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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Pc 3, 4 activity and interplanetary field orientation

Analysis of Pc 3, 4 micropulsation wave forms recorded at Calgary in September 1969 shows a tendency toward signal enhancement in cases where the interplanetary magnetic field is at a small angle to the sun-earth line. Scatter plots of hourly Pc 3, 4 amplitudes exhibit a definite trend toward large signals when this angle is less than 50 or 60 degrees, and a corresponding disappearance of significant amplitudes when the angle is greater than 60 degrees. There is, however, an appreciable variability in individual cases. Power density spectrograms improved the correlation of pulsation strength with low angle in some cases.

Greenstadt, E. W.↗
NASA NTRS1981

An approximate model for backscattering and emission from land and sea

A two-scale model for near nadir backscattering based upon Semyonov's paper is developed for random surfaces with rms surface height smaller than one-half the incident wavelength. Reasonable estimates are obtained when this model is applied to interpret both land and sea measurements. By generalizing this approximate model (not corrected for scattering by small-scale roughnesses for scattering at large incident angles) to the bistatic case and then using it for emission calculations, reasonable estimates near nadir are also obtained when applied to emissions from the bare ground and the sea surfaces.

Fung, A. K.↗
NASA NTRS1972

Elastic scattering of 600-MeV protons from H, D, He-3, and He-4.

Study of the elastic scattering of 600-MeV protons from light nuclei. Differential cross sections have been obtained for the scattering of protons from hydrogen, deuterium, helium-3, and helium-4. Polarization was measured for deuterium and He-4 nuclei. The p-p cross-section data are in excellent agreement with the predictions from the Livermore phase shifts. Small-angle p-D, p-He-3 elastic scattering data are compared with calculations based on the multiple-scattering theories of Watson and Glauber.

Boschitz, E. T.↗
NASA NTRS1988

Shutters and slats for the integral sunshade of an optical reception antenna

Optical reception antennas used at a small Sun-Earth-probe angle (small solar elongation E) require sunshading to prevent intolerable scattering of light from the surface of the primary mirror. An integral sunshade consisting of hexagonal tubes aligned with the segmentation of a large mirror was proposed for use down to E = 12 degrees. For smaller angles, asterisk-shaped vanes inserted into the length of the hexagonal tubes would allow operation down to about 6 degrees with a fixed obscuration of 3.6 percent. Two alternative methods are investigated to extend the usefulness of the integral sunshade to smaller angles by adding either variable-area shutters to block the tube corners that admit off-axis sunlight or by inserting slats (partial vanes) down the full length of some tubes. Slats are effective for most operations down to 6 degrees, and obscure only 1.2 percent. For E between 10.75 and 12 degrees, shutters cause even less obscuration.

Kerr, E. L.↗
NASA NTRS1993

The infrared light curve of Periodic Comet Halley 1986 III and its relationship to the visual light curve, C2, and water production rates

The near-IR light curve of Periodic Comet Halley 1986 III is analyzed and compared with C2 production, water production, and the visual light curve. This is the most complete IR light curve compiled to date for any comet. The scattering phase function at small sun-comet-earth angles is shown to affect the slope of near-IR light curve significantly. P/Halley's dust production, as inferred from the IR light curve showed an increased production rate near perihelion which appears to be correlated with the onset of significant jet activity. The near-IR light curve, visual light curve, C2, and water production rates displayed different heliocentric variations, suggesting that one parameter cannot be accurately estimated from another. This is particularly true of the early preperihelion visual light curve. A peak of 0.3-0.5 magnitude in the visual magnitude, representing the integrated brightness of the comet's visible coma, lagged the other parameters by about a day. The near-IR color, J-H, was less red during periods of strong dust activity.

