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NASA NTRS1991

Surface scattering properties estimated from modeling ASAS multiple emission angle reflectance data over the lunar crater volcanic field, Nevada

ASAS multispectral data were acquired over Lunar Lake playa at 12 different lighting and viewing geometries. Atmospheric models and field-based data demonstrate that atmospheric contributions to ASAS data were relatively small and were thus ignored in analyses of surface photometric properties. ASAS data of an artificially roughened playa and cobble-strewn surfaces exhibit trends of increasing reflectance as phase angle decreases. The reflectance of a natural playa surface exhibits Lambertian scattering properties that cannot be replicated with the Hapke (1986) photometric function. The Hapke model does fit the rough and cobble-strewn playa data with single-scattering albedo, asymmetry factor, and roughness terms consistent with known properties of these surfaces. No published photometric model explains the reflectance variations of all three surfaces.

Guinness, Edward A.↗

NASA NTRS1990

Effect of cloud boundaries on reflected radiation

The effects of cloud boundaries on cloud reflectivity are evaluated by comparing satellite-observed reflectivities for layered clouds that are uniform over scales that are large compared with the field of view of the satellite radiometer and those of the same clouds where they are broken on scales which are small compared with the field of view. Results are given for 1 km AVHRR data. The reflectivities are compared at 0.63 micron for which droplets in stratiform water clouds are nonabsorbing, and at 3.7 microns where the single-scattering albedo of the droplets is typically about 0.9 micron.

Coakley, James A., Jr.↗

NASA NTRS1991

Relativistic enhancement of the Compton-reflected component in active galactic nuclei

Compton reflection may be an important process in AGNs, since it provides an improvement over power-law fits to AGN spectra observed by Ginga and can also explain the spectrum of the cosmic X-ray background. The fraction of the total X-ray luminosity which is incident upon a thin steady alpha-disk if the X-rays are produced by inverse-Compton scattering of soft, disk photons off relativistic electrons located above the disk is calculated. This fraction is called the Compton reflection covering factor, f, and it is found that it can range between 0.5 and 0.88. This large range in f, due to a relativistic kinematic effect first calculated in this connection by Ghisellini et al (1991) is sufficient to explain the typical covering (about 0.5) observed in bright AGNs by Ginga as well as the large covering factor (about 0.9) required to explain the cosmic X-ray background in the Compton reflection model.

Rogers, R. D.↗

NASA NTRS1992

Effects of grain size and shape in modeling reflectance spectra of mineral mixtures

An 'isograin model' of reflectance from particulate material is presented. The model treats reflectance as a series of grain interactions, incorporating optical constants of each material and two parameters that model the activity of scattering and absorption in the vertical and the horizontal directions, respectively. Mixing ratios of components in mixture are treated as variables and are calculated using least-squares fit. Grain size and shape have a significant effect on the accuracy of predicting mixing ratios but can be modeled by introducing the concept of effective grain size. The model calculates spectra in good agreement with measured spectra and models reflectance within 1 percent and mixing ratios within 5 wt pct.

Hiroi, T.↗

NASA NTRS1992

Reflectivity (visible and near IR), Moessbauer, static magnetic, and X ray diffraction properties of aluminum-substituted hematites

The effect of substituting iron by aluminum in polymorphs of Fe2O3 and FeOOH on their reflectivity characteristics was investigated by comparing data on visible and NIR reflectivities and on static magnetic, XRD, and Moessbauer properties for a family of aluminum-substituted hematites alpha-(Fe,Al)2O3, with compositions where the values of the Al/(Al+Fe) ratio were up to 0.61. Samples were prepared by oxidation of magnetite, dehydroxylation of goethite, and direct precipitation. The analytical methods used for obtaining diffuse reflectivity spectra (350-2200 nm), Moessbauer spectra, and static magnetic data are those described by Morris et al. (1989).

Morris, Richard V.↗

NASA NTRS1992

Light-trap design using multiple reflections and solid-angle attenuation - Application to a spaceborne electron spectrometer

The design and performance of a new light trap for a spaceborne electron spectrometer are described. The light trap has a measured photon-rejection ratio of 2 x 10 exp -11, allowing only one in 5 x 10 exp 10 incident photons to reach the sensitive area of the instrument. This rejection is more than sufficient because the ambient UV in earth orbit requires a rejection no better than 10 exp -8 to maintain the photon interference to less than 10 count/s. The light trap uses triple reflections to keep most of the light passing through the entrance slit away from the sensitive area of the spectrometer. It is shown that the average reflectance of all the internal surfaces must be less than 0.006, which is consistent with the data on the black coating applied to all surfaces. The analysis makes it possible to compare the photon contributions of each of the internal reflecting areas and to estimate the effective scattering width of the metallic electrode edge.

Herrero, Federico A.↗

NASA NTRS1992

Reflection of interannual Rossby waves at the maritime western boundary of the tropical Pacific

The Geosat altimetric sea level time sequences taken between November 1986 and August 1989 are examined for the evidence of interannual Rossby waves reflecting at the maritime western boundary of the tropical Pacific in the Northern Hemisphere. The analyses applied to the observations are similar to those used by White et al. (1990) but are extended to include an interpretation in terms of the linear theory of Rossby wave reflection. It is shown that the amplitude of the first-mode Rossby wave (i.e., mode 2) accounts for 70-80 percent of the Kelvin wave amplitude, and the second symmetric Rossby wave mode explains another 20-30 percent. The results are consistent with theoretical estimates of Rossby mode contributions to the reflected Kevin wave obtained by Kessler (1991).

White, Warren B.↗

NASA NTRS1993

Multiwavelength Pyrometry To Correct For Reflections

Computerized curve fitting yields more and better information on thermal radiation. Multiwavelength pyrometry involves measurement of spectrum of thermal radiation emitted by and reflected from specimen. Auxiliary source enables determination of spectral reflectance of specimen. Spectral reflectance and temperature of specimen obtained from spectral measurements by nonlinear least-squares curve-fitting routine.

Ng, Daniel↗

NASA NTRS1993

Pyrometry Without Spurious Reflections

Temperature of heated spherical specimen of opaque material with specular surface measured by optical pyrometry, without need to correct for effect of reflected thermal radiation upon measured radiances, provided reflections eliminated. To prevent unwanted reflections, apparatus must be arranged so field of view of pyrometer does not contain any lines of sight to image of wall. Particularly useful for making accurate noncontact measurements of levitated specimens of molten metal and other materials in containerless processing.

Thomas, Andrew S. W.↗

NASA NTRS1993

Low cloud investigations for project FIRE: Island studies of cloud properties, surface radiation, and boundary layer dynamics. A simulation of the reflectivity over a stratocumulus cloud deck by the Monte Carlo method

The radiation field over a broken stratocumulus cloud deck is simulated by the Monte Carlo method. We conducted four experiments to investigate the main factor for the observed shortwave reflectively over the FIRE flight 2 leg 5, in which reflectivity decreases almost linearly from the cloud center to cloud edge while the cloud top height and the brightness temperature remain almost constant through out the clouds. From our results, the geometry effect, however, did not contribute significantly to what has been observed. We found that the variation of the volume extinction coefficient as a function of its relative position in the cloud affects the reflectivity efficiently. Additional check of the brightness temperature of each experiment also confirms this conclusion. The cloud microphysical data showed some interesting features. We found that the cloud droplet spectrum is nearly log-normal distributed when the clouds were solid. However, whether the shift of cloud droplet spectrum toward the larger end is not certain. The decrease of number density from cloud center to cloud edges seems to have more significant effects on the optical properties.

Ackerman, Thomas P.↗

NASA NTRS1992

Mineral equilibria and the high radar reflectivity of Venus mountaintops

The relationship between altitude and microwave emissivity in 10 highland regions of Venus is investigated on the basis of the Magellan data set. Highlands on Venus are found to display high radar reflectivity. The required change in surface electrical properties occurs abruptly at a 'critical altitude,' whose value varies from one highland area to another. Critical altitudes range from 4.75 km to 2.49 km. Differences in reflectivity are caused by differences in the surface mineral assemblage, which determines the dielectric constant of surface material. The mineral responsible for high radar reflectivity on mountaintops is pyrite, which occurs in weathered mineral assemblages at high altitudes. Conductive pyrite occurs dispersed in insulating materials, forming a loaded dielectric material.

Klose, K. B.↗

NASA NTRS1993

Seasonal variation in the structure of red reflectance of leaves from yellow poplar, red oak, and red maple

The light scattered from leaves was measured as a function of view angle in the principal plane for yellow poplar, red oak, and red maple. The source was a parallel-polarized helium-neon laser. Yellow poplar leaves had the highest reflectance of the three species, which may have been due to its shorter palisade cells and more extensive spongy mesophyll. Prior to senescence, there was a significant decrease, but not total extinction, in the reflectance of the beam incident at 60 deg from nadir on the adaxial side of the leaves of all three species. Low-temperature SEM observations showed differences in the surface wax patterns among the three species but did not indicate a cause of the reflectance changes other than possibly the accumulation and aging of the wax.

Brakke, Thomas W.↗

NASA NTRS1993

On Compton reflection in the sources of the cosmic X-ray background

Consideration is given to recent models for the cosmic X-ray background that assume that it originates from unresolved AGN emitting spectra due to enhanced Compton reflection of a power-law photon spectrum incident on cold matter. The parameter space of the Compton reflection model is studied, and the allowed parameter space is found to be severely constrained by physical and cosmological effects. For an incident power-law energy index alpha is greater than about 1, the X-ray peak in the observed spectrum from a population of AGN is necessarily at an energy less than that of the observed peak. Two examples of improved fits to the X-ray background are shown. It is concluded that the Compton reflection models proposed to date do not provide a straightforward explanation of the X-ray background spectrum.

Zdziarski, Andrzej A.↗

NASA NTRS1993

Influence of horizontal inhomogeneity in the ionosphere on the reflection of Alfven waves

A study of the reflection of Alfven waves at a horizontally inhomogeneous ionosphere has been carried out. In this study, the Alfven speed above the ionosphere is assumed to be uniform and the ionosphere is treated as a height-integrated conducting slab. Analytical and numerical results indicate that the horizontal nonuniformity of the ionospheric conductivity can lead to a rotation of the reflected wave fields and cause field-aligned currents that originate in the ionosphere. A strong conductivity nonuniformity on the direction perpendicular to the incident wave field, large Hall to Pedersen conductivity ratios, and low conductivity values lead to a large rotation of the reflected wave field in the range from a few to 40 deg. The implications of the theoretical results for some ionospheric phenomena are presented, including comments on the Harang discontinuity and sun-aligned arcs.

Zhu, L.↗

NASA NTRS1993

The continuum slope of Mars - Bidirectional reflectance investigations and applications to Olympus Mons

Two primary causes of near-IR continuum slope variations have been observed in an investigation of the bidirectional reflectance characteristics of ferric coatings on the continuum slope of Mars. First, the presence of a thin ferric coating on a dark substrate produces a negative continuum slope due to the wavelength-dependent transparency of the ferric coating. Second, wavelength-dependent directional reflectance occurs when the surface particles are tightly packed, particle sizes are on the order of or smaller than the wavelength of light, or the surface is otherwise smooth on the order of the wavelength of light. Based on these results, the annuli on the flanks of Olympus Mons which are defined by reflectance and continuum slope are consistent with spatial variations in surface texture and possibly with spatial variations in the thickness of a ferric dust coating or rind.

Fischer, Erich M.↗

NASA NTRS1993

Lunar regolith analogues - Spectral reflectance properties of compositional variations

The effect is analyzed of variations in glass, end member abundance, and compositional variations on the reflectance spectra of glass and mafic silicate-bearing mineral mixtures chosen to reproduce spectral reflectance properties of lunar regoliths. The spectral properties of the glasses appear to be sensitive to parameters such as fusion techniques and initial composition. Variations in these parameters can yield classes which show spectral variations far in excess of those found for naturally occurring lunar impact-derived glasses and surface fines. The addition of glass to a mafic silicate-bearing assemblage results in a reduction in overall reflectance, band depths, and band area ratios, as well as a general decrease in band II minimum wavelength position and an increase in interband peak position.

Cloutis, Edward A.↗

NASA NTRS1993

Design and fabrication of the All-Reflecting H-Lyman-alpha Coronagraph/Polarimeter

We have designed, analyzed, and are now fabricating an All-Reflecting H-Lyman-alpha Coronagraph/Polarimeter for solar research. This new instrument operates in a narrow bandpass centered at 215.7 A - the neutral hydrogen Ly-alpha line. It is shorter and faster than the telescope which produced solar Ly-alpha images as a part of the MSSTA payload that was launched on May 13, 1991. The Ly-alpha line is produced and linearly polarized in the solar corona by resonance scattering, and the presence of a magnetic field modifies this polarization according to the Hanle effect. The Lyman-alpha Coronagraph/Polarimeter instrument has been designed to measure coronal magnetic fields by interpreting, via the Hanle effect, the measured linear polarization of the coronal Ly-alpha line. Ultrasmooth mirrors, polarizers, and filters are being flow-polished for this instrument from CVD silicon carbide substrates. These optical components will be coated using advanced induced transmission and absorption thin film multilayer coatings to optimize the reflectivity and polarization properties at 1215.7 A. We describe some of the solar imaging results obtained with the MSSTA Lyman-alpha coronagraph. We also discuss the optical design parameters and fabrication plans for the All-Reflecting H-Lyman-alpha Coronagraph/Polarimeter.

Hoover, Richard B.↗

NASA NTRS1992

Simulation of space measurements of vegetation canopy bidirectional reflectance factors

A vegetation canopy receives both monodirectional and scattered radiation which is then reflected back into the atmosphere according to its bidirectional reflectance distribution function (BRDF). Remote measurements are typically the angular and spectral distributions of radiations exiting the atmosphere. The evaluation of vegetation canopy bidirectional reflectance factors requires a numerical solution of the canopy radiative transfer equation. A horizontally homogeneous vegetation canopy, of finite physical depth, filled densely with small leaves and bounded by a flat Lambertian ground surface is considered. The canopy is illuminated spatially uniformly by monodirectional radiation. The scattered intensity distribution emerging at the top of the canopy in all directions in the upper hemisphere is evaluated. The magnitude, sign and angular distribution of atmospheric effects are wavelength specific and depend on the composition of the atmosphere. The net atmospheric effect is positive (negative) at the red (near-infrared) wavelength due to strong scattering (absorption) in the atmosphere. The angular distribution of these effects is bowl-shaped. Thus, off-nadir remote observations, especially at shorter wavelengths, are most affected by atmospheric perturbations.

Myneni, R. B.↗