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NASA NTRS1996

A Numerical Simulation of Scattering from One-Dimensional Inhomogeneous Dielectric Random Surfaces

In this paper, an efficient numerical solution for the scattering problem of inhomogeneous dielectric rough surfaces is presented. The inhomogeneous dielectric random surface represents a bare soil surface and is considered to be comprised of a large number of randomly positioned dielectric humps of different sizes, shapes, and dielectric constants above an impedance surface. Clods with nonuniform moisture content and rocks are modeled by inhomogeneous dielectric humps and the underlying smooth wet soil surface is modeled by an impedance surface. In this technique, an efficient numerical solution for the constituent dielectric humps over an impedance surface is obtained using Green's function derived by the exact image theory in conjunction with the method of moments. The scattered field from a sample of the rough surface is obtained by summing the scattered fields from all the individual humps of the surface coherently ignoring the effect of multiple scattering between the humps. The statistical behavior of the scattering coefficient sigma(sup 0) is obtained from the calculation of scattered fields of many different realizations of the surface. Numerical results are presented for several different roughnesses and dielectric constants of the random surfaces. The numerical technique is verified by comparing the numerical solution with the solution based on the small perturbation method and the physical optics model for homogeneous rough surfaces. This technique can be used to study the behavior of scattering coefficient and phase difference statistics of rough soil surfaces for which no analytical solution exists.

Sarabandi, Kamal↗

NASA NTRS1993

On the evaluation of air mass factors for atmospheric near-ultraviolet and visible absorption spectroscopy

The interpretation of UV-visible twilight absorption measurements of atmospheric chemical constituents is dependent on how well the optical path, or air mass factor, of light collected by the spectrometer is understood. A simple single scattering model and a Monte Carlo radiative transfer scheme have been developed to study the effects of multiple scattering, aerosol scattering, surface albedo and refraction on air mass factors for scattered light observations. At fairly short visible wavelengths (less than about 450 nm), stratospheric air mass factors are found to be relatively insensitive to multiple scattering, surface albedo and refraction, as well as aerosol scattering by background aerosols. Longer wavelengths display greater sensitivity to refraction and aerosol scattering. Tropospheric air mass factors are found to be highly dependent on aerosol scattering, surface albedo and, at long visible wavelengths (about 650 nm), refraction. Absorption measurements of NO2 and O4 are shown to support these conclusions.

Perliski, Lori M.↗

NASA NTRS1985

Light Scattering Tests of Fundamental Theories of Transport Properties in the Critical Region

The objective of this program is to measure the decay rates of critical density fluctuations in a simple fluid (xenon) very near its liquid-vapor critical point using laser light scattering and photon correlation spectroscopy. Such experiments have been severely limited on Earth by the presence of gravity which causes large density gradients in the sample when the compressibility diverges approaching the critical point. The goal is to measure decay rates deep in the critical region where the scaled wavevector is the order of 1000. This will require loading the sample to 0.01% of the critical density and taking data as close as 3 microKelvin to the critical temperature (Tc = 289.72 K). Other technical problems have to be addressed such as multiple scattering and the effect of wetting layers. The ability to avoid multiple scattering by using a thin sample (100 microns) was demonstrated, as well as a temperature history which can avoid wetting layers satisfactory temperature control and measurement, and accurate sample loading. Thus the questions of experimental art are solved leaving the important engineering tasks of mounting the experiment to maintain alignment during flight and automating the state-of-the-art temperature bridges for microcomputer control of the experiment.

Gammon, R. W.↗

NASA NTRS1992

Inversion of snow parameters from passive microwave remote sensing measurements by a neural network trained with a multiple scattering model

Simultaneous inversion of the three parameters was performed which included mean-grain size of ice particles in snow, snow density, and snow temperatures from five brightness temperatures. Good results for the inversion of parameters were obtained using the neural network based on the simulated data computed from the dense media radiative transfer equation that takes into account the effects of multiple scattering.

