Two-sensor microtremor SPAC method: potential utility of imaginary spectrum components

2019 ◽  
Author(s):  
Ikuo Cho
Keyword(s):  
Imaginary Part ◽  
Quality Data ◽  
Potential Utility ◽  
Coefficient Estimates ◽  
Direction Parallel ◽  
Seismic Arrays ◽  
Site Field ◽  
Relative Errors ◽  
The Waves ◽  
Array Axis

Summary We build a model of discretization errors, known as directional aliasing, to theoretically evaluate how biases in the microtremor spatial autocorrelation (SPAC) coefficient, or the real part of the SPAC spectrum of microtremor analysis, are related to the magnitudes of the imaginary part when a seismic array of only two sensors is used. By using this model, we investigate the potential utility of the imaginary spectrum component as an indicator of applicability of the two-sensor SPAC method to the field of microtremors generated at an observation site. Field data of microtremors from compact seismic arrays (1–15 m) are used to test the model. It is found that, when the imaginary components are very large in magnitude (where the threshold depends on the rk, the array radius times the wavenumber), the field of microtremors is dominated by waves arriving from a single direction parallel to the array axis and the SPAC coefficients tend to be underestimated in small rk ranges (i.e. rk < 3.8; the range considered throughout this study). In the present study, which is based on the observations of 400 microtremor arrays, the underestimates seldom exceeded 30 per cent. The SPAC coefficient estimates could be corrected in that case by using information on the imaginary part. When the imaginary components are very modest in magnitude, by contrast, there are two possible scenarios: either (i) the waves are arriving predominantly from a single direction perpendicular to the array axis and the SPAC coefficients are wildly overestimated (i.e. there was a small percentage of low-quality data, with relative errors exceeding +50 per cent, based on the observed data analyses), or (ii) the wavefield is close to isotropic and the SPAC coefficients are unbiased (i.e. 70–90 per cent of all observed data fell within the relative error range of ±20 per cent). It is difficult in that case to have SPAC coefficient estimates corrected by using information on the imaginary part alone.

Multimode Resonators with a Small Fresnel Number (Lowest-Order Eigenmodes)

1965 ◽  
Vol 20 (1) ◽  
pp. 38-48 ◽  
Author(s):  
Helmut K. V. Lotsch
Keyword(s):  
Imaginary Part ◽  
Traveling Wave ◽  
General Type ◽  
Wave Type ◽  
Fresnel Number ◽  
Fabry Perot ◽  
Diffracted Waves ◽  
Resonator System ◽  
The Waves ◽  
Confocal Resonator

The Fabry-Pèrot Interferometer, the confocal and the spherical resonator systems are investigated. The lowest-order traveling-wave type eigenmodes are calculated. Numerical values for the diffraction losses are given. The smallest diffraction losses are obtained for the general-type eigenmode of a confocal resonator system. The eigenfunctions of an open-walled resonator show a point of inflection as their characteristic feature. They are complex if the Fresnel Number is finite. When calculate over appropriate surfaces, their imaginary part, in the region close to the axis, decreases as F increases. In that region the waves resonate between the reflectors. Towards the rim of the system the imaginary part increases rapidly as do the diffracted waves associated with the imaginary part.

Observations and simulations of foreshock waves during magnetic clouds

2020 ◽  
Author(s):  
Lucile Turc ◽  
Owen Roberts ◽  
Martin Archer ◽  
Minna Palmroth ◽  
Markus Battarbee ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Numerical Simulations ◽  
Wave Field ◽  
Geomagnetic Storms ◽  
Wave Activity ◽  
Bow Shock ◽  
Magnetic Clouds ◽  
Direction Parallel ◽  
High Magnetic Field Strength ◽  
The Waves

&lt;p&gt;The foreshock is a region of intense wave activity, situated upstream of the quasi-parallel sector of the terrestrial bow shock. The most common type of waves in the Earth's ion foreshock are quasi-monochromatic fast magnetosonic waves with a period of about 30 s. In this study, we investigate how the foreshock wave field is modified when magnetic clouds, a subset of coronal mass ejections driving the most intense geomagnetic storms, interact with near-Earth space. Using observations from the Cluster constellation, we find that the average period of the fast magnetosonic waves is significantly shorter than the typical 30 s during magnetic clouds, due to the high magnetic field strength inside those structures, consistent with previous works. We also show that the quasi-monochromatic waves are replaced by a superposition of waves at different frequencies. Numerical simulations performed with the hybrid-Vlasov model Vlasiator consistently show that an enhanced upstream magnetic field results in less monochromatic wave activity in the foreshock. The global view of the foreshock wave field provided by the simulation further reveals that the waves are significantly smaller during magnetic clouds, both in the direction parallel and perpendicular to the wave vector. We estimate the transverse extent of the waves using a multi-spacecraft analysis technique and find a good agreement between the numerical simulations and the spacecraft measurements. This suggests that the foreshock wave field is structured over smaller scales during magnetic clouds. These modifications of the foreshock wave properties are likely to affect the regions downstream - the bow shock, the magnetosheath and possibly the magnetosphere - as foreshock waves are advected earthward by the solar wind.&lt;/p&gt;

A note on the formation of Debye potential well in current-carrying plasmas

1971 ◽  
Vol 6 (1) ◽  
pp. 223-228 ◽  
Author(s):  
J. R. Kan
Keyword(s):  
Thermal Equilibrium ◽  
Resonant Interaction ◽  
Landau Damping ◽  
Potential Well ◽  
Direction Parallel ◽  
Debye Potential ◽  
Collective Interaction ◽  
Test Charge ◽  
Net Flux ◽  
The Waves

The field of a test charge at rest in a plasma is known to be shielded via the collective interaction between the test charge and the particles of the plasma. In a thermal equilibrium plasma, the shielded potential has a spatial variation of the Debye form exp (— r)/r. In this note we will show that, in a collisionless current-carrying plasma, the potential profile along the radial direction parallel to the current changes from the Debye form to a potential well form, and causes a reversal of the ele ctric field of the test charge. The formation of the Debye potential well is attributed to the resonant interaction between the particles and the waves stationary in the test charge frame when the net flux of charged particles is different from zero. This is the same type of resonant interaction which leads to the well-known Landau damping (Landau 1946).

Anisotropic Optical Properties of 2D Silicon Telluride

MRS Advances ◽  
2020 ◽  
Vol 5 (35-36) ◽  
pp. 1881-1889 ◽  
Author(s):  
Romakanta Bhattarai ◽  
Jiyang Chen ◽  
Thang B. Hoang ◽  
Jingbiao Cui ◽  
Xiao Shen
Keyword(s):  
Dielectric Constant ◽  
Imaginary Part ◽  
Optical Anisotropy ◽  
Optical Measurement ◽  
Computational Study ◽  
Gw Approximation ◽  
Direction Parallel ◽  
New Device ◽  
Out Of Plane ◽  
Potential Applications

ABSTRACTSilicon telluride (Si2Te3) is a silicon-based 2D chalcogenide with potential applications in optoelectronics. It has a unique crystal structure where Si atoms form Si-Si dimers to occupy the “metal” sites. In this paper, we report an ab initio computational study of its optical dielectric properties using the GW approximation and the Bethe-Salpeter equation (BSE). Strong in-plane optical anisotropy is discovered. The imaginary part of the dielectric constant in the direction parallel to the Si-Si dimers is found to be much lower than that perpendicular to the dimers. The optical measurement of the absorption spectra of 2D Si2Te3 nanoplates shows modulation of the absorption coefficient under 90-degree rotation, confirming the computational results. We show the optical anisotropy originates from the particular compositions of the wavefunctions in the valence and conduction bands. Because it is associated with the Si dimer orientation, the in-plane optical anisotropy can potentially be dynamically controlled by electrical field and strain, which may be useful for new device design. In addition, BSE calculations reduce GW quasiparticle band gap by 0.3 eV in bulk and 0.6 eV in monolayer, indicating a large excitonic effect in Si2Te3. Furthermore, including electron-hole interaction in bulk calculations significantly reduces the imaginary part of the dielectric constant in the out-of-plane direction, suggesting strong interlayer exciton effect in Si2Te3 multilayers.

scholarly journals Characteristics of Nonstatic Quantum Light Waves: The Principle for Wave Expansion and Collapse

Photonics ◽  
2021 ◽  
Vol 8 (5) ◽  
pp. 158
Author(s):  
Jeong Ryeol Choi
Keyword(s):  
Imaginary Part ◽  
Exponential Function ◽  
Time Varying ◽  
Electromagnetic Parameters ◽  
Fock States ◽  
Wave Expansion ◽  
Light Waves ◽  
The Waves ◽  
Periodical Change

