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At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Separability of spatiotemporal spectra of image sequences

The spatiotemporal power spectrum was calculated of 14 image sequences in order to determine the degree to which the spectra are separable in space and time, and to assess the validity of the commonly used exponential correlation model found in the literature. The spectrum was expanded by a Singular Value Decomposition into a sum of separable terms and an index was defined of spatiotemporal separability as the fraction of the signal energy that can be represented by the first (largest) separable term. All spectra were found to be highly separable with an index of separability above 0.98. The power spectra of the sequences were well fit by a separable model. The power spectrum model corresponds to a product of exponential autocorrelation functions separable in space and time.

Eckert, Michael P.↗
NASA NTRS1992

Charged particle LET-spectra measurements aboard LDEF

The linear energy transfer (LET) spectra of charged particles was measured in the 5 to 250 keV/micron (water) interval with CR-39 and in the 500 to 1500 keV/micron (water) interval with polycarbonate plastic nuclear track detectors (PNTDs) under different shielding depths in the P0006 experiment. The optimal processing conditions were determined for both PNTDs in relation to the relatively high track densities due to the long term exposure in space. The total track density was measured over the selected samples, and tracks in coincidence on the facing surfaces of two detector sheets were selected for measuring at the same position on each sheet. The short range (SR) and Galactic Cosmic Ray (GCR) components were measured separately with CR-39 PNTDs and the integral dose and dose rate spectra of charged particles were also determined. The high LET portion of the LET spectra was measured with polycarbonate PNTDs with high statistical accuracy. This is a unique result of this exposure due to the low flux of these types of particles for typical spaceflight durations. The directional dependence of the charged particles at the position of the P0006 experiment was also studied by four small side stacks which surrounded the main stack and by analyzing the dip angle and polar angle distributions of the measured SR and GCR particle tracks in the main stack.

Csige, I.↗
NASA NTRS1992

Turbulent spectra, fluxes, stability and growth of the mixed layer in the boundary layer of Mars

Spectra of wind and temperature from high frequency measurements in the atmospheric surface layer of Mars are presented for the first time. Heat and momentum fluxes, and stability are calculated for early spring from estimates of the surface temperature and mean Viking Lander 2 temperature and wind at 44 degrees N, using similarity theory. This study provides the first estimates of (1) the height of the mixed layer, (2) spectra of wind and temperature, (3) the validity of similarity theories on Mars, and (4) bounds of effective measuring height and surface roughness. It confirms and extends the universiality of model wind spectra. Finally and more important, we provide the ability to estimate and determine the self-consistency of Martian atmospheric mixed layer fluxes, stability and heights on a diurnal and annual basis. This can be used to initialize and validate the various models, which previously could only be compared with each other or the efforts of Sutton, Leovy and Tillman: these were restricted in season and limited by the results from the surface temperature model available at that time. Analysis of additional data can better estimate z(sub 0) values directly from the measurements and provide the parameters necessary to calculate the vertical profiles of wind and temperature on a daily, seasonal and annual basis.

Tillman, James E.↗
NASA NTRS1992

Measurements of LET spectra and comparison to models

Measurements of LET spectra for near-earth orbits with various inclinations and altitudes are presented. A comparison with calculated LET spectra shows that the contribution from direct ionizing galactic cosmic rays is well described by the models. An additional contribution to the spectra originates from stopping protons and from nuclear interactions of particles with material. In the case of an interaction, a large amount of energy is deposited in a small volume by target recoils or target fragments. These events will be called short range (SR) events. For a low-inclination orbit radiation belt protons are the main source of these events while galactic protons become more important when increasing the inclination to near-polar orbits. It is shown that the contribution of SR events for orbits with low altitude (324 km) and 57-deg inclination is comparable to that for an orbit with 28-deg inclination at a high altitude (510 km).

Wiegel, B.↗
NASA NTRS1992

Energy spectra of ions from impulsive solar flares

A study of the energy spectra of ions from impulsive solar flares in the 0.1-100 MeV region is reported. Most of the events studied are dominated by He and these He spectra show a persistent steepening or break above about 10 MeV resulting in an increase in the power-law spectral indices from about 2 to about 3.5 or more. Spectra of H, He-3, O, and Fe have spectral indices that are consistent with a value of about 3.5 above about 2 MeV/amu. One event, dominated by protons, shows a clear maximum in the spectrum near 1 MeV. If the rollover in the spectrum below 1 MeV is interpreted as a consequence of matter traversal in the solar atmosphere, then the source of the acceleration would lie only about 800 km above the photosphere, well below the corona. Alternative interpretations are that trapping in the acceleration region directly causes a peak in the resulting ion spectrum or that low-energy particles encounter significant additional scattering during transport from the flare.

Reames, D. V.↗
NASA NTRS1992

Analysis of the photospheric epoch spectra of type 1a supernovae SN 1990N and SN 1991T

This paper presents an LTE analysis of the photospheric epoch optical and IUE UV spectra of the recent Type Ia supernovae SN 1990N and SN 1991T. It is found that SN 1990N has matter moving as fast as 40,000 km/s, while SN 1991T has matter moving as fast as 20,000 km/s. The exponential density profile with e-folding velocity of 3160 km/s suggested by deflation and delayed/late-detonation explosion models is adequate for calculating fits to the observed spectra. Findings on the iron peak elements suggest that in both SNe some nuclear burning continues into the outer ejecta or the newly synthesized elements are mixed into the outer ejecta. The observed spectra of SN 1990N are consistent with a composition of the inner envelope like that of model W7. Intermediate-mass elements with abundances higher than solar are probably in the outer ejecta of both SNe, while silicon, sulfur, and calcium are underabundance in SN 1991T relative to SN 1990N by factors of order 3, 3, and 120, respectively.

Jeffery, David J.↗
NASA NTRS1992

CCD reflectance spectra of selected asteