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Results for “ELECTROMAGNETIC SCATTERING”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 451 records · Page 25
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NASA NTRS1987

Effective propagation constants in dense random media under effective medium approximation

The effective medium approximation is applied to study the effective propagation constants in a dense random medium. The dyadic Green's function is introduced to establish the effective medium approximation formalism for electromagnetic waves. The multiple scattering equations and the Lippmann-Schwinger equations for the transition operator are obtained in the configuration average form. The dispersion equations of multiple scattering is derived by using a standard method in quantum mechanics. To obtain an expression for the effective propagation constants the matrix elements of the configuration average dyadic transiton operator are calculated in momentum representation. Numerical illustations are carried out to demonstrate the difference in the effective propagation constants between the use of this approximation and the well-known quasi-crystalline approximation. A comparison is made with measured loss tangent in dry snow.

Zhu, P. Y.↗
NASA NTRS1987

Wind-fetch dependence of Seasat scatterometer measurements

This work is focused on the effects of large-scale wind-generated gravity waves on scatterometer wind-measurement accuracy. Theoretical and experimental evidence of the important role played by the degree of wind-wave coupling in surface geometry and therefore in microwave signatures is presented. Seasat scatterometer wind-measurement errors are analyzed, and an error bias is found to be related to the degree of wind-wave development. Attention is focused on a dynamic measure of wind-wave development that characterizes the rate of energy transfer from the mean wind to the energy-carrying portion of the wave spectrum. An explanation of the bias is suggested based on the consideration of an additional component of surface scattering caused by electromagnetic-wave diffraction at the crests of individual, sufficiently steep wavelets.

Glazman, Roman E.↗
NASA NTRS1992

Interpreting asteroid photometry and polarimetry using a model of shadowing and coherent backscattering

The shadow-hiding models for the opposition effect and negative polarization of atmosphereless solar system bodies do not explain some experimental findings, such as the enhancing opposition effect and negative polarization with decreasing particle size down to wavelength scales. The enhancement for laboratory photometric and polarimetric data on artificial glass samples with different particle size is shown. These results are in agreement with the so-called coherent backscattering or interference mechanism proposed for the interpretation of the opposition effect and negative polarization. Two different approaches for describing the opposition effect and negative polarization produced by the shadow-interference mechanism were developed. One is based on exact electromagnetic solutions for simple scattering systems that include dipole-dipole and dipole-surface coupling; The other is based on a point-scatterer approximation characterized by model photometric and polarimetric phase functions, and the mutual shadowing effect is derived using virtual volumes associated with the point-scatterers. Both approaches yield qualitatively similar results, although neither is entirely satisfactory. We regard them as prototypes for a future unified model of shadowing and coherent backscattering. The sharp opposition effect of 44 Nysa and the asteroid albedo-polarization rule are here explained using the point-scatterer approach.

Shkuratov, Y. G.↗
NASA NTRS1993

A time domain, weighted residual formulation of Maxwell's equations

A finite element model is developed and used to simulate two-dimensional electromagnetic wave propagation and scattering. The spatial discretization of the time-domain electrodynamic equations is accomplished by a Galerkin approach. The semi-discrete equations are solved explicitly using a second-order Runge-Kutta scheme. Both the electric and magnetic fields are discretized using a single grid, with the divergence-free conditions satisfied through a correction approach. Examples depicting the scattering of plane waves in 2D geometries are given to demonstrate the validity of the methodology.

Young, Jeffrey L.↗
NASA NTRS2007

Paired Pulse Basis Functions for the Method of Moments EFIE Solution of Electromagnetic Problems Involving Arbitrarily-shaped, Three-dimensional Dielectric Scatterers

A pair of basis functions is presented for the surface integral, method of moment solution of scattering by arbitrarily-shaped, three-dimensional dielectric bodies. Equivalent surface currents are represented by orthogonal unit pulse vectors in conjunction with triangular patch modeling. The electric field integral equation is employed with closed geometries for dielectric bodies; the method may also be applied to conductors. Radar cross section results are shown for dielectric bodies having canonical spherical, cylindrical, and cubic shapes. Pulse basis function results are compared to results by other methods.

