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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Saturn's rings: Particle composition and size distribution as constrained by microwave observations. I - Radar observations

The radar backscattering characteristics of compositional and structural models of Saturn's rings are calculated and compared with observations of the absolute value, wavelength dependence, and degree of depolarization of the rings' radar cross section (reflectivity). The doubling method is used to calculate reflectivities for systems that are many particles thick using optical depths derived from observations at visible wavelengths. If the rings are many particles thick, irregular centimeter- to meter-sized particles composed primarily of water ice attain sufficiently high albedos and scattering efficiencies to explain the radar observations. In that case, the wavelength independence of radar reflectivity implies the existence of a broad particle size distribution; a narrower size distribution is also a possibility. Particles of primarily silicate composition are ruled out by the radar observations. Purely metallic particles may not be ruled out on the basis of existing radar observations. A monolayer of very large ice 'particles' that exhibit multiple internal scattering may not yet be ruled out.

Cuzzi, J. N.↗
NASA NTRS2002

Foam Optics and Mechanics

The Foam Optics and Mechanics (FOAM) project will exploit the microgravity environment to more accurately measure the rheological and optical characteristics of wet aqueous foams. Using both rheology and laser light scattering diagnostics, the goal is to quantify the unusual elastic character of foams in terms of their underlying microscopic structure and dynamics. Of particular interest is determining how the elastic character vanishes, i.e., how the foam 'melts' into a simple viscous liquid, as a function of both increasing liquid content and increasing shear strain rate. The unusual elastic character of foams will be quantified macroscopically by measurement of the shear stress as a function of shear strain rate and of time following a step strain. Such data will be analyzed in terms of a yield stress, shear moduli, and dynamical time scales. Microscopic information about bubble packing and rearrangement dynamics, from which the macroscopic non-Newtonian properties ultimately arise, will be obtained non-invasively by multiple-light scattering: diffuse transmission spectroscopy (DTS) and diffusing wave spectroscopy (DWS). Quantitative trends with materials parameters, most importantly average bubble size and liquid content, will be sought in order to elucidate the fundamental connection between the microscopic structure and dynamics and the macroscopic rheology.

Durian, Douglas J.↗
NASA NTRS2000

Recent Trends in the Analysis of Quasioptical Systems

The recent trend in microwave instruments is the use of multiple millimeter and submillimeter wavelength bands. These systems are typically analyzed by using physical optics, Gaussian beams or ray tracing techniques. Physical optics offers high accuracy at the expense of computation time. This trade-off becomes particularly apparent in the analysis of multiple reflector antennas, such as beam waveguide antennas, where physical optics is used to compute the current on each reflector from the current on the previous reflector. At the other end of the spectrum is ray tracing approaches that ignore diffraction effects entirely. These methods are fast but sacrifice the ability to predict some effects accurately. An intermediate approach is to use an appropriate set of expansion functions to model the field between the reflectors. If the set is chosen wisely only a few coefficients need to be determined from each reflector current. The field is then computed at the next reflector through the use of the expansion functions and their coefficients rather than by using the previous reflector current. For a beam waveguide system with no enclosing tubes an excellent set of expansion functions is the Gaussian beam mode set. In many cases a preliminary design which includes the effects on diffraction may be obtained by considering only the fundamental mode and a thin lens model for the reflectors. Higher-order modes are included to model the effects of the curved reflector, which include asymmetric distortion of the beam, cross polarization, and beam truncation. This paper describes a computer code implementing higher-order Gaussian beam scattering by multiple reflector systems. There are four essential steps in the algorithm. (1) Compute the current on the first reflector using physical optics using either a feed model or by an incident set of Gaussian beam modes. (2) Find the direction of propagation for the reflected Gaussian beam-set using ray tracing. (3) Determine the waist size and location for the output beam set by examining the amplitude and phase distribution of the current on the reflector. (4) Compute the amplitudes of the individual modes in the output mode set. These steps are then repeated for each addition reflector in the chain. In each of these cases the previous Gaussian beam set provides the input field for the current calculation. Details of the four steps discussed above will be discussed. Examples will compare results from the Gaussian beam approach to pure physical optics, illustrating both its merits and limitations. Hybrid approaches capable of eliminating some of the limitations will also be discussed.