Nonstatic quantum light waves arise in time-varying media in general. However, from a recent report, it turned out that nonstatic waves can also appear in a static environment where the electromagnetic parameters of the medium do not vary in time. Such waves in Fock states exhibit a belly and a node in turn periodically in the graphic of their evolution. This is due to the wave expansion and collapse in quadrature space, which manifest a unique nonstaticity of the wave. The principle for wave expansion and collapse is elucidated from rigorous analyses for the basic nonstatic waves which are dissipative and amplifying ones. The outcome of wave nonstaticity can be interpreted in terms of the coefficient of the quadratic exponent in the exponential function appearing in the wave eigenfunction; if the imaginary part of the coefficient is positive, the wave expands, whereas the wave collapses when it is negative. Using this principle, we further analyze novel nonstatic properties of light waves which exhibit complicated time behaviors, i.e., for the case that the waves not only undergo the periodical change of nodes and bellies but their envelopes exhibit gradual dissipation/expansion as well.

Numerical study of internal wave reflection from sloping boundaries

1999 ◽  
Vol 396 ◽  
pp. 183-201 ◽  
Author(s):  
A. JAVAM ◽  
J. IMBERGER ◽  
S. W. ARMFIELD
Keyword(s):  
Internal Wave ◽  
Critical Frequency ◽  
Wave Reflection ◽  
Numerical Study ◽  
Buoyancy Frequency ◽  
Direction Parallel ◽  
Reflected Waves ◽  
Reflected Wave ◽  
Resonant Interactions ◽  
The Waves

The breaking of internal waves propagating in a stratified fluid of constant buoyancy frequency on a sloping boundary was investigated numerically. It was found that at the boundary, nonlinear non-resonant interactions between the incident and reflected waves produced higher-mode waves. These modes had frequencies greater than the local buoyancy frequency and so could not radiate from the interaction region. The energy level of trapped waves increased with time and subsequently led to overturning of the density field. At the critical frequency, when the reflected wave propagated in a direction parallel to the slope, wave overturning occurred near the wall, but the point of overturning moved off the bottom as the propagation angle changed away from that of the bottom slope as the waves became increasingly supercritical. The internal wave reflection coefficient generally increased as the effects of nonlinearity and viscosity decreased, but depended strongly on the forcing frequency and the angle of the sloping boundary.

Analytic integration of contrast transfer functions

1988 ◽  
Vol 46 ◽  
pp. 822-823
Author(s):  
Peter Rez
Keyword(s):  
Wave Function ◽  
Transfer Function ◽  
Transfer Functions ◽  
Spherical Aberration ◽  
Continuous Distribution ◽  
Objective Lens ◽  
Direction Parallel ◽  
Scattering Angle ◽  
Analytic Integration ◽  
Image Position

In high resolution microscopy the image amplitude is given by the convolution of the specimen exit surface wave function and the microscope objective lens transfer function. This is usually done by multiplying the wave function and the transfer function in reciprocal space and integrating over the effective aperture. For very thin specimens the scattering can be represented by a weak phase object and the amplitude observed in the image plane is1where fe (Θ) is the electron scattering factor, r is a postition variable, Θ a scattering angle and x(Θ) the lens transfer function. x(Θ) is given by2where Cs is the objective lens spherical aberration coefficient, the wavelength, and f the defocus.We shall consider one dimensional scattering that might arise from a cross sectional specimen containing disordered planes of a heavy element stacked in a regular sequence among planes of lighter elements. In a direction parallel to the disordered planes there will be a continuous distribution of scattering angle.

Evidence-Based Practice & Practice-Based Evidence Applied to Adult, Medical Speech-Language Pathology

2013 ◽  
Vol 18 (1) ◽  
pp. 14-26 ◽  
Author(s):  
Rik Lemoncello ◽  
Bryan Ness
Keyword(s):  
Evidence Based Practice ◽  
Single Case ◽  
Quality Data ◽  
Evidence Based ◽  
Speech Language Pathology ◽  
Clinical Practices ◽  
High Quality ◽  
Randomized Controlled Clinical Trials ◽  
Relative Paucity ◽  
Language Pathology

In this paper, we review concepts of evidence-based practice (EBP), and provide a discussion of the current limitations of EBP in terms of a relative paucity of efficacy evidence and the limitations of applying findings from randomized controlled clinical trials to individual clinical decisions. We will offer a complementary model of practice-based evidence (PBE) to encourage clinical scientists to design, implement, and evaluate our own clinical practices with high-quality evidence. We will describe two models for conducting PBE: the multiple baseline single-case experimental design and a clinical case study enhanced with generalization and control data probes. Gathering, analyzing, and sharing high-quality data can offer additional support through PBE to support EBP in speech-language pathology. It is our hope that these EBP and PBE strategies will empower clinical scientists to persevere in the quest for best practices.

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