Multiple Scattering of Electromagnetic Waves by an Array of Parallel Gyrotropic Rods

We study multiple scattering of electromagnetic waves by an array of parallel gyrotropic circular rods and show that such an array can exhibit fairly unusual scattering properties and provide, under certain conditions, a giant enhancement of the scattered field. Among the scattering patterns of such an array at its resonant frequencies, the most amazing is the distribution of the total field in the form of a perfect self-similar structure of chessboard type. The scattering characteristics of the array are found to be essentially determined by the resonant properties of its gyrotropic elements and cannot be realized for arrays of nongyrotropic rods. It is expected that the results obtained can lead to a wide variety of practical applications.Comment: 5 pages, 6 figure

Photoproduction des mesons D dans l'experience NA14 : un test de QCD

SIGLECNRS T 61722 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Hamiltonian models for resonant wave-particle interaction processes in magnetized and inhomogeneous plasmas

The kinetic theory of plasmas, based on the Vlasov-Poisson system of equations, can efficiently solve only some aspects of the extremely large panel of problems involving wave-particle and wave-wave interaction processes in plasmas. Therefore the dynamics of charged particles and waves has been modeled by other approaches as, for example, Hamiltonian models describing the self-consistent wave-particle and wave-wave interactions in homogeneous or inhomogeneous magnetized plasmas. Various physical problems could be efficiently studied by such methods, concerning nonlinear and turbulent stages of different instabilities of electron or ion distributions, wave packets’ saturation and particles fluxes’ relaxation processes, particle trapping and detrapping mechanisms by waves, wave-particle interactions at multiple resonances, quasilinear diffusion processes of particles in waves, wave turbulence in randomly inhomogeneous plasmas, acceleration of particles, wave focusing, scattering, reflection and decay, etc. In particular, the aim of the paper, after a brief description of such Hamiltonian models, is to present the most recent simulation results obtained when studying Langmuir turbulence in the presence of electron beams propagating in inhomogeneous plasmas as the solar wind, where random density fluctuations with average levels up to several percents of the background plasma density have been measured

Mycoplasma pneumonia in children and adolescents

Mycoplasma pneumoniae is a common cause of community-acquired pneumonia in children and adolescents and needs to be recognized by physicians. Findings associated with M pneumoniae infection are nonspecific and difficult to distinguish from other causes of respiratory tract infections. Chest radiography is nonspecific and findings are variable. (4)(5)(6)(7) No firm recommendation exists at this time, but testing for children with suspected M pneumoniae pneumonia should be reserved for those who are hospitalized or in whom testing will alter management. (2)(16) Based on evidence, choice of test depends on the location of the patient (inpatient versus outpatient), availability, and cost. The currently recommended method of diagnosis is by polymerase chain reaction testing, but based on cost and availability, this should be reserved for inpatients. (2)(16) Treatment of M pneumoniae is best accomplished with azithromycin in children whose clinical picture is consistent with atypical pneumonia, but other regimens are available

Electromagnetic Wave Emissions from a Turbulent Plasma with Density Fluctuations

International audienceIn the solar wind, Langmuir turbulence can generate electromagnetic waves at the fundamental plasma frequency omega p . This process can likely result from either linear wave transformations on the ambient random density inhomogeneities or resonant three-wave interactions involving Langmuir waves and ion acoustic oscillations. In the presence of sufficiently intense plasma density fluctuations of scales much larger than the Langmuir wavelengths, the first mechanism may be more efficient than the second one. A new approach to calculate the electromagnetic wave emissions by Langmuir wave turbulence in plasmas with background density fluctuations is developed. The evolution of the Langmuir turbulence is studied by numerically solving the Zakharov equations in such a two-dimensional plasma. The dynamics of the spatial distributions of the electric currents with frequencies close to omega p is calculated, as well as their emission into electromagnetic waves. The efficiency of this radiation is determined as a function of the level of the Langmuir turbulence, the characteristics of the density fluctuations, the background plasma temperature, the position of the satellite receiver, and the durations of the source's emissions and spacecraft's observations. The results obtained by the theoretical modeling and numerical simulations are successfully compared with space observations of electromagnetic waves radiated during Type III solar radio bursts

Propagation of VLF waves guided by plane density trough in the magnetosphere

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Carbon-Based Composite Microwave Antennas

International audienceApplications of metamaterials to microwave antennas are reviewed over the past decade. The manufacturing of microwave antennas using graphene-containing carbon composite materials was developed and prototypes of dipole and horn antennas made from such materials were created. The radiation properties of the designed antennas and their metal analogs were measured and compared. The standing wave ratios, the radiation patterns and the amplitude-frequency characteristics were analyzed for horn antennas at frequencies 1.6 GHz and 5 GHz and for dipole antennas in the frequency range 0.2–0.6 GHz. The polarization characteristics of the horn antennas were studied. The effects of different carbon composite materials’ structures (fiber or fabric) on the antennas’ parameters were estimated. It is shown that antennas made from graphene-containing composite materials are able to operate efficiently and exhibit almost the same radiation properties as conventional metal antennas of the same geometry and size. However, the carbon-based antennas have much smaller weights and enhanced stability in a wide range of temperatures. In the future, such antennas should replace the conventional ones for many applications, especially for the excitation and reception of electromagnetic waves in space plasmas

THEORY OF A STRIP ANTENNA LOCATED AT THE INTERFACE OF AN ISOTROPIC MEDIUM AND A MAGNETOPLASMA

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Differentiating SCT and inattentive symptoms in ADHD using fMRI measures of cognitive control.

Attention Deficit/Hyperactivity Disorder (ADHD) is associated with different impairment profiles in the symptom domains of hyperactivity/impulsivity and/or inattention. An additional symptom domain of sluggish cognitive tempo (SCT) has also been proposed. Although there is a degree of correlation between the SCT symptom domain and inattention, it has been proposed as a distinct disorder independent of ADHD. The objective of this study was to examine the neural substrates of cue-related preparatory processes associated with SCT symptoms versus inattentive symptoms in a group of adolescents with ADHD. We also compared cue-related effects in the entire ADHD group compared with a group of typically developing (TD) peers. A modified cued flanker paradigm and fMRI examined brain activity associated with attention preparation and motor response preparation. Between group contrasts between the ADHD and TD group revealed significant hypoactivity in the ADHD group during general attention preparation in the supplementary motor area (SMA) and in the right superior parietal lobe (SPL) during response preparation. In the ADHD group, greater numbers of SCT symptoms were associated with hypoactivity in the left SPL to cues in general whereas greater numbers of inattentive symptoms were associated with greater activity in the SMA to cues that provided no information and less activity in the thalamus during response preparation. Hypoactivity in the SPL with increasing SCT symptoms may be associated with impaired reorienting or shifting of attention. Altered activity in the SMA and thalamus with increasing inattention may be associated with a general problem with response preparation, which may also reflect inefficient processing of the response preparation cue. Our results support a degree of differentiation between SCT and inattentive symptom profiles within adolescents with ADHD

Nonlinear interaction of whistler waves in a magnetized plasma with density ducts

International audienceThe nonlinear resonant interactions between whistler waves guided bydensity ducts surrounded by a uniform magnetized plasma are studied. Itis shown that, under specific conditions that are determined, a time-harmonic external electromagnetic field can drive the parametricinstability of guided whistlers. Both cases of cylindrical and planarducts are considered, of either decreased or increased plasma density.The frequency interval where the magnetized plasma may be resonant isanalyzed. The growth rate and the threshold of the parametricinstability are determined. Numerical calculations are presented forparameters typical of "space plasmas and" modeling laboratoryexperiments where guided whistler propagation was observed