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At least 1,171 records · Page 65

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NASA NTRS1989

Gravitational effects on planetary neutron flux spectra

The effects of gravity on the planetary neutron flux spectra for planet Mars, and the lifetime of the neutron, were investigated using a modified one-dimensional diffusion accelerated neutral-particle transport code, coupled with a multigroup cross-section library tailored specifically for Mars. The results showed the presence of a qualitatively new feature in planetary neutron leakage spectra in the form of a component of returning neutrons with kinetic energies less than the gravitational binding energy (0.132 eV for Mars). The net effect is an enhancement in flux at the lowest energies that is largest at and above the outermost layer of planetary matter.

Feldman, W. C.↗

NASA NTRS1988

Raman spectra of shocked minerals. I - Olivine

The Raman spectra of olivine contained in a chip of the Twin Sisters Peak (Washington) dunite shocked to 22.2 GPa is shown to be identical to that of unshocked olivine in the same rock. The Raman spectra of powder of the rock shocked to 20.1 GPa and of chips shocked to 59.5 GPa and 60.7 GPa display strong and broad low-frequency features with crests at 475/cm, 556/cm, and 572/cm, and broad high-frequency features near 1100/cm. It is suggested that these features are due to the formation of olivine glass with a considerable degree of three-dimensional Si-O-Si linkage having scattered domains of greatly variable grain size, internal structure, and chemical composition.

Heymann, D.↗

NASA NTRS1989

Monte Carlo computations of F-region incoherent radar spectra at high latitudes and the use of a simple method for non-Maxwellian spectral calculations

Monte Carlo simulations of ion velocity distributions in the high-latitude F region have been performed in order to improve the calculation of incoherent radar spectra in the auroral ionosphere. The results confirm that when the ion temperature becomes large due to frictional heating in the presence of collisions with the neutral background constituent, F region spectra evolve from a normal double hump, to a triple hump, to a spectrum with a single maximum. An empirical approach is developed to overcome the inadequacy of the Maxwellian assumption for the case of radar aspect angles of between 30 and 70 deg.

Kikuchi, K.↗

NASA NTRS1989

Production of self-absorbed synchrotron spectra steeper than nu to the 5/2

Self-absorbed synchrotron radiation produced by electrons with a power-law distribution of energies has a unique spectral shape: intensity is proportional to nu to the 5/2, where nu is the frequency, irrespective of the power law index of the electrons. It has been asserted that the measurement of a spectral index greater than 5/2 at frequencies below the 'far-IR turnovers' observed in the spectra of many radio-quiet AGN is incompatible with the physics of self-absorbed synchrotron sources. It is shown here that plausible electron energy distributions can lead to self-absorbed synchrotron spectra which are steeper than nu to the 5/2 over 1-1.5 orders of magnitude in frequency. This indicates that none of the existing observations are in fact incompatible with self-absorbed synchrotron radiation as the source.

De Kool, Martijn↗

NASA NTRS1989

Stratospheric HBr mixing ratio obtained from far infrared emission spectra

Emission features of HBr isotopes have been identified in high-resolution FIR emission spectra obtained with a balloon-borne Fourier-transform spectrometer in the spring of 1979 at 32 deg N latitude. When six single-scan spectra at a zenith angle of 93.2 deg were averaged, two features of HBr isotopes at 50.054 and 50.069/cm were obtained with a signal-to-noise ratio of 2.5. The volume mixing ratio retrieved from the average spectrum is 2.0 x 10 to the -11th, which is assumed to be constant above 28 km, with an uncertainty of 35 percent. This stratospheric amount of HBr is about the same as the current level of tropospheric organic bromine compounds, 25 pptv. Thus HBr could be the major stratospheric bromine species.

Park, J. H.↗

NASA NTRS1989

Absorption features in the 3 micron spectra of protostars

Low-resolution spectra have been obtained for a selection of infrared protostars and one object located behind the Taurus dark cloud. Most of the differences in the spectra can be attributed to different H2O ice temperatures combined with additional broad absorption between 3.3 and 3.5 microns plus another absorption in the 2.8-2.9 micron range. An NH3-H2O ice mixture, scattering by H2O ice-coated grains, and hydrated silicates are ruled out as explanations for the last type of absorption. The most plausible explanation is still some form of hydrocarbon in the grain mantles.