Tsang, Leung↗

NASA NTRS2008

Modeling the Diffuse Cloud-Top Optical Emissions from Ground and Cloud Flashes

A number of studies have indicated that the diffuse cloud-top optical emissions from intra-cloud (IC) lightning are brighter than that from normal negative cloud-to-ground (CG) lightning, and hence would be easier to detect from a space-based sensor. The primary reason provided to substantiate this claim has been that the IC is at a higher altitude within the cloud and therefore is less obscured by the cloud multiple scattering medium. CGs at lower altitudes embedded deep within the cloud are more obscured, so CG detection is thought to be more difficult. However, other authors claim that because the CG source current (and hence luminosity) is typically substantially larger than IC currents, the greater CG source luminosity is large enough to overcome the effects of multiple scattering. These investigators suggest that the diffuse cloud top emissions from CGs are brighter than from ICs, and hence are easier to detect from space. Still other investigators claim that the detection efficiency of CGs and ICs is about the same because modern detector sensitivity is good enough to "see" either flash type no matter which produces a brighter cloud top emission. To better assess which of these opinions should be accepted, we introduce an extension of a Boltzmann lightning radiative transfer model previously developed. It considers characteristics of the cloud (geometry, dimensions, scattering properties) and specific lightning channel properties (length, geometry, location, current, optical wave front propagation speed/direction). As such, it represents the most detailed modeling effort to date. At least in the few cases studied thus far, it was found that IC flashes appear brighter at cloud top than the lower altitude negative ground flashes, but additional model runs are to be examined before finalizing our general conclusions.

Solakiewicz, Richard↗

NASA NTRS1977

Multiple scattering in reflection nebulae. I - A Monte Carlo approach. II - Uniform plane-parallel nebulae with foreground stars. III - Nebulae with embedded illuminating stars

A method for calculating the surface brightness distribution on a plane-parallel reflection nebula of uniform density illuminated by a star located either in front of, behind, or arbitrarily inside the scattering medium is proposed. The Monte Carlo technique is used to find solutions to the radiative transfer problem. The scattering properties of the nebular particles are parameterized by the albedo for single scattering and a three-parameter analytic phase function. Calculations are then presented for the surface brightness distribution across the face of such nebulae with (1) a foreground star and (2) immersed stars. The calculations include the full effects of multiple scattering, are independent of a particular assumed grain material or size distribution, and are applicable to any wavelength region for which observations can be obtained.

Witt, A. N.↗

NASA NTRS1974

Satellite determination of nature and microstructure of atmospheric aerosols

A method is presented for the determination of aerosol physical parameters on the basis of scattered radiance measurements. The reconstruction of aerosol particle size distribution in the case of a spectral forward scattering method is considered, taking into account an inverse diffraction integral expression, a model cloud, the effect of lower wavenumber cut-off, the effect of higher wavenumber cut-off, the effect of spectral resolution, and the effects of multiple scattering and background noise. The determination of the aerosol complex refractive index with the aid of a minimization search method is also discussed.

Fymat, A. L.↗

NASA NTRS1998

Calculations of multiple scattering-induced errors in the GLAS mission

The Geoscience Lidar Altimeter System (GLAS) is a laser altimeter designed to measure temporal changes in the topography and mass balance of the Earth's ice sheets. GLAS is scheduled to be launched as part of the Earth Observing System (EOS) in 2001. Due to the large pulse width of GLAS and the slope (and roughness) of the ice sheets, the mean elevation of the laser's surface spot will be estimated from the centroid of the return pulse. While the accuracy of individual altimetry measurements is expected to be 15 cm, a cross-over technique that averages the elevation differences at selected points on the ice sheets will allow GLAS to detect mean ice elevation changes as small as 2 cm per year. One factor that may degrade the proposed accuracy of the altimeter measurements is the effect of multiple scattering by thin clouds and aerosols over the ice sheets. In the return pulse, some photons from the lidar are slightly deflected by cloud particles but still return to the instrument detector. Since these photons travel a longer path than photons that pass directly to and from the surface, the mean travel time of the pulse is lengthened and the centroid of the return pulse is shifted toward a later time . This study intends to determine the importance of this source of altimetry error in the GLAS measurements. Two methods will be used to estimate path delays in the GLAS lidar for a range of cloud conditions. An analytic expression of a doubly scattered return signal (Eloranta, 1972) will be compared to Monte Carlo simulations that estimate path delays by cloud, aerosol and molecular scattering. The results from both methods will be discussed in relationship to current knowledge of Arctic cloud climatology, and the impact of multiple scattering on GLAS altimeter measurements will be evaluated.

