:Vol. 59, No. 3 (2016)

Authors:B. V. Sveshnikov; A. S. Bagdasaryan Pages: 225 - 233 Abstract: We develop a physical model allowing one to analyze reflection of the inhomogeneous beams of surface acoustic waves from metal strips in a planar waveguide on the piezoelectric substrate. Analytical relationships for determining the coefficients of scattering and mutual conversion of the transverse waveguide modes during their interaction with the spatially limited Bragg reflectors are obtained. The waveguide-reflector characteristics are shown to depend on the ratio of the waveguide aperture to its maximum value for which only the fundamental transverse mode is excited. It is established that the developed model strictly corresponds to the energy conservation law, i.e., in the absence of dissipation, the power of the inhomogeneous beam, which is incident on the finite reflector, is equal to the total power of all the scattered fields of the discrete and continuous waveguide spectra. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9691-9 Issue No:Vol. 59, No. 3 (2016)

Authors:N. A. Denisova Pages: 234 - 245 Abstract: We describe a method for finding new classes of monotonic and nonmonotonic profiles of the refractive index in inhomogeneous layers, which allow one to obtain exact analytical solutions of the direct problem of electromagnetic-field reflection. The possibility of using inhomogeneous layers for optical antireflection coating is discussed. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9692-8 Issue No:Vol. 59, No. 3 (2016)

Authors:V. E. Nazarov; S. B. Kiyashko; A. V. Radostin Pages: 246 - 256 Abstract: We present the results of analytical and numerical studies of propagation of longitudinal acoustic waves in micro-inhomogeneous media with different-modulus nonlinearity and relaxation. The solutions for evolution of the profiles and spectral characteristics of the initially harmonic waves are obtained. PubDate: 2016-08-01 DOI: 10.1007/s11141-016-9693-7 Issue No:Vol. 59, No. 3 (2016)

Authors:A. V. Dedov; V. G. Nazarov Pages: 43 - 47 Abstract: We study the electrical properties of multilayer radar absorbing materials obtained by adding nonwoven sheets of dielectric fibers with an intermediate layer of electrically conductive carbon fibers. Multilayer materials that absorb electromagnetic radiation in a wide frequency range are obtained by varying the content of the carbon fibers. The carbon-fiber content dependent mechanism of absorption of electromagnetic radiation by sheets and multilayer materials is considered. PubDate: 2016-07-20 DOI: 10.1007/s11141-016-9674-x Issue No:Vol. 59, No. 1 (2016)

Authors:K. N. Aleshin; V. V.Matrosov; V. D. Shalfeev Pages: 48 - 58 Abstract: We study collective dynamics of a small-sized chain of the unidirectionally coupled phase-locked loop. The conditions for the synchronous-regime existence are found, the asynchronous selfoscillation regimes and the transitions among them are studied, and the property of inheriting the structure of the parameter space of the chain when a new element is added to it is established. PubDate: 2016-07-25 DOI: 10.1007/s11141-016-9675-9 Issue No:Vol. 59, No. 1 (2016)

Authors:A. G. Sazontov; I. P. Smirnov; A. S. Chashchin Abstract: We consider the problem of determining the location of a coherent acoustic source in a shallowwater channel using a partially calibrated antenna array. To solve this problem, we develop a reduced-rank stable projection algorithm, which is the generalization of the RARE method to the case of signal reception under the conditions of incomplete information on the propagation medium. The results of experimental validation of the proposed approach, which show its efficiency under the conditions of an actual shallow-water area, are presented. PubDate: 2016-08-11 DOI: 10.1007/s11141-016-9678-6

Authors:S. N. Vlasov; S. V. Katin; E. V. Koposova; L. V. Lubyako; L. I. Prokofyev Abstract: Abstract
We present a prototype of a quasioptical Mach—Zehnder interferometer with wave beam splitters in the form of reflective diffraction gratings. Characteristics of the diplexer are studied experimentally at frequencies of about 140 GHz. The measurement results are close to the calculated ones. PubDate: 2016-08-11 DOI: 10.1007/s11141-016-9683-9

Authors:A. N. Kuftin; V. N. Manuilov Abstract: We describe specific features of modeling numerically the operation of magnetron-injection guns, which form high-quality helical electron beams in gyrotrons operated in the short-wave part of the millimeter-wave band (at a wavelength of 1 mm). As an example, we consider the gun of a gyrotron having an operating frequency of 263 GHz designed for spectroscopic research. It is shown that there are good reasons to perform calculations and optimization of the magnetroninjection un in two steps. At the first step, a simplest two-dimensional model can be used, which allows only for the influence of the field of the electrodes and the intrinsic space charge of the beam on the beam parameters. At the second, final stage one should allow for such factors as roughness of the emitting surface and thermal velocities of electrons. The electron distribution function in oscillatory velocities and the coefficient of electron reflection from the magnetic mirror should be calculated. It is demonstrated that the magnetron-injection gun, which is optimized by the method presented, is sufficiently universal and can be operated both at the first and second cyclotron-frequency harmonics. This opens up the possibility of developing gyrotrons for spectroscopy applications at frequencies of 263 and 526 GHz, respectively, which are required for biological and medical research. PubDate: 2016-08-11 DOI: 10.1007/s11141-016-9682-x