Smith, Robert G.↗

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

Trigonometric Polynomials For Estimation Of Spectra

Orthogonal sets of trigonometric polynomials used as suboptimal substitutes for discrete prolate-spheroidal "windows" of Thomson method of estimation of spectra. As used here, "windows" denotes weighting functions used in sampling time series to obtain their power spectra within specified frequency bands. Simplified windows designed to require less computation than do discrete prolate-spheroidal windows, albeit at price of some loss of accuracy.

Greenhall, Charles A.↗

NASA NTRS1990

Infrared spectra of van de Waals complexes of importance in planetary atmospheres

It has been suggested that (CO2)2 and Ar-CO2 are important constituents of the planetary atmospheres of Venus and Mars. Recent results on the laboratory spectroscopy of CO2 containing van der Waals complexes which may be of use in the modeling of the spectra of planetary atmospheres are presented. Sub-Doppler infrared spectra were obtained for (CO2)2, (CO2)3, and rare-gas-CO2 complexes in the vicinity of the CO2 Fermi diad at 2.7 micrometers using a color-center-laser optothermal spectrometer. From the spectroscopic constants the geometries of the complexes have been determined and van der Waals vibrational frequencies have been estimated. The equilibrium configurations are C2h, C3h, and C2v, for (CO2)2, (CO2)3, and the rare-gas-CO2 complexes, respectively. Most of the homogeneous linewidths for the revibrational transitions range from 0.5 to 22 MHz, indicating that predissociation is as much as four orders of magnitude faster than radiative processes for vibrational relaxation in these complexes.

Fraser, G. T.↗

NASA NTRS1989

Cyclotron resonant scattering in the spectra of gamma-ray bursts

Fits of theoretical spectra from Monte Carlo radiation-transfer calculations to dips at approximately 20 and 40 keV in a spectrum of the gamma-ray burst source GB 880 205 give best-fit values and 68 percent-confidence intervals for the magnetic field of (1.71 + or - 0.07) x 10 to the 12th G, the electron density of (1.2 + or - 0.6) x 10 to the 21st electrons/cm-squared, and the cosine of the viewing angle relative to the field of 0.31 + or - 0.05. The dips observed at approximately 20 keV in the spectra are interpreted as cyclotron resonant scattering, in which electrons undergo radiative 0 to 1 to 0 Landau transitions initiated by photons near the first harmonic. Physical self-consistency fixes the temperature, and the equilibrium temperature equals 5.3 + 0.3 or - 0.2 keV. These results suggest that this gamma-ray burst and many others which exhibit a low-energy dip originate from strongly magnetic neutron stars and are galactic in origin.

Wang, J. C. L.↗

NASA NTRS1988

Observed and theoretical spectra in the 10-100 A interval

The soft X-ray spectra recorded in two sounding-rocket flights in 1982 and 1985 are compared with predicted spectra. The processed densitometer trace of the full spectrum is presented, together with the new spectrum from the 1985 experiment. The intensities of the lines are then compared with predictions.

Brown, W. A.↗

NASA NTRS1989

Phyllosilicate absorption features in main-belt and outer-belt asteroid reflectance spectra

Absorption features having depths up to 5 percent are identified in high-quality, high-resolution reflectance spectra of 16 dark asteroids in the main belt and in the Cybele and Hilda groups. Analogs among the CM2 carbonaceous chondrite meteorites exist for some of these asteroids, suggesting that these absorptions are due to iron oxides in phyllosilicates formed on the asteroidal surfaces by aqueous alteration processes. Spectra of ten additional asteroids, located beyond the outer edge of the main belt, show no discernible absorption features, suggesting that aqueous alteration did not always operate at these heliocentric distances.

