2,349 research outputs found

    New lycids from China (Coleoptera, Lycidae)

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    Thirteen new lycid species (Coleoptera, Lycidae) are described f rom China: Pyropterus (Helcophorus) tricolor n. sp., Xylobanellus gansuensis n. sp., Cautires bulenoides n. sp., C. yengi n. sp., C. mao n. sp., C. vigens n. sp., C. sichuanensis n. sp., C. galae n. sp., Xylobanus montiphionus n. sp., Libnetis xilingensis n. sp., L. monachus n. sp., L. latrunculus n. sp. and L. confucius n. sp. This is the first record of Xylobanellus Kleine, 1930, in China. Only basally sclerotized paramerae are for the first time described in Pyropterus Mulsant, 1838. Key words: Coleoptera, Lycidae, New species, China.Thirteen new lycid species (Coleoptera, Lycidae) are described f rom China: Pyropterus (Helcophorus) tricolor n. sp., Xylobanellus gansuensis n. sp., Cautires bulenoides n. sp., C. yengi n. sp., C. mao n. sp., C. vigens n. sp., C. sichuanensis n. sp., C. galae n. sp., Xylobanus montiphionus n. sp., Libnetis xilingensis n. sp., L. monachus n. sp., L. latrunculus n. sp. and L. confucius n. sp. This is the first record of Xylobanellus Kleine, 1930, in China. Only basally sclerotized paramerae are for the first time described in Pyropterus Mulsant, 1838. Key words: Coleoptera, Lycidae, New species, China.Thirteen new lycid species (Coleoptera, Lycidae) are described f rom China: Pyropterus (Helcophorus) tricolor n. sp., Xylobanellus gansuensis n. sp., Cautires bulenoides n. sp., C. yengi n. sp., C. mao n. sp., C. vigens n. sp., C. sichuanensis n. sp., C. galae n. sp., Xylobanus montiphionus n. sp., Libnetis xilingensis n. sp., L. monachus n. sp., L. latrunculus n. sp. and L. confucius n. sp. This is the first record of Xylobanellus Kleine, 1930, in China. Only basally sclerotized paramerae are for the first time described in Pyropterus Mulsant, 1838. Key words: Coleoptera, Lycidae, New species, China

    Is nonhelical hydromagnetic turbulence peaked at small scales?

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    Nonhelical hydromagnetic turbulence without an imposed magnetic field is considered in the case where the magnetic Prandtl number is unity. The magnetic field is entirely due to dynamo action. The magnetic energy spectrum peaks at a wavenumber of about 5 times the minimum wavenumber in the domain, and not at the resistive scale, as has previously been argued. Throughout the inertial range the spectral magnetic energy exceeds the kinetic energy by a factor of about 2.5, and both spectra are approximately parallel. At first glance, the total energy spectrum seems to be close to k^{-3/2}, but there is a strong bottleneck effect and it is suggested that the asymptotic spectrum is k^{-5/3}. This is supported by the value of the second order structure function exponent that is found to be \zeta_2=0.70, suggesting a k^{-1.70} spectrum.Comment: 6 pages, 6 figure

    Self-similar turbulent dynamo

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    The amplification of magnetic fields in a highly conducting fluid is studied numerically. During growth, the magnetic field is spatially intermittent: it does not uniformly fill the volume, but is concentrated in long thin folded structures. Contrary to a commonly held view, intermittency of the folded field does not increase indefinitely throughout the growth stage if diffusion is present. Instead, as we show, the probability-density function (PDF) of the field strength becomes self-similar. The normalized moments increase with magnetic Prandtl number in a powerlike fashion. We argue that the self-similarity is to be expected with a finite flow scale and system size. In the nonlinear saturated state, intermittency is reduced and the PDF is exponential. Parallels are noted with self-similar behavior recently observed for passive-scalar mixing and for map dynamos.Comment: revtex, 4 pages, 5 figures; minor changes to match published versio

    A uniqueness result for the inverse problem of identifying boundaries from weighted Radon transform

