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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 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Radiative transfer theory for inhomogeneous media with random extinction and scattering coefficients

The small-angle scattering approximation of the scalar radiative transfer equation (RTE) is examined for the case where the extinction and scattering coefficients have a component that is a deterministic function of position along the propagation path and a component that is a random function of position transverse to the propagation direction. It is found that the resulting stochastic RTE can be reduced to a system of two stochastic integrodifferential equations for the average and fluctuating components of the radiant intensity. Two transfer equations are obtained describing the average radiant intensity and the spatial correlation function of the intensity fluctuations. The average intensity equation is then solved and applied to a simple propagation scenario. An approximate solution is also derived for the equation giving the correlation function. The developed equations can be applied to problems involving short wavelength electromagnetic wave propagation through media possessing the variable characteristics of turbulence and turbidity, such as plasmas, the atmosphere, and the ocean.

Manning, Robert M.↗
NASA NTRS1990

Scattering properties of woven fibrous insulations - Effects of interference for normal incidence

This paper examines the scattering properties of woven fibrous materials in order to model the interactions between the scattered radiation from individual fibers. A normally incident plane electromagnetic wave is considered and different representative geometries are analyzed theoretically. Experimental results are presented for specific woven thermal insulation fabrics. Previous studies in the literature have mostly ignored the effect of constructive or destructive addition of scattered waves from individual fibers with the exception of the case of parallel fibers. Whereas the light-scattering characteristics of a single long fiber depend only on the size parameter and on the material optical constants, the additional mechanism of interference between the scattered waves from fibers of parallel or perpendicular orientation introduces an additional dependence on fiber locations. In this study, analytical models are developed for obtaining the radiative scattering characteristics of woven fibrous media taking into account the mechanism of interference. Experimental measurements of forward- and backward-scattered light from a woven silica cloth over a broad range of wavelengths in the infrared are obtained and compared with the predicted results for a system of two orthogonal families of infinite parallel fibers.

Kumar, Sunil↗
NASA NTRS1990

Scattering by an impedance sphere coated with a chiral layer

The scattering of a plane, linearly polarized electromagnetic wave by a sphere on whose surface an impedance boundary condition holds, and that is covered with a concentric layer of chiral material, is considered. Exact, explicit expressions are derived for the scattered field coefficients. The co-polarized and cross-polarized components of the far backscattered field are determined and discussed. The value of this canonical problem as a benchmark for computer codes is pointed out.

Uslenghi, Piergiorgio L. E.↗
NASA NTRS1991

Effects Of Interference On Scattering By Parallel Fibers

Report discusses radiative transfer of heat through fibrous materials, focusing on interactions between electromagnetic waves scattered from individual fibers. Equations for coherent addition of scattered waves presented and used to derive equations for intensity of scattered radiation, with emphasis on cases of evenly spaced coplanar fibers and randomly positioned fibers. Theory predicts scattering efficiency of randomly positioned fibers less than that predicted by independent-scattering theory.

White, Susan M.↗
NASA NTRS1990

Remote sensing of Earth terrain

Remote sensing of earth terrain is examined. The layered random medium model is used to investigate the fully polarimetric scattering of electromagnetic waves from vegetation. The model is used to interpret the measured data for vegetation fields such as rice, wheat, or soybean over water or soil. Accurate calibration of polarimetric radar systems is essential for the polarimetric remote sensing of earth terrain. A polarimetric calibration algorithm using three arbitrary in-scene reflectors is developed. In the interpretation of active and passive microwave remote sensing data from the earth terrain, the random medium model was shown to be quite successful. A multivariate K-distribution is proposed to model the statistics of fully polarimetric radar returns from earth terrain. In the terrain cover classification using the synthetic aperture radar (SAR) images, the applications of the K-distribution model will provide better performance than the conventional Gaussian classifiers. The layered random medium model is used to study the polarimetric response of sea ice. Supervised and unsupervised classification procedures are also developed and applied to synthetic aperture radar polarimetric images in order to identify their various earth terrain components for more than two classes. These classification procedures were applied to San Francisco Bay and Traverse City SAR images.