Hoppe, D. J.↗
NASA NTRS1993

Theoretical studies of the radar properties of the icy Galilean moons of Jupiter

The icy Galilean satellites of Jupiter - Europa, Ganymede, and Callisto - have unusual radar scattering properties compared with those of the terrestrial planets or Earth's Moon. There are three main features of the data that distinguish these targets: (1) the radar cross-section normalized by the geometrical cross-section is an order of magnitude larger than that of any terrestrial planet; (2) the reflected power is almost evenly distributed between two orthogonal polarizations with more power being returned in the same circular polarization as was transmitted whereas virtually all of the power returned from the terrestrial planets is contained in the opposite circular polarization to the one that was transmitted; and (3) the echo power spectra have a broad shape indicating a nearly uniformly radar-bright surface in contrast to the spectra from the terrestrial planets that contain a strong quasi-specular component from the vicinity of the sub-radar point and very little reflected power from the rest of the surface. The normalized radar cross-sections decrease as the areal water ice coverage decreases from Europa to Ganymede to Callisto. Recently, radar echoes from the polar caps of Mars and Mercury, and from Saturn's satellite Titan imply similarly strong cross-sections and have classically unexpected polarization properties and it is also thought that this is due to the presence of ice on the surface. A model called the radar glory model is analyzed and it is shown that the main features of the radar echoes calculated from this model agree well with the observations from all three icy Galilean satellites. This model involves long radar paths in the ice below the surface and special structures in which the refractive index decreases abruptly at a hemispherical boundary. It is not known whether such structures exist or how they could be created, but possible scenarios can be imagined such as the formation of an impact crater followed by deposition of a frost layer followed by a resurfacing event in which a layer of solid ice is placed above the layer of frost. Regardless of the exact geophysical processes required to create such structures, the superior ability of this model to account for all of the important observations with very few adjustable parameters and with no ad hoc assumptions is a compelling argument in support of at least the electromagnetic model. The key features of the electromagnetic model are multiple subsurface scattering events, total internal reflection, and a low degree of randomness imposed on a deterministic geometry that strongly favors backscattering.

Eshleman, Von R.↗
NASA NTRS1972

Momentum transfer interaction of a laser-produced plasma with a low-pressure background.

The expansion of a laser-produced metallic plasma into a photoionized hydrogen background has been experimentally studied. Langmuir probe and microwave diagnostics have detected an interaction front which decelerates with a dependence on background density and time consistent with a momentum coupling between the laser plasma and the ionized fraction of the background. An ion percursor has also been observed. Calculations of scattering cross sections indicate that multiple-encounter Coulomb scattering will dominate collisional momentum transfer. The leading edge of the laser plasma contains multiply charged ions of charge state z greater than or equal to 5, and collisional effects appear adequate to explain the principal features of the momentum coupling. The ion precursor may have a collisionless origin.

Koopman, D. W.↗
NASA NTRS1974

Radiative transfer in real atmospheres

The problem of multiple radiation scattering in an atmosphere characterized by various amounts of aerosol absorption and different particle size distributions was investigated. The visible part of the spectrum was emphasized, including the effect of ozone absorption. An atmosphere bounded by a nonhomogenous, Lambertian surface was also studied, along with the effect of background radiation on target in terms of various atmopheric and geometric conditions. Results of the investigation indicate that comtaminated atmospheres can change the radiation field by a considerable amount, and that the effect of non-uniform surface significantly alters the intrinsic radiation from a target element. The implications of these results for the recognition processing of multispectral remote sensing data is discussed.

Turner, R. E.↗
NASA NTRS1974

Investigation of electron dynamics in the magnetosphere with electron beams injected from sounding rockets

This paper describes an empirical study of the basic mechanisms by which electrons precipitate from the geomagnetic field to produce 'auroral X rays' during periods of geomagnetic activity. The study was based on data obtained by the Echo satellites. Echo I, launched in 1970, injected 40 keV, 0.1 amp electron pulses at low latitude (L ? 2.6) and successfully measured the returning pulses from the conjugate region. Electric fields and multiple Coulomb scattering were studied. Echo II, launched in 1972 from high latitude (L ? 8) studied the interaction of the beams with background radiation and the detailed motion of the beams near the rocket. Evidence for a beam plasma instability was obtained. Echo III launched in April 1974, (L ? 5.5) detected a series of conjugate echoes during the presence of a strong convective field in the magnetosphere. It was shown that the electric field measurement in the ionosphere using the incoherent backscatter radar and detectors on the rocket was transferred to the equatorial plane as though field lines were equipotentials.

Winckler, J. R.↗
NASA NTRS1975

The width of electron-photon cascades in air

Results of a Monte-Carlo simulation of electron-photon cascades are presented with particular emphasis on cascade lateral spread. The effects of single and multiple Coulomb scattering are represented by giving the particles suitable displacements in position and angle at each interaction point. Some of the more oblique particle tracks are first divided into a number of steps between interaction points and the scattering procedure is applied to each step separately. To represent the particle defections due the geomagnetic field, a field of 0.3 gauss is supposed present and the appropriate helical path is imposed on each particle between interaction points. The results are in good agreement with the calculations of Nordheim et al., with simulations using the Berger-Seltzer electron photon transport program and with experimental measurements in water.

Allan, H. R.↗
NASA NTRS1985

LPM effect and primary energy estimations

The distortion of the electron cascade development under LPM effects is now admitted; it consists of an increase of depth of showers origin, of shower maximum T sum max, a decrease of the number of particles at maximum N sub max and results in a flattening and a widening of the cascade transition curve. Connected with the influence of multiple Coulomb scattering on basic electromagnetic processes (bremsstrahlung, pair production), this effect appears at high energy with a threshold dependent on the density of the medium (more than 10 TeV for lead, more than 10 sup 6 TeV in air). Consequently, the electromagnetic components of hadron induced showers in lead and EAS in air calculated for the same hadronic cascades in the different alternative, including or not the LPM effect are examined here.

Bourdeau, M. F.↗
NASA NTRS1985

The Nagoya cosmic-ray muon spectrometer 3, part 2: Track detector

The twelve wide gap spark chambers were utilized as the track detectors of the Nagoya cosmic-ray muon spectrometer not only to obtain the precise locations of particles, but also to get some information about the correspondences between segments of trajectories. The area of each chamber is 150 x 70 sq cm and the width of a gap is 5 cm. The gas used is He at the atmospheric pressure. Each three pairs of them are placed on both sides of the deflection magnet. All images of sparks for each event are projected through the mirror system and recorded by two cameras stereoscopically. The mean detection efficiency of each chamber is 95 + or - 2% and the spacial resolution (jitter and drift) obtained from the prototype-experiment is 0.12 mm. Maximum detectable momentum of the spectrometer is estimated at about 10 TeV/c taking into account these characteristics together with the effects of the energy loss and multiple Coulomb scattering of muons in the iron magnet.

Shibata, S.↗
NASA NTRS1986

Infrared imaging and photometry of Comet Giacobini-Zinner

Infrared images and photometry were obtained to determine the spatial distribution and physical characteristics (temperature, albedo, size distribution, total mass, etc.) of the grains in the coma of Comet GZ. A 10.8 m image of Comet GZ obtained on August 4 represents the first groundbased thermal-infrared image of a Comet. Among the most significant results are: (1)An estimate of the number of grains that the ICE spacecraft must have encountered, which led the plasma wave team to conclude that they could only detect impacts on the antennae and not on the whole body of the ICE spacecraft; (2)The discovery of a population of large grains (radius > 100 micrometer), not observed in most other comets, which formed a curved tail near the nucleus (within 80 arcsec or 34,000 km); and (3)the detection of structure in the spatial distribution in the coma of the particle albedo, which was tentatively attributed to the presence of very fluffy grains which are likely to have multiple internal scattering of incident sunlight. The albedo map of Comet GZ was obtained by combining the 10.8 micrometer image shown with a simultaneous image taken at 0.68 micrometer, a bandpass which isolates the scattered continuum.

Campins, H.↗
NASA NTRS1986

Thermal-infrared and visual imaging of comet Giacobini-Zinner

Thermal-infrared and visual images obtained simultaneously of comet Giacobini-Zinner on UT 1985 August 4.4 are presented. The 10.8-micron map, which spans several arc minutes with 8-arcsec resolution, is the first ground-based infrared image of a comet. Considerable spatial structure is evident in the map, including a curved dust tail consisting primarily of dust grains larger than 100 microns. The width of this large-grain tail implies that these grains depart the nucleus with velocities less than 3 m/s. The distribution of grain albedo is determined for the central 1 arcmin. It is speculated that an observed increase in the albedo by a factor of 2 with distance from the nucleus and prolonged radius vector results from the confinement to the orbital plane of large fluffy grains previously considered for GZ by others; the lower albedo of this dust could result from multiple internal scattering increasing the probability of absorption of incident light. It is estimated that dust was departing the nucleus at the rate of about 8 x 10 to the 5th g/s, comparable to that derived for many other comets.