Morris, Charles S.↗
NASA NTRS1991

Character of the opposition effect and negative polarization

Photometric and polarimetric properties at small phase angles were measured for silicates with controlled surface properties in order to distinguish properties that are associated with surface reflection from those that are associated with multiple scattering from internal grain boundaries. These data provide insight into the causes and conditions of photometric properties observed at small phase angles for dark bodies of the solar system. Obsidian was chosen to represent a silicate dielectric with no internal scattering boundaries. Because obsidian is free of internal scatterers, light reflected from both the rough and smooth obsidian samples is almost entirely single and multiple Fresnel reflections form surface facets with no body component. Surface structure alone cannot produce an opposition effect. Comparison of the obsidian and basalt results indicates that for an opposition effect to occur, surface texture must be both rough and contain internal scattering interfaces. Although the negative polarization observed for the obsidian samples indicates single and multiple reflections are part of negative polarization, the longer inversion angle of the multigrain inversion samples implies that internal reflections must also contribute a significant negative polarization component.

Pieters, Carle M.↗
NASA NTRS1984

The interplanetary scattering mean free path - Collisionless wave-damping effects

The role of dissipation in the scattering of charged particles in the interplanetary medium (IPM) is analyzed to obtain a model for the interplanetary magnetic turbulence spectrum that yields particle free paths (PEP) which agree with observational data. The scattering processes are attributed to waves with small wavelengths intersected by particles with zero pitch angles. The waves, being strongly damped by collisionless cyclotron damping in the ambient thermal plasma, produce reduced scattering and longer MFPs. The model, which includes the damping factor, was used to generate proton propagation maps at kinetic energy levels of under, over, and within the 5 MeV-2 GeV range. The results, when compared with observational data, displayed good agreement. The same held true for the MFP propagation of energetic electrons.

Davila, J. M.↗
NASA NTRS1972

Anisotropic pitch angle distribution of 50 eV to 50 keV particles at synchronous altitude.

At times, the electron pitch angle distributions at synchronous orbit have been observed to be highly anisotropic. In the local morning region, distributions concentrated near 90 deg are often observed in particles of less than approximately 2000 V. This anisotropy decreases with increasing energy from 1 keV to the detector's limit at 50 keV. The time development of anisotropy is consistent with production by pitch angle scattering processes which are not effective on electrons with small velocities parallel to the magnetic field. Another type of distribution has been observed with the low-energy (below 1000 V) electrons concentrated parallel and antiparallel to the magnetic field. These distributions are only seen in the dusk sector, but this may be an orbital artifact.

Deforest, S. E.↗
NASA NTRS1981

Reduction of the one-dimensional X-ray scattering data on mirror flats at NASA/Marshall Space Flight Center

Data on the scattering of X-rays from various mirror flats obtained from different vendors. The scattering data discussed in this section is obtained by a one dimensional proportional counter at X-ray energies of 8.06, 6.4 and 2.99 keV. X-rays are generated at one end of a 312.4m long tube which is under vacuum. This long tube provides a parallel beam of X-rays which is then collimated by a slit and allowed to scatter from a pair of mirror flats at small grazing angles of incidence. The slit and mirrors assembly is placed 182.4 meters from the other end of the tube where the proportional counter detector is located. The detector is sitting on an optical table which is pivoted so that it can rotate to intercept X-rays at a range of spatial locations. The optical table pivot is located 2.78 meters in front of the detector, and has an operational angular range of 350 arc minutes. Measured from the position of the mirror, this total angular range of the data is (350) (2.78/182.4) = 5.33 arcminutes.

Williams, A. C.↗
NASA NTRS2005

Impact of Cirrus Crystal Shape on Solar Spectral Irradiance: A Case Study for Subtropical Cirrus

Profiles of in situ measurements of ice crystal size distribution of subtropical cirrus were used to calculate solar spectral irradiances above and below the clouds. Spheres and nonspherical ice crystal habits (columns, hollows, plates, bullets, and aggregates) were assumed in the calculations. The simulation results were compared to irradiance measurements from the NASA Solar Spectral Flux Radiometer. The microphysical and radiation data were collected by three aircraft during CRYSTAL-FACE. Two cirrus cases (optical thickness of about 1 and 7) from two mission dates (26 and 23 July 2002) were investigated in detail. The measured downwelling and upwelling irradiance spectra above the cirrus could mostly be reproduced by the radiation model to within +/- 5-10% for most ice crystal habits. Below the cirrus the simulations disagreed with the measured irradiances due to surface albedo variability along the flight track, and nonoptimal colocation between the microphysical and irradiance measurements. The impact of shape characteristics of the crystals was important for the reflected irradiances above the optically thin cirrus, especially for small solar zenith angles, because in this case single-scattering dominated the solar radiation field. For the cirrus of moderate optical thickness the enhanced multiple scattering tended to diminish particular shape features caused by nonspherical single-scattering. Within the ice absorption bands the shape-related differences in the absorption characteristics of the individual nonspherical ice crystals were amplified if multiple scattering prevailed. Furthermore, it was found that below the cloud the shape sensitivity of the downwelling irradiance spectra is larger compared to the nonsphericity effects on reflected irradiances above the cirrus. Finally, it was shown that the calculated cirrus solar radiative forcing could vary by as much as 26% depending on the ice crystal habit.

Wendisch, Manfred↗
NASA NTRS1997

Polarization of Line Emission from an Accretion Disk and Application to ARP 102B

We model the polarization properties of line emission from an accretion disk under a range of assumptions about the source function and electron-scattering optical depth tau(sub es). For small values of tau(sub es) and modest viewing angles, polarization can be in excess of the Chandrasekhar result for tau(sub es) = infinity. The polarization vector can be either parallel or perpendicular to the projected direction of the disk axis. The polarization properties of the double-peaked H-alpha emission line of the broad-line radio galaxy Arp 102B observed by Antonucci, Hurt, and Agol can be understood in terms of electron scattering and line broadening within the line-emitting region if tau(sub es) is of order unity, and if the position angle of polarization is parallel to the projected disk axis. The required small tau(sub es) is consistent with the hypothesis that the Balmer lines in Arp 102B are produced by photoionization of the disk atmosphere.

Chen, Kaiyou↗
NASA NTRS2018

Development of Low-Scatter Optical Edges for Starshades

Starshades, combined with future space telescopes, provide the ability to detect Earth-like exoplanets in the habitable zone by producing high contrast ratios at small inner working angles. The primary function of a starshade is to suppress light from a target star such that its orbiting planets are revealed. In order to do so, the optical edges of the starshade must maintain their precise in-plane profile to produce the necessary apodization function. However, an equally important consideration is the interaction of these edges with light emanating from our own Sun as scattered and/or diffracted sunlight can significantly degrade the achievable contrast. This paper describes the technical efforts performed to obtain precision, low-scatter optical edges for future starshades. Trades between edge radius (i.e. sharpness) and surface reflectivity have been made and small-scale coupons have been produced using scalable manufacturing processes. A custom scattered light testbed has been developed to quantify the magnitude of scattered light over all sun angles. Models have also been developed to make predictions on the level of reflected and/or diffracted light for various edge architectures. The results of these studies have established a current baseline approach which implements photochemical etching techniques on thin metal foils.

Bradley, Christine↗
NASA NTRS1981

The ultraviolet properties of dust in the Orion Nebula

The continuum in the Orion Nebula was observed with the IUE at 16 positions varying from 30 arcsec to 5 arcmin in angular separation from the central star Theta(1) Orionis C. The atomic continuum was estimated from the H-beta brightnesses. The remaining scattered starlight was analyzed by multiple-scattering models in which the main parameters are: (1) the albedo of the dust; (2) the scattering phase function parameter, the averaged cosine of the angle of scattering; and (3) the density of dust grains at small distances from the star. It is found that (a) dust must be depleted near the star; (b) the albedo is fairly well determined and is constant across the 2200 A extinction feature; and (c) grains are quite forward-throwing at 1300 A, as they have been found at 4600 A earlier. The scattering is more isotropic at 2400 A. However, the absolute value of the scattering phase function parameters at all wavelengths is quite uncertain because it depends sensitively upon the density of grains close to the star.

Mathis, J. S.↗
NASA NTRS1980

Dependence of the spectral surface irradiance on aerosol properties and surface reflectivity

A reduction in global surface irradiance occurs with increasing aerosol loadings when the aerosols are absorbing. For scattering aerosols, a reduction is pronounced for isotropic scattering (characteristic of small particles) but reduction is not as significant for scattering with a high anisotropy of a large forward peak (characteristic of large particles). This distinction between isotropic and anisotropic scattering becomes small or null over highly reflecting terrain; and for reflectivities higher than 0.5 and solar elevation angles close to the zenith, the global irradiance can be slightly higher for isotropic scattering than in the case of an anisotropy of a forward peak. Under such conditions, which can be encountered in reflective infrared bands over dense vegetation or over sandy deserts (close to noon, in low latitudes) the surface irradiance becomes nearly independent of the aerosol optical thickness.

Otterman, J.↗
NASA NTRS1990

Optical scattering methods applicable to drops and bubbles

An overview of optical scattering properties of drops and bubbles is presented. The properties lead to unconventional methods for optically monitoring the size or shape of a scatterer and are applicable to acoustically levitated objects. Several of the methods are applicable to the detection and measurement of small amplitude oscillations. Relevant optical phenomena include: (1) rainbows; (2) diffraction catastrophes from spheroids; (3) critical angle scattering; (4) effects of coatings; (5) glory scattering; and (6) optical levitation.

Marston, Philip L.↗
NASA NTRS1975

Thermal diffuse X-ray scattering in simple metals

Calculations are reported for the ionic structure factor and X-ray scattering cross section of sodium (at T=0 K and 90 K) and lithium (both isotopes at T=0 K) within the harmonic approximation. An evaluation of the appropriate displacement- displacement correlation function by the special point method circumvents the need for a multiphonon expansion. In the case of sodium, the structure in the one-phonon scattering was straightforwardly accounted for, and an approximate expansion was obtained for all multiphonon scattering. By treating core and conduction electrons on an equal footing, it is shown that information on the conduction electron system is present in the forward scattering component. In lithium the one-phonon cross section at small angles aids in the determination of the effective electron-ion interaction.

Straus, D. M.↗
NASA NTRS1980

Polar nephelometer for atmospheric particulate studies

A polar nephelometer for use in studying atmospheric aerosols was developed. The nephelometer detects molecular scatter from air and measures scattering from very clean air using pure molecular scattering for calibration. A compact system using a folded light path with an air cooled argon laser for the light source was designed. A small, sensitive detector unit permits easy angular rotation for changing the scattering angle. A narrow detector field of view of + or - 1/4 degree of scattering along with a single wavelength of incident light is used to minimize uncertainties in the scattering theory. The system is automated for data acquisition of the scattering matrix elements over an angular range from 2 degrees to 178 degrees of scattering. Both laser output and detector sensitivity are monitored to normalize the measured light scattering.

Hansen, M. Z.↗
NASA NTRS1988

Optical investigation of Comet Halley

The physical properties of Comet P/Halley are being monitored from the ground using state of the art optical detectors. The long time base (1982-present) of the observations provides a unique record of the development of activity in a single comet. In addition, physical properties of other comets are measured to provide a reference against which Comet Halley may be compared. Results for the past year include: (1) publication of the first results on the surface brightness profiles of dust comets; (2) charge coupled device measurements of Comet Bowell show that the activity of comets can persist at least to heliocentric distances R = 13.6 AU; (3) a study of the back-scattering phase angle dependences of 5 comets (including Halley) shows evidence for small linear phase coefficients in each case; (4) temporal monitoring of P/Halley continues.

Jewitt, David↗