Search Results - (Author, Cooperation:V. Shapiro)

2013-08-09

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0028-0836

1476-4687

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1089-7674

AIP Digital Archive

Physics

In this paper numerical solutions of Zakharov-type equations for lower-hybrid (LH) waves, including pumping at the long wavelengths and dissipation at short wavelengths in the form of dissipative cavitons are described. The caviton is a quasistationary structure undergoing many sequences of collapse due to dissipation, created by ion–wave interactions, which is compensated for by constant pump action. The possibility of trapping of short-wavelength LH oscillations by much broader density cavitons is investigated both analytically and numerically. Analytic self-similar solutions corresponding to collapse of such cavitons are constructed and demonstrate cascading to shorter wavelengths, which develops faster than the three-dimensional (3-D) quasiclassical cavity contraction. Numerical solutions show the development of deep caviton modulation due to the instability of quasiclassical collapse. Results of the numerical and analytical investigation are used to explain the recent observations of cavity formation in the auroral ionosphere, and show that the measured structures could indeed arise from quasiclassical LH collapse. © 1995 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

Experiments are described on the interaction of a weak warm beam with a broad spectrum of unstable waves on a traveling wave tube. The wave–particle interactions are similar to those in beam–plasma systems, and are traditionally described by quasilinear theory. The precise wave evolution is obtained by launching a specified waveform, allowing it to interact with the beam, and analyzing the received waveform. Significant mode coupling is observed, resulting in saturated waves correlated less than 0.5 with their launch values. Experimentally, each wave is separated into a component proportional to the launch amplitude and a component due solely to mode coupling. The measured properties of these separate components agree quantitatively with a four-wave coupling model. Strongest coupling is observed between waves whose wave numbers match within about an inverse turbulent trapping length. In the linear growth regime, the measured ensemble-averaged wave growth rates and beam velocity diffusion rates agree reasonably with quasilinear and resonance-broadening theory; in the nonlinear regime near saturation, the discrepancies become larger. © 1995 American Institute of Physics.

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1089-7674

AIP Digital Archive

Physics

Tilting instability is an instability of convective motion in two-dimensional (2-D) ideal fluid transforming convection into sheared flow. An analytical model of the tilting instability is proposed that clearly exhibits inverse cascade phenomenon, conserving both energy and enstrophy. Obtained solution describes the evolution of the nonlinear stage in which initial fluid convection is transformed completely into the large-scale flow.

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1089-7674

AIP Digital Archive

Physics

The linear and nonlinear dynamics of modulational interaction between small-scale drift waves and large-scale trapped ion convective cells are investigated. This example is a paradigm of the more general problem of describing the self-consistent interaction of small-scale fluctuations with mean sheared flows. The growth rate of modulational instability is determined by spectral properties of drift waves and can exceed the linear growth rate of the trapped ion mode. An anisotropic spectrum of drift waves is always modulationally unstable. The spatial orientation of the convective cell pattern and structure (i.e., shear strength) is determined by drift wave spectrum anisotropy and propagation direction. In the presence of a sheared magnetic field, which pins small-scale drift waves to mode rational surfaces, the modulational growth rate becomes intrinsically anisotropic, on account of the modified radial structure of drift waves. © 1995 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

Recently it was shown that the light absorption spectrum of (CH2)2(NH3)2MnCl4 (EDAMnCl4) differs strongly from those of other compounds of this family (CH2)n(NH3)2MnCl4. Additional peculiarities in the spectral distributions of two-magnon Raman scattering and the coefficient of exciton-magnon absorption were found. It was also determined that the pure exciton band in the absorption spectrum is of the electric dipole type of polarization. All these peculiarities are out of the framework of the usual tetragonal two-sublattice antiferromagnet model and indicate the inverse center loss and magnetic cell doubling. Results of studying the absorption spectrum of EDAMnCl4 presented in the report show that not less than three Davydov components of the exciton band σA1g(σS)→4Eg(4G) optical transition may be detected with confidence in magnetic fields oriented along the c axis. It confirms the four-sublattice magnetic structure of EDAMnCl4. The presence of exciton line splitting at H=0 and its nonlinear character at H(parallel)α (that is practically parallel to l) allows us to make a conclusion about the resonance interaction between ions with antiparallel spins and, probably, about the initial noncollinearity of magnetic structure. Parameters characterizing the magnetic Davydov splitting are determined using comparison of the experimental data and theoretical analysis.