Kong, Jin AU↗
NASA NTRS1990

X ray sensitive area detection device

A radiation sensitive area detection device is disclosed which comprises a phosphor-containing film capable of receiving and storing an image formed by a pattern of incoming x rays, UV, or other radiation falling on the film. The device is capable of fluorescing in response to stimulation by a light source in a manner directly proportional to the stored radiation pattern. The device includes: (1) a light source capable of projecting light or other appropriate electromagnetic wave on the film so as to cause it to fluoresce; (2) a means to focus the fluoresced light coming from the phosphor-containing film after light stimulation; and (3) at least one charged coupled detector or other detecting element capable of receiving and digitizing the pattern of fluoresced light coming from the phosphor-containing film. The device will be able to generate superior x ray images of high resolution from a crystal or other sample and will be particularly advantageous in that instantaneous near-real-time images of rapidly deteriorating samples can be obtained. Furthermore, the device can be made compact and sturdy, thus capable of carrying out x ray or other radiation imaging under a variety of conditions, including those experienced in space.

Carter, Daniel C.↗
NASA NTRS1990

Analytical study of the effects of clouds on the light produced by lightning

Researchers consider the scattering of visible and infrared light due to lightning by cubic, cylindrical and spherical clouds. The researchers extend to cloud physics the work by Twersky for single and multiple scattering of electromagnetic waves. They solve the interior problem separately to obtain the bulk parameters for the scatterer equivalent to the ensemble of spherical droplets. With the interior solution or the equivalent medium approach, the multiple scattering problem is reduced to that of a single scatterer in isolation. Hence, the computing methods of Wiscombe or Bohren specialized to Mie scattering with the possibility for absorption were used to generate numerical results in short computer time.

Phanord, Dieudonne D.↗
NASA NTRS1991

Radiation sensitive area detection device and method

A radiation sensitive area detection device for use in conjunction with an X ray, ultraviolet or other radiation source is provided which comprises a phosphor containing film which releases a stored diffraction pattern image in response to incoming light or other electromagnetic wave. A light source such as a helium-neon laser, an optical fiber capable of directing light from the laser source onto the phosphor film and also capable of channelling the fluoresced light from the phosphor film to an integrating sphere which directs the light to a signal processing means including a light receiving means such as a photomultiplier tube. The signal processing means allows translation of the fluoresced light in order to detect the original pattern caused by the diffraction of the radiation by the original sample. The optical fiber is retained directly in front of the phosphor screen by a thin metal holder which moves up and down across the phosphor screen and which features a replaceable pinhole which allows easy adjustment of the resolution of the light projected onto the phosphor film. The device produces near real time images with high spatial resolution and without the distortion that accompanies prior art devices employing photomultiplier tubes. A method is also provided for carrying out radiation area detection using the device of the invention.

Carter, Daniel C.↗
NASA NTRS1991

A dynamic analysis of the radiation excitation from the activation of a current collecting system in space

Current collecting systems moving in the ionosphere will induce electromagnetic wave radiation. The commonly used static analysis is incapable of studying the situation when such systems undergo transient processes. A dynamic analysis has been developed, and the radiation excitation processes are studied. This dynamic analysis is applied to study the temporal wave radiation from the activation of current collecting systems in space. The global scale electrodynamic interactions between a space-station-like structure and the ionospheric plasma are studied. The temporal evolution and spatial propagation of the electric wave field after the activation are described. The wave excitations by tethered systems are also studied. The dependencies of the temporal Alfven wave and lower hybrid wave radiation on the activation time and the space system structure are discussed. It is shown that the characteristics of wave radiation are determined by the matching of two sets of characteristic frequencies, and a rapid change in the current collection can give rise to substantial transient radiation interference. The limitations of the static and linear analysis are examined, and the condition under which the static assumption is valid is obtained.