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    We study the problem of the integral geometry, in which the functions are integrated over hyperplanes in the nn-dimensional Euclidean space, n=2m+1n=2m+1. The integrand is the product of a function of nn variables called the density and weight function depending on 2n2n variables. Such an integration is called here the weighted Radon transform, which coincides with the classical one if the weight function is equal to one. It is proved the uniqueness for the problem of determination of the surface on which the integrand is discontinuous.Comment: 10 pages, 1 figur

    Magnetic Field Amplification by Small-Scale Dynamo Action: Dependence on Turbulence Models and Reynolds and Prandtl Numbers

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    The small-scale dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus is expected to depend crucially on the nature of turbulence. In this work, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(l) ~ l^theta, where l and v(l) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e., the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB-approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rm_crit, using our model of the turbulent velocity correlation function. For Kolmogorov turbulence (theta=1/3), we find that the critical magnetic Reynolds number is approximately 110 and for Burgers turbulence (theta=1/2) approximately 2700. Furthermore, we derive that the growth rate of the small-scale magnetic field for a general type of turbulence is Gamma ~ Re^((1-theta)/(1+theta)) in the limit of infinite magnetic Prandtl numbers. For decreasing magnetic Prandtl number (down to Pm approximately larger than 10), the growth rate of the small-scale dynamo decreases. The details of this drop depend on the WKB-approximation, which becomes invalid for a magnetic Prandtl number of about unity.Comment: 13 pages, 8 figures; published in Phys. Rev. E 201

    Stochastic Flux-Freezing and Magnetic Dynamo

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    We argue that magnetic flux-conservation in turbulent plasmas at high magnetic Reynolds numbers neither holds in the conventional sense nor is entirely broken, but instead is valid in a novel statistical sense associated to the "spontaneous stochasticity" of Lagrangian particle tra jectories. The latter phenomenon is due to the explosive separation of particles undergoing turbulent Richardson diffusion, which leads to a breakdown of Laplacian determinism for classical dynamics. We discuss empirical evidence for spontaneous stochasticity, including our own new numerical results. We then use a Lagrangian path-integral approach to establish stochastic flux-freezing for resistive hydromagnetic equations and to argue, based on the properties of Richardson diffusion, that flux-conservation must remain stochastic at infinite magnetic Reynolds number. As an important application of these results we consider the kinematic, fluctuation dynamo in non-helical, incompressible turbulence at unit magnetic Prandtl number. We present results on the Lagrangian dynamo mechanisms by a stochastic particle method which demonstrate a strong similarity between the Pr = 1 and Pr = 0 dynamos. Stochasticity of field-line motion is an essential ingredient of both. We finally consider briefly some consequences for nonlinear MHD turbulence, dynamo and reconnectionComment: 29 pages, 10 figure

    Nonlinear magneto-optical resonances at D1 excitation of 85Rb and 87Rb in an extremely thin cell

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    Nonlinear magneto-optical resonances have been measured in an extremely thin cell (ETC) for the D1 transition of rubidium in an atomic vapor of natural isotopic composition. All hyperfine transitions of both isotopes have been studied for a wide range of laser power densities, laser detunings, and ETC wall separations. Dark resonances in the laser induced fluorescence (LIF) were observed as expected when the ground state total angular momentum F_g was greater than or equal to the excited state total angular momentum F_e. Unlike the case of ordinary cells, the width and contrast of dark resonances formed in the ETC dramatically depended on the detuning of the laser from the exact atomic transition. A theoretical model based on the optical Bloch equations was applied to calculate the shapes of the resonance curves. The model averaged over the contributions from different atomic velocity groups, considered all neighboring hyperfine transitions, took into account the splitting and mixing of magnetic sublevels in an external magnetic field, and included a detailed treatment of the coherence properties of the laser radiation. Such a theoretical approach had successfully described nonlinear magneto-optical resonances in ordinary vapor cells. Although the values of certain model parameters in the ETC differed significantly from the case of ordinary cells, the same physical processes were used to model both cases. However, to describe the resonances in the ETC, key parameters such as the transit relaxation rate and Doppler width had to be modified in accordance with the ETC's unique features. Agreement between the measured and calculated resonance curves was satisfactory for the ETC, though not as good as in the case of ordinary cells.Comment: v2: substantial changes and expanded theoretical model; 13 pages, 10 figures; accepted for publication in Physical Review
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