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1089-7666

AIP Digital Archive

Physics

This paper presents a model of beam–plasma discharge resulting from the quasilinear heating of plasma electrons by Langmuir waves which are excited by beam–plasma interactions. The heating is made possible by the spectral transformation of waves propagating radially from the central beam-occupied region toward the region of lower plasma density. In this paper equations describing the wave spectral density, the distribution function of a high-energy electron tail, and its stationary density profile are obtained and numerically solved; to do so a balanced diffusion and ionization is assumed. The possibility of significant plasma density enhancement in beam–plasma discharge is demonstrated.

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1089-7666

AIP Digital Archive

Physics

The modulational instability and collapse of waves in the vicinity of the lower-hybrid resonance including both magnetosonic and lower-hybrid waves are investigated by analytical and numerical methods. The mechanism leading to the modulational instability is the nonlinear coupling of lower-hybrid waves with the much lower-frequency quasineutral density perturbations via the ponderomotive force. The result is a filamentation of the high-frequency field producing elongated, cigar-shaped nonlinear wave packets aligned along the magnetic field with the plasma expelled outside (cavities). The analytical self-similar solutions describing cavity collapse are obtained and compared with the results of numerical simulation for both two- and three-dimensional cavity geometries. It is shown that in three-dimensional solutions the transverse, with respect to the magnetic field, contraction remains prevailing. The possibility of ion acceleration as the result of the lower-hybrid collapse is discussed and detailed comparison is made with the observations of the phenomena in the auroral ionosphere.

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1089-7666

AIP Digital Archive

Physics

The Cohen–Kulsrud–Burgers equation (CKB) is used to consider the nonlinear evolution of resistive, quasiparallel Alfvén waves subject to a long-wavelength, plane-polarized, monochromatic instability. The instability saturates by nonlinear steepening, which proceeds until the periodic waveform develops an interior scale length comparable to the dissipation length; a fast or an intermediate shock then forms. The result is a periodic train of Alfvén shocks of one or the other type. For propagation strictly parallel to the magnetic field, there will be two shocks per instability wavelength. Numerical integration of the time-dependent CKB equation shows that an initial, small-amplitude growing wave asymptotes to a stable, periodic stationary wave whose analytic solution specifies how the type of shock embedded in the shock train, and the amplitude and speed of the shock train, depend on the strength and phase of the instability. Waveforms observed upstream of the Earth's bowshock and cometary shocks resemble those calculated here.

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1089-7666

AIP Digital Archive

Physics

One of the main experiments of the Active Magnetospheric Particle Tracer Explorers (AMPTE) [J. Geophys. Res. 91, 10013 (1986)] satellite mission was the release of neutral barium atoms in the solar wind. The barium atoms ionized by photoionization extremely rapidly forming a dense, expanding, plasma cloud that interrupted the solar wind flow creating diamagnetic cavities. On the upstream side of the cavity a region of compressed plasma and enhanced magnetic field was created as the result of being produced by the slowing down and deflection of the solar wind, and magnetic field line draping. Intense electrostatic and magnetic turbulence was observed by both the IRM [J. Geophys. Res. 91, 10 013 (1986)] and UKS [J. Geophys. Res. 91, 1320 (1986)] satellites at the boundary of the diamagnetic cavity, with the most intense waves being detected near the outer boundary of the compressed region. This paper examines how the newly created expanding plasma couples to the solar wind by means of plasma–beam and current-driven instabilities. In particular, it is shown how lower-hybrid and lower-hybrid drift waves are generated by cross-field proton–barium streaming instabilities and cross-field electron currents. The saturation mechanism for these waves is considered to be the modulational instability, this instability can also lead to filamentation and coupling to magnetosonic modes, which are also observed. As the result of modulational instability the k(parallel) component increases, which allows the heating and acceleration of electrons that is consistent with the observations.