Duda, David P.↗

NASA NTRS1974

Models of Saturn's rings which satisfy the optical observations

A theoretical model of Saturn's rings is investigated which includes the shadowing effect and realistic anisotropic phase functions for the ring particles. The effects of multiple scattering and the finite size of the sun, including the penumbra, are rigorously included. The permissible range of the relevant parameters, including optical thickness, single scattering albedo, volume density, and phase function are investigated by comparing the theoretical results to observations of the ring brightness vs phase angle, wavelength, and elevation of the sun and earth. Anisotropic scattering by the ring particles is necessary in order to match the observations. The color dependence of the opposition effect is interpreted in terms of the albedo spectrum of the ring particles.

Kawata, Y.↗

NASA NTRS1994

The axisymmetric stellar wind of AG Carinae

We present optical linear spectropolarimetry of the Luminous Blue Variable AG Carinae obtained after a recent visual brightness increase. The absence of He II lambda 4686 emission, together with the weakening of the He I spectrum and the appearance of Fe lines in the region around 5300 A, confirm that AG Car has started a new excursion across the HR diagram. The H alpha line profile exhibits very extended line wings that are polarized differently in both amount and position angle from either the continuum or the line core. The polarization changes across H alpha, together with variable continuum polarization, indicate the presence of intrinsic polarization. Coexistence of the line-wing polarization with extended flux-line wings evidences that both are formed by electron scattering in a dense wind. The position angle rotates across the line profiles, in a way that presently available models suggest is due to rotation and expansion of the scattering material. AG Car displays very large variations of its linear polarization with time, Delta P approximately 1.2%, indicating significant variations in envelope opacity. We find that the polarization varies along a preferred position angle of approximately 145 deg (with a scatter of +/- 10 deg) which we interpret as a symmetry axis of the stellar wind (with an ambiguity of 90 deg). This position angle is co-aligned with the major axis of the AG Car ring nebula and perpendicular to the AG Car jet. Our observations thus suggest that the axisymmetric geometry seen in the resolved circumstellar environment at various distances already exists within a few stellar radii of AG Car. From the H alpha polarization profile we deduce an interstellar polarization of Q = 0.31%, U = -1.15% at H alpha. The inferred interstellar polarization implies that the intrinsic polarization is not always of the same sign. This indicates either significant temporal changes in the envelope geometry, or it may arise from effects of multiple scattering in conjunction with density variations.

Schulte-Ladbeck, Regina E.↗

NASA NTRS1979

A coupled radiative-convective-photochemical model of the stratosphere

A one-dimensional atmospheric model is described. The neutral photochemical processes of the troposphere and stratosphere are presented by a reaction set of 87 kinetic processes and 28 photolytic reactions, with vertical transport being taken into account by an eddy diffusion parameter. The vertical temperature profiles are coupled with the chemical calculations by means of a radiative-convective code. The model has the capability of including effects of multiple scattering and diurnal variations in species concentrations. The numerical solution methodology is described in detail, and current input data are given along with results obtained in a sample calculation.

Boughner, R. E.↗

NASA NTRS1983

Development of a nuclear technique for monitoring water levels in pressurized vehicles

A new technique for monitoring water levels in pressurized stainless steel cylinders was developed. It is based on differences in attenuation coefficients of water and air for Cs137 (662 keV) gamma rays. Experimentally observed gamma ray counting rates with and without water in model reservoir cylinder were compared with corresponding calculated values for two different gamma ray detection theshold energies. Calculated values include the effects of multiple scattering and attendant gamma ray energy reductions. The agreement between the measured and calculated values is reasonably good. Computer programs for calculating angular and spectral distributions of scattered radition in various media are included.