Authors:A. G. Luchinin; L. S. Dolin Abstract: We study frequency dependences of the phase and group velocities of the photon-density waves in an anisotropic scattering turbid medium of the sea water type. It is shown that such a medium has an anomalous dispersion in relation to these waves, and their phase and group velocities are functions of the distance to a radiation source. The possibility of time focusing of the photondensity waves is considered for a linear frequency modulation of the radiated pulse. It is shown that full compression of the modulated signal is not achievable due to the frequency dependence of the refractive index of the photon-density wave. The degree of compression of the chirp signals with different parameters has been estimated. PubDate: 2016-08-11 DOI: 10.1007/s11141-016-9684-8

Authors:I. V. Pleshakov; P. S. Popov; Yu. I. Kuzmin; V. I. Dudkin Abstract: We consider a spin echo processor that uses a magnetically ordered material (ferrite) as a working substance. It is shown that it is possible to achieve suppression of the crosstalk (spurious signals) excited by radio-frequency pulses from different chains arriving at the system if the working substance is affected by sufficiently long magnetic field pulses. Thus, time-division multiplexing of the information processes can be carried out. PubDate: 2016-08-10 DOI: 10.1007/s11141-016-9686-6

Authors:V. E. Nazarov; S. B. Kiyashko Abstract: We study theoretically the nonlinear wave processes during excitation of a longitudinal harmonic wave in an unbounded medium and the rod resonator with inelastic hysteresis and saturation of the amplitude-dependent loss. The nonlinear-wave characteristics in such systems, namely, the amplitude-dependent loss, variation in the wave-propagation velocity, the resonant-frequency shift, and the higher-harmonic amplitudes are determined. The results of the theoretical and experimental studies of nonlinear effects in the rod resonator of annealed polycrystalline copper are compared. The effective parameters of the hysteretic nonlinearity of this metal are evaluated. PubDate: 2016-08-10 DOI: 10.1007/s11141-016-9680-z

Authors:V. L. Frolov; G. P. Komrakov; Ya. V. Glukhov; E. S. Andreeva; V. E. Kunitsyn; G. A. Kurbatov Abstract: We consider the experimental results obtained by studying the large-scale structure of the HF-disturbed ionospheric region. The experiments were performed using the SURA heating facility. The disturbed ionospheric region was sounded by signals radiated by GPS navigation satellite beacons as well as by signals of low-orbit satellites (radio tomography). The results of the experiments show that large-scale plasma density perturbations induced at altitudes higher than the F2 layer maximum can contribute significantly to the measured variations of the total electron density and can, with a certain arrangement of the reception points, be measured by the GPS sounding method. PubDate: 2016-08-10 DOI: 10.1007/s11141-016-9677-7

Authors:B. V. Sveshnikov; A. S.Bagdasaryan Abstract: We develop a self-consistent model allowing one to analyze the properties of the interdigital transducer of the surface acoustic waves as a symmetric five-layered waveguide on a piezoelectric substrate with three possible values of the phase velocity of the acoustic-wave propagation along the longitudinal axis of the system. The transcendental dispersion relation for describing the waves in such a system is derived and the method for its instructive graphic analysis is proposed. The condition under which only the fundamental transverse mode is excited in the waveguide is formulated. The method for calculating the normalized power and the transverse distribution of the field of the continuous-spectrum waves radiated from the considered waveguide is described. It is shown that the characteristic spatial scale of the longitudinal damping of the amplitude of this field at the waveguide center can be a qualitative estimate of the transverse-mode formation length. The efficiency of a new method for suppressing the higher-order transverse waveguide modes is demonstrated. PubDate: 2016-08-10 DOI: 10.1007/s11141-016-9679-5

Authors:S. V. Mikhaylovich; Yu.V. Fedorov Abstract: We perform a computational and analytical study of how the thickness of the barrier layer in nanoheterostructures and the gate-drain capacitance C
gd influence the microwave parameters (limiting frequency of current amplification and maximum generation frequency) and noise parameters (noise factor) of a field-effect AlGaN/GaN high electron mobility transistor. The results of complex measurements of the parameters of such transistors based on nanoheterostructures with a barrier layer thickness of 3.5–15.7 nm, which were performed within the framework of four technological routes in the range 0.1–67 GHz, are presented. It is shown that in order to reduce the noise ratio and improve the microwave parameters, it is necessary to optimize both the parameters of nanoheterostructures and the manufacturing techniques. In particular, the thickness of the barrier layer should be reduced, and the gate length should be chosen such as to maximize the product of the squared maximum current amplification frequency in the interior of the transistor and the output impedance between the drain and the source. Additionally, attention should be given to the shape of the gate to reduce the capacitance C
gd. Under certain conditions of manufacture of nitride field-effect HEMT, one can achieve a lower noise factor compared with the transistors based on arsenide nanoheterostructures. PubDate: 2016-08-10 DOI: 10.1007/s11141-016-9685-7

Authors:A. V. Shatsky Abstract: We consider the problem of the influence of diffraction effects on the result of measuring the absorption coefficient of ultrasound in weakly absorbing liquids by the pulse method. Diffraction attenuation of an ultrasonic signal in a measuring cell using solid-state delay lines is calculated. It is shown that the use of delay lines of the ultrasonic signal leads to a considerable distortion of the measured absorption coefficient in the low-frequency range from the true value and can either overestimate or underestimate the results. PubDate: 2016-08-09 DOI: 10.1007/s11141-016-9681-y