Vilas, Faith↗

NASA NTRS1989

Average electric wave spectra across the plasma sheet and their relation to ion bulk speed

Using 4 months of tail data obtained by the ELF/MF spectrum analyzer of the wave experiment and the three-dimensional plasma instrument on board the AMPTE/IRM satellite, a statistical survey on the electric wave spectral density in the earth's plasma sheet has been conducted. More than 50,000 10-s-averaged electric wave spectra were analyzed with respect to differences between their values in the inner and outer central plasma sheet and the plasma sheet boundary layer as well as their dependence on radial distance and ion bulk speed. High-speed flows are dominated by broadband electrostatic noise with highest spectral densities in the plasma sheet boundary, where broadband electrostatic noise also exists during periods of low-speed flows. The broadband electrostatic noise has a typical spectral index of about -2. During low-speed flows the spectra in the central plasma sheet show distinct emissions at the electron cyclotron odd half-harmonic and upper hybrid frequency. Wave intensities during episodes of fast perpendicular flows are higher than those associated with fast parallel flows.

Baumjohann, W.↗

NASA NTRS1989

Optical Stark effect in the four-wave mixing and stimulated Raman spectra of N2

The influence of the optical Stark effect on spectral line shapes in four-wave-mixing Raman spectroscopy (FWMRS) and stimulated Raman spectroscopy (SRS) is investigated experimentally and theoretically. Using an experimental setup capable of rapid alternation between the simultaneous measurement of coherent Stokes Raman spectroscopy and inverse Raman spectroscopy at low and high intensities, together with a sophisticated frequency reference scheme, it was possible to perform a rather direct comparison between Stark-broadened and non-Stark-broadened spectra of both classes of Raman spectroscopies. The results demonstrate that SRS spectra show more Stark shift and broadening than their FWMRS counterparts. A discrepancy with theoretical results is pointed out, and an attempt is made to explain it.

Moosmuller, H.↗

NASA NTRS1990

A catalogue of low-resolution IUE spectra of dwarf novae and nova-like stars

A catalogue is presented of all the low-resolution IUE spectra of dwarf novae and nova-like stars that were recorded until the end of 1987. All spectra have been reduced, and are displayed, in a homogeneous way. In addition to details about these data, to the extent available, their position in the outburst light curves is given, physical information about each system, and a comprehensive list of references for published observations in all wavelength ranges.

La Dous, Constanze↗

NASA NTRS1989

Multifractal spectra in shear flows

Numerical simulations of three-dimensional homogeneous shear flow and fully developed channel flow, are used to calculate the associated multifractal spectra of the energy dissipation field. Only weak parameterization of the results with the nondimensional shear is found, and this only if the flow has reached its asymptotic development state. Multifractal spectra of these flows coincide with those from experiments only at the range alpha less than 1.

Keefe, L. R.↗

NASA NTRS1990

Charge-coupled device spectra of asteroids. I - Near-earth and 3:1 resonance asteroids

Main-belt asteroids and cometary nuclei are both frequently suggested sources of the relatively short-lived near-earth asteroids (NEAs). The asteroidal origin hypothesis is studied by obtaining new high-quality reflectivity spectra of 19 NEAs and 23 asteroids near the 3:1 resonance, and by performing a systematic comparison of these two groups of spectra. Nonparametric statistical tests are used to check for consistency between the color distributions of the two populations. The tests indicate that the difference between the two distributions is generally not significant at the 3 sigma level. However, this conclusion is dependent on the (unknown) size distribution of the NEAs. The observations are compatible with the hypothesis that the NEAs are derived from the 3:1 resonance region, but do not imply that this is the only possible source region.

Luu, Jane X.↗

NASA NTRS1990

Unexpected effects of a trap in CCD echelle spectra of B-type stars

Because of the nature of echelle spectra, cosmetic defects such as traps may mimic real spectral features. An example from spectra taken at CTIO with a GEC CCD is presented, and it is shown how the affected pixels can be eliminated from the reduced spectrum, at a slight cost in signal-to-noise ratio.

Morrison, Nancy D.↗