Wang, J.↗
NASA NTRS1991

The interaction of collisionless shocks in astrophysical plasmas

The interaction between collisionless shocks, such as may arise in a number of situations in space plasmas, is examined by means of hybrid numerical simulations. The production of energetic particles by shock collisions is investigated. When the shocks are quasi-perpendicular, ions with energies greater than 10E(0) are produced, where E(0) is the kinetic energy of the unshocked ions. The effect is optimized when the two colliding shocks have roughly equal strengths. As the shocks become closer to quasi-parallel, the number and energy of the accelerated ions increases. Energies in excess of 30E(0) are obtained in the quasi-parallel regime, with a small fall-off when the shocks are exactly parallel. For quasi-parallel collisions, the ion energization can be attributed to the interaction of hot ions with both strong electromagnetic waves and the motional electric field located at the shocks. These results are applied to shock collisions in the distant solar wind, at planetary bow shocks, as well as to the impulsive phase of solar flares.

Cargill, P. J.↗
NASA NTRS1991

Inverse problems in diffraction

A two-dimensional problem of diffraction of a plane electromagnetic wave on a smooth 2 pi-periodic surface is considered. A numerical algorithm solving this problem is developed. An inverse problem of determination of the shape of 2 pi-periodic surface using the performance data of reverse scattering is considered. The inverse problem was solved by means of minimization of the residual functional with the help of the gradient descent method. The initial data were calculated with the help of the numerical method. On each step of the iterative method of minimization, the residual functional was calculated approximately with the help of the small slope method. The examples of the shape determination are considered.

Mikheev, Andrew G.↗
NASA NTRS1991

Ulysses navigation

Ulysses, a cooperative NASA and ESA project, measures charged and neutral particles, magnetic fields, and electromagnetic wave emissions from the Sun's polar regions and the out of elliptic heliospheric environment. ESA supplied the spacecraft and one half of the instruments, and is in control of the spacecraft and its attitude. NASA supplied the power supply, one half of the instruments, the launch vehicle, and tracks the spacecraft and performs the navigation. Orbit determination results mapped to the time of Jupiter's arrival show improved accuracy and consistency over the course of the mission to date. Solar radiation pressure model uncertainties are major contributors to orbit determination errors, but are being estimated more accurately as data are accumulated.

Gordon, H. J.↗
NASA NTRS1992

Investigation of the structure of the electromagnetic field and related phenomena, generated by the active satellite

A short review is given for the general frequency and angle distribution of the electric field radiated by an electric dipole E = E(sub 0)cos(omega)t, in a magnetoplasma. Detailed results of numerical calculations of (E) were made in the Very Low Frequency (VLF) and the Low Frequency (LF) bands 0.02f(sub b) is less than or equal to F is less than or equal to 0.5f(sub b) (F is approximately (4-500) kHz) in the ionosphere and magnetosphere in the altitude region Z = (800-6000) km; f(sub b) is the electron gyro-frequency of the plasmas in the discussed region f(sub b) is approximately equal to (1.1 to 0.2) MHz. The amplitudes of the electric field have large maxima in four regions: close to the direction of the Earth's magnetic field line (B(sub 0)), it is the so called Axis field (E(sub 0)) and in the Storey (E(sub St)), Reversed Storey (E(sub RevSt)), and Resonance (E(sub Res)) Cones. The maximal values of E(sub 0), E(sub Res), and E(sub RevSt) are very pronounced close to the low hybrid frequency, F approximately F(sub L). The flux of the electric field is concentrated in very narrow regions, the apex angles of the cones delta(beta) is approximately equal to (0.1 - 1) degree. The enhancement and focusing of the electric field is growing up, especially quickly at Z greater than 800 km. At Z is greater than 1000 up to 6000 km, the relative value of (E), in comparison with its value at Z = 800 km is about (10(exp 2) to 10(exp 4)) times larger. Thus, the flux of VLF and LF electromagnetic waves in the Earth magnetoplasma produces and is guided by very narrow pencil beams, similar, let us say, to laser beams.