Singh, J. J.↗

NASA NTRS1984

The 1984 ASEE-NASA summer faculty fellowship program (aeronautics and research)

The 1984 NASA-ASEE Faculty Fellowship Program (SFFP) is reported. The report includes: (1) a list of participants; (2) abstracts of research projects; (3) seminar schedule; (4) evaluation questionnaire; and (5) agenda of visitation by faculty programs committee. Topics discussed include: effects of multiple scattering on laser beam propagation; information management; computer techniques; guidelines for writing user documentation; 30 graphics software; high energy electron and antiproton cosmic rays; high resolution Fourier transform infrared spectrum; average monthly annual zonal and global albedos; laser backscattering from ocean surface; image processing systems; geomorphological mapping; low redshift quasars; application of artificial intelligence to command management systems.

Dah-Nien, F.↗

NASA NTRS1984

Change of calibration of diffraction-based particle sizers in dense sprays

A calibration procedure for diffraction-based particle-sizing instruments using multielement detectors is briefly reviewed. The effects of multiple scattering due to dense sprays on the calibration of these instruments are then examined over a range of sprays with Sauter mean diameters from 20 to 55 microns. An empirical scheme to correct for changes in calibration due to dense sprays or other scattering media is presented.

Dodge, L. G.↗

NASA NTRS1986

Temperature retrievals by Rayleigh backscatter lidar signals

Differences between retrieved temperatures from lidar and atmospheric model temperatures are calculated for four problems: (1) the effect of a transient thin aerosol layer at any altitude on temperature retrieved by a single-wavelength lidar; (2) the effect of residual aerosols on temperature retrieved by a two-wavelength lidar; (3) the errors due to splicing two backscatter signals which are obtained separately; and (4) the effect of multiple scattering. The results show that temperatures can be retrieved for + or - 3 K if Rayleigh backscatter can be measured for + or - 1.5 percent. The LITE lidar in a 240 km orbit aboard the Shuttle should be able to make a + or -3 K measurements from 10-40 km with vertical resolutions of 1 km and horizontal resolutions of 300 km.

Uchino, O.↗

NASA NTRS1988

CSDUST3 - A radiation transport code for a dusty medium with 1-D planar, spherical or cylindrical geometry

The program solves the radiation transport problem in a dusty medium with one-dimensional planar, spherical or cylindrical geometry. It determines self-consistently the effects of multiple scattering, absorption, and re-emission of photons on the temperature of dust grains and the characteristics of the internal radiation field. The program can treat radiation field anisotropy, linear anisotropic scattering, and multi-grain components. The program output consists of the dust-temperature distribution, flux spectrum, surface brightness at each frequency and the observed intensities (involving a convolution with a telescope beam pattern).

Egan, Michael P.↗

NASA NTRS1988

An improved method for determining the vertical ozone distribution using satellite measurements

A procedure was developed for determining vertical ozone distributions from spectral measurements of UV radiances diffusely reflected from the terrestrial atmosphere. In this procedure, the inversion method of Aruga and Igarashi (1976) is improved by incorporating the effects of multiple scattering and the lower boundary of the atmosphere. The improved inversion algorithm was tested using data of NIMBUS-4 backscattered UV experiments. The results indicate that by using this method, the vertical distributions of atmospheric ozone can be obtained with high accuracy over a wide range of altitude.

Aruga, Tadashi↗

NASA NTRS1991

Corona-producing cirrus cloud properties derived from polarization lidar and photographic analyses

Polarization lidar data are used to demonstrate that clouds composed of hexagonal ice crystals can generate multiple-ringed colored coronas. Although relatively uncommon in the mid-latitude cirrus sample, the coronas are associated with unusual cloud conditions that appear to be effective in generating the displays. Invariably, the cirrus cloud tops are located at or slightly above elevated tropopauses at temperatures between -60 and -70 C. The cloud top region also generates relatively strong laser backscattering and unusually high 0.5-0.7 linear depolarization ratios. Color photograph analysis of corona ring angles indicates crystals with mean diameters of from 12 to 30 microns. The cirrus cloud types were mainly subvisual to thin cirrostratus, but also included fibrous cirrus. Estimated cloud optical thicknesses at the 0.694-micron laser wavelength ranged from about 0.001 to 0.2, where the upper limit reflects the effects of multiple scattering and/or unfavorable changes in particle characteristics in deep cirrus clouds.

Sassen, Kenneth↗