MacKenzie, Anne I.↗
NASA NTRS2018

An Empirical Model of Radiation Belt Electron Pitch Angle Distributions Based On Van Allen Probes Measurements

Based on over 4 years of Van Allen Probes measurements, an empirical model of radiation belt electron equatorial pitch angle distribution (PAD) is constructed. The model, developed by fitting electron PADs with Legendre polynomials, provides the statistical PADs as a function of L-shell (L = 1-6), magnetic local time, electron energy (~30 keV to 5.2 MeV), and geomagnetic activity (represented by the Dst index) and is also the first empirical PAD model in the inner belt and slot region. For megaelectron volt electrons, model results show more significant day-night PAD asymmetry of electrons with higher energies and during disturbed times, which is caused by geomagnetic field configuration and flux radial gradient changes. Steeper PADs with higher fluxes around 90° pitch angle and lower fluxes at lower pitch angles for higher energy electrons and during active times are also present, which could be due to electromagnetic ion cyclotron wave scattering. For hundreds of kiloelectron volt electrons, cap PADs are generally present in the slot region during quiet times and their energy-dependent features are consistent with hiss wave scattering, while during active times, cap PADs are less significant especially at outer part of slot region, which could be due to the complex energizing and transport processes. The 90°-minimum PADs are persistently present in the inner belt and appear in the slot region during active times, and minima at 90° pitch angle are more significant for electrons with higher energies, which could be a critical evidence in identifying the underlying physical processes responsible for the formation of 90°-minimum PADs.

Zhao, H.↗
NASA NTRS2019

EMIC Wave-Driven Bounce Resonance Scattering of Energetic Electrons in the Inner Magnetosphere

While electromagnetic ion cyclotron (EMIC) waves have been long studied as a scattering mechanism for ultrarelativistic (megaelectron volt) electrons via cyclotron‐resonant interactions, these waves are also of the right frequency to resonate with the bounce motion of lower‐energy (approximately tens to hundreds of kiloelectron volts) electrons. Here we investigate the effectiveness of this bounce resonance interaction to better determine the effects of EMIC waves on subrelativistic electron populations in Earth's inner magnetosphere. Using wave and plasma parameters directly measured by the Van Allen Probes, we estimate bounce resonance diffusion coefficients for four different events, illustrative of wave and plasma parameters to be encountered in the inner magnetosphere. The range of electron energies and pitch angles affected is examined to better assess the realistic effects of EMIC‐driven bounce resonance on energetic electron populations based on actual, locally observed event‐based parameters. Significant local diffusion coefficients (~ > 10(exp −6) s(exp −1)) for 50‐ to 100‐keV electrons are achieved for both H+ band wave events as well as He+ band, with diffusion coefficients peaking for near‐90° pitch angles but remaining elevated for intermediate ones as well. Diffusion coefficients for higher‐energy 200‐keV electrons are typically multiple orders of magnitude lower (ranging from 10(exp −11) to 10(exp −6) s(exp −1)) and often peak at lower pitch angles (~20–30°). These results suggest that both H+ and He+ band EMIC waves can play a role in shaping lower‐energy electron dynamics via bounce‐resonant interactions, in addition to their role in relativistic electron loss via cyclotron resonance.

Blum, L. W.↗
NASA NTRS1991

Experimental measurements of Space Station antenna patterns at 60 GHz for EM analysis verification

Complex body scattering is a major problem facing the electromagnetic researcher today. Computer codes are one important method for predicting such scattering. With funding from Langley, Ohio State University has developed such a code. A 30:1 scale model of the Space Station was constructed to be used as a scattering target in the verification of this code. The purpose here is to document the methods used to make these measurements and present the results which will be used by others for code verification.

Young, W. Robert↗
NASA NTRS1973

Biomass in the upwelling areas along the northwest coast of Africa as viewed with ERTS-1

Light penetration in water is affected by plankton, algae, and dissolved and suspended matter. As a consequence, the composition of backscattered light from below the air-sea interface is determined by the nature of the constituents in the water column. In contrast to the absorption spectrum of chemically pure chlorophyll in solution, algae suspensions absorb and scatter light more uniformly throughout the visible part of the electromagnetic spectrum. Because of their spectral absorption and scattering properties plankton concentration can be estimated by measuring the spectral backscattered radiance over water. Experiments using this approach were performed in upwelling regions along the northwest coast of Africa.

Szekielda, K.↗
NASA NTRS2018

Radiative Transfer in a Discrete Random Medium Adjacent to a Half-Space with a Rough Interface

For a macroscopically plane-parallel discrete random medium, the boundary conditions for the specific coherency dyadic at a rough interface are derived. The derivation is based on a modification of the Twersky approximation for a scattering system consisting of a group of particles and the rough surface, and reduces to the solution of the scattering problem for a rough surface illuminated by a plane electromagnetic wave propagating in a discrete random medium with non-scattering boundaries. In a matrix-form setting, the boundary conditions for the specific coherency dyadic imply the boundary conditions for specific intensity column vectors which in turn, yield the expressions for the reflection and transmission matrices. The derived expressions are shown to be identical to those obtained by applying a phenomenological approach based on a facet model to the solution of the scattering problem for a rough surface illuminated by a plane electromagnetic wave.

Adrian Doicu↗
NASA NTRS1992

Experimental measurement and theoretical modeling of microwave scattering and the structure of the sea surface influencing radar observations from space

The electromagnetic bias is an error present in radar altimetry of the ocean due to the non-uniform reflection from wave troughs and crests. A study of the electromagnetic bias became necessary to permit error reduction in mean sea level measurements of satellite radar altimeters. Satellite radar altimeters have been used to find the upper and lower bounds for the electromagnetic bias. This report will present a theory using physical optics scattering and an empirical model of the short wave modulation to predict the electromagnetic bias. The predicted electromagnetic bias will be compared to measurements at C and Ku bands.

Arnold, David↗
NASA NTRS2003

Resonant Spectra of Malignant Breast Cancer Tumors Using the Three-Dimensional Electromagnetic Fast Multipole Model

An intensive numerical study for the resonance scattering of malignant breast cancer tumors is presented. The rigorous three-dimensional electromagnetic model, based on the equivalence theorem, is used to obtain the induced electric and magnetic currents on the breast and tumor surfaces. The results show that a non-spherical malignant tumor can be characterized based its spectra regardless of its orientation, the incident polarization, or the incident or scattered directions. The tumor's spectra depend solely on its physical characteristics (i.e., the shape and the electrical properties), however, their locations are not functions of its burial depth. This work provides a useful guidance to select the appropriate frequency range for the tumor's size.

El-Shenawee, Magda↗
NASA NTRS1983

Scattering from a random layer with applications to snow, vegetation and sea ice

Two approaches for computing scattering from a random layer with an irregular interface are shown using the radiative-transfer principle. One approach is applied to a random layer to develop a scattering model for snow and sea ice, while the other is used to generate a scattering model for vegetation. It is noted that to model the scattering characteristics of a special medium, it is necessary to relate the electromagnetic parameters to the measurable parameters of the scatterers in the medium. Such relations are given and used to calculate theoretical estimates to compare with measurements acquired by microwave scatterometers.

Fung, A. K.↗
NASA NTRS1988

Coherent electromagnetic waves in the presence of a half space of randomly distributed scatterers

The present investigation of coherent field propagation notes, upon solving the Foldy-Twersky integral equation for a half-space of small spherical scatterers illuminated by a plane wave at oblique incidence, that the coherent field for a horizontally-polarized incident wave exhibits reflectivity and transmissivity consistent with the Fresnel formula for an equivalent continuous effective medium. In the case of a vertically polarized incident wave, both the vertical and longitudinal waves obtained for the coherent field have reflectivities and transmissivities that do not agree with the Fresnel formula.

Karam, M. A.↗
NASA NTRS1991

Microwave absorption by a lossy dielectric sphere in a rectangular cavity

A new theory of absorption of microwave power by a lossy dielectric sphere in a resonant rectangular chamber is developed here. In this treatment, resonant modes of the cavity are resolved into traveling plane-wave fields, and Mie's theory of scattering and absorption of a plane electromagnetic wave by a sphere is utilized. The fields in the cavity containing the sphere are found by superposition of results for the individual scattering problem for each traveling plane wave. Absorbed power is then calculated by evaluating the time-averaged Poynting vector and integrating over the surface of the sphere. Applications of microwave power absorption are discussed, with particular emphasis on processing of materials. Numerical studies based on the newly derived formulas are presented. The results exhibit the manner in which microwave absorption is influenced by a number of different experimentally controllable parameters. The examples considered were chosen with a view toward designing experiments and apparatus for containerless processing of materials in the microgravity environment of space.

Jackson, H. W.↗
NASA NTRS1986

Microwave emission and scattering from Earth surface and atmosphere

Nonlinear Electromagnetic (EM) wave interactions with the upper atmosphere were investigated during the period 15 December 1985 to 15 June 1986. Topics discussed include: the simultaneous excitation of ionospheric density irregularities and Earth's magnetic field fluctuations; the electron acceleration by Langmuir wave turbulence; and the occurrence of artificial spread F. The role of thermal effects in generating ionospheric irregularities by Whistler waves, intense Quasi-DC electric fields, atmospheric gravity waves, and electrojets was investigated. A model was developed to explain the discrete spectrum of the resonant ultralow frequency (ULF) waves that are commonly observed in the magnetosphere.

Kong, J. A.↗