Alpert, Yakov L.↗
NASA NTRS1992

Inhibition of electron thermal conduction by electromagnetic instabilities

Heat flux inhibition by electromagnetic instabilities in a hot magnetized plasma is investigated. Low-frequency electromagnetic waves become unstable due to anisotropy of the electron distribution function. The chaotic magnetic field thus generated scatters the electrons with a specific effective mean free path. Saturation of the instability due to wave-wave interaction, nonlinear scattering, wave propagation, and collisional damping is considered. The effective mean free path is found self-consistently, using a simple model to estimate saturation level and scattering, and is shown to decrease with the temperature gradient length. The results, limited to the assumptions of the model, are applied to astrophysical systems. For some interstellar clouds the instability is found to be important. Collisional damping stabilizes the plasma, and the heat conduction can be dominated by superthermal electrons.

Levinson, Amir↗
NASA NTRS1992

The mirror and ion cyclotron anisotropy instabilities

The linear dispersion equation for fully electromagnetic waves and instabilities at arbitrary directions of propagation relative to a background magnetic field B(0) in a homogeneous Vlasov plasma is solved numerically for bi-Maxwellian particle distributions. For isotropic plasmas the dispersion and damping of the three modes below the proton cyclotron frequency are studied as functions of Beta(i) and T(e)/T(i). The transport ratios of helicity, cross-helicity, Alfven ratio, compressibility, and parallel compressibility are defined. Under the condition that the proton temperature perpendicular to B(0) is greater than the parallel temperature, the growth rates and transport ratios of the mirror instability and the ion cyclotron anisotropy instability are examined and compared. Both the proton parallel compressibility and the proton Alfven ratio are significantly different for the two growing modes.

Gary, S. P.↗
NASA NTRS1989

Evaluation of reflection and transmission coefficients for multi-layered chiral structures

The paper examines a structure consisting of an arbitrary number of layers of different chiral materials, backed by either a perfect conductor or a penetrable half-space. A plane electromagnetic wave of arbitrary polarization is obliquely incident on the structure. The reflection and transmission coefficients of the structure can be determined by a chain-matrix algorithm.

Tamirisa, P.↗
NASA NTRS1992

Polarization effects and multipolarization SAR

Imaging radar polarimeters are usually implemented using a Synthetic Aperture Radar (SAR) approach to give a high resolution image in two dimensions: range and azimuth. For each pixel in the image a polarimetric SAR gives sufficient information to characterize the polarimetric scattering properties of the imaged area (or target) as seen by the radar. Using a polarimetric SAR system as opposed to a single-polarization SAR system provides significantly more information about the target scattering mechanisms and allows better discrimination between different types of surfaces. In these notes a brief overview of SAR polarimetry is offered. The notes are intended as a text to accompany a lecture on SAR polarimetry as part of an AGARD-NATO course. Covered in the notes are the following: the polarization properties of electromagnetic waves; the concepts of radar scattering and measuring radar backscatter with a SAR; polarization synthesis; scattering matrix, Stokes matrix, and covariance matrix representations of polarimetric SAR data; polarization signature plots; design and calibration of polarimetric SAR systems; polarization filtering for target detection; fitting a simple model to polarimetric SAR measurements of naturally occurring features; and supervised classification of polarimetric SAR data.

Freeman, Anthony↗
NASA NTRS1992

Remote sensing of Earth terrain

Research findings are summarized for projects dealing with the following: application of theoretical models to active and passive remote sensing of saline ice; radiative transfer theory for polarimetric remote sensing of pine forest; scattering of electromagnetic waves from a dense medium consisting of correlated Mie scatterers with size distribution and applications to dry snow; variance of phase fluctuations of waves propagating through a random medium; theoretical modeling for passive microwave remote sensing of earth terrain; polarimetric signatures of a canopy of dielectric cylinders based on first and second order vector radiative transfer theory; branching model for vegetation; polarimetric passive remote sensing of periodic surfaces; composite volume and surface scattering model; and radar image classification.

Kong, J. A.↗