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1089-7666

AIP Digital Archive

Physics

The comments of Montgomery and Matthaeus on the authors' paper1, have been answered. The results do not contradict those of Montgomery and Matthaeus but for simple case considered, transition to shear flow state and shape of initial vortices is explained. (AIP)

Electronic Resource

1089-7666

AIP Digital Archive

Physics

A periodic array of convection cells is subject to a "shear flow'' instability. The generation of the sheared flow is a consequence of "peeling'' of the convection cells. Fluid simulations demonstrate that the efficiency of shear flow generation is high. Implications for understanding poloidal rotation in tokamaks are discussed.

Electronic Resource

0143-8166

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Electrical Engineering, Measurement and Control Technology

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Physics

Technology

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract The Giotto spacecraft is scheduled to intercept comet P/Grigg-Skjellerup on July 10, 1992. The observed outgassing rate of this comet is over an order of magnitude smaller than comet Giacobini-Zinner and over two orders of magnitude smaller than that of comet Halley. Consequently, the new data obtained during the upcoming encounter will strengthen our understanding of how the solar wind interaction with comets depends upon the neutral gas production rate. In this brief note, we make predictions of the location of the flow transition regions — i.e., the bow shock and the ionopause, and discuss the expected level of wave turbulence.

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1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The spacecraft Vega 1 and Vega 2 encountered comet Halley on 6 and 9 March 1986. Their scientific payload comprised 14 instruments, which collected data concerning the comet's optical characteristics, dust emission, and neutral gas, plasma and electromagnetic field environment. The main ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Each dust particle that hits the solid target of the SP-1 detector gives rise to a charged plasma cloud5; the amplitude of each charge pulse is measured and recorded by decade counters. A preliminary estimate for the mass range Amn of the nth channel is obtained by using an empirical conversion ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The techniques used evolved from the near-Earth cosmic dust studies carried out on Kosmos satellites from 1966 to 19723. The SP-2 experiment4 comprises two types of particle impact sensor: an acoustic sensor and an ionization (plasma) sensor. The acoustic sensor consists of a thin circular plate ...

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1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] The heart of the PUMA instrument is a time-of-flight mass spectrometer as described, for example, in ref. 2. When a dust particle strikes the target in front of the spectrometer, ions are formed, and those with positive charge are mass-analysed. Onboard electronics and microprocessor-control allow ...

Electronic Resource

1432-0924

Springer Online Journal Archives 1860-2000

Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics

Abstract  The R-Function Method (RFM) solution structure is a functional expression that satisfies all given boundary conditions exactly and contains some undetermined functional components. It is complete if there exists a choice of undetermined component that transform the solution structure into an exact solution. Such a structure was used by Kantorovich (Kantorovich and Krylov, 1958) and his students to solve boundary value problems with homogeneous boundary conditions on geometrically simple domains. RFM is based on the theory of R-functions (Rvachev, 1982) that allows construction of a set of functions vanishing on the boundary and can be applied to problems with arbitrarily complex domains and boundary conditions. The resulting solution method is essentially meshfree, in the sense that the spatial discretization no longer needs to conform to the geometry of the domain, and can be completely automated. This paper summarizes the main principles of RFM, proves its completeness, and presents numerical results for several simple test problems.

Electronic Resource

1572-9540

Springer Online Journal Archives 1860-2000

Physics

Abstract Particles in resonance electromagnetic fields experience a sharp increase of the recoil forces. The quasi-elastic and cooling action of the recoil has found application in traps and optics of particles. This paper aims at drawing attention to the vortical recoil action; it can dominate and has peculiar features deserving interest for the application.

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