Telesco, C. M.↗
NASA NTRS1988

P-Cygni features and photospheric velocities

The photospheric velocities implied from the P-Cygni features of H-alpha, H-beta, and H-gamma in SN 1987A differ. This implies that the 'photosphere' relevant to P-Cygni formation may be frequency dependent. To obtain a P-Cygni feature from above a fixed surface, the photons leaving there must not undergo other frequency dependent absorption processes (bound-free or free-free) or be smeared in frequency due to multiple Thomson scattering. The paper delineates different regimes when the frequency dependence of this surface should be manifest, and those when it would not be.

Bildsten, Lars↗
NASA NTRS1989

Formation of cyclotron lines in gamma-ray burst spectra

A transmission model of gamma-ray burst sources is studied using the relativistic QED magnetic-resonant opacities including multiple photon scattering, incorporated into a discrete-ordinate radiative-transport scheme. The physics of the cyclotron line-producing region is discussed in general, and the expected line profiles, relative harmonic strengths, and polarizations are indicated under various conditions. The calculated spectra for these models show good agreement with the spectra reported from Ginga for GB 880205 and GB 870303.

Alexander, S. G.↗
NASA NTRS1990

The sensitivity of EGRET to gamma ray polarization

A Monte Carlo simulation shows that EGRET (Energetic Gamma-Ray Experimental Telescope) does not even have sufficient sensitivity to detect 100 percent polarized gamma-rays. This is confirmed by analysis of calibration data. A Monte Carlo study shows that the sensitivity of EGRET to polarization peaks around 100 MeV. However, more than 10 (exp 5) gamma-ray events with 100 percent polarization would be required for a 3 sigma significance detection - more than available from calibration, and probably more than will result from a single score source during flight. A drift chamber gamma ray telescope under development (Hunter and Cuddapah 1989) will offer better sensitivity to polarization. The lateral position uncertainty will be improved by an order of magnitude. Also, if pair production occurs in the drift chamber gas (xenon at 2 bar) instead of tantalum foils, the effects of multiple Coulomb scattering will be reduced.

Mattox, John R.↗
NASA NTRS1989

Biophysical aspects of heavy ion interactions in matter

The biological effects of high energy, high charge nuclei (HZE particles) occupy a central role in the management of space radiation hazards due to galactic cosmic rays. For the energy range of interest, the mean free path for nuclear interactions of these heavy ions is comparable to the thickness of the material traversed, and a significant fraction of stopping particles will undergo a nuclear reaction with the nuclei of the stopping material. Transport methods for HZE particles are dependent on models of the interaction of man-made systems with the space environment to an even greater extent than methods used for other types of radiation. Hence, there is a major need to validate these transport codes by comparison with experimental data. The basic physical properties of HZE particles will be reviewed and illustrated with the results of nuclear fragmentation experiments performed with 670A MeV neon ions incident on a water absorber and with measurements of multiple Coulomb scattering of uranium beams in copper. Finally, the extent to which physical measurements yield radiobiological predictions is illustrated for the example of neon.

Schimmerling, Walter↗
NASA NTRS1991

Background in X-ray astronomy proportional counters

The authors report the results of an investigation into the nature of background events in proportional counters sensitive to X-ray photons having energy of less than 150 keV. Even with the use of thick shields composed of high-atomic-number material, a significant flux background in the detector's energy region can result from multiple Compton scattering in the mass surrounding the active region of the detector. The importance of the selection of detector components in the reduction of the background by more than an order of magnitude is emphasized. Experimental results are shown to agree qualitatively with Monte Carlo simulations. It is concluded that escape gating is a powerful means of determining the nature of background in flight detectors: the single/pair ratios reveal whether the detected events are charged particles or photons.

Bower, C. R.↗
NASA NTRS1992

Resonant Compton cooling and annihilation line production in gamma-ray bursts

Attention is given to a synchrotron self-Compton emission model for gamma-ray bursts which produces narrow annihilation features for a variety of field strengths, primary electron injection energies, and injection rates. In this model, primary electrons are injected and cooled by synchrotron emission in a strong, homogeneous magnetic field, resulting in a pair cascade. Multiple resonant scattering with cyclotron photons efficiently traps and cools pairs in the ground state to an average energy where the Compton energy loss rate is zero, which is in agreement with previous estimates of a Compton temperature. The particle distributions in the ground state are determined by numerically solving the Fokker-Planck equation in the steady state. In the case of isotropic injection of primary electrons, a significant narrow-line feature appears in the overall emission. In the case of beamed injection, the annihilation line is broadened to the extent that it would not be observable.

Preece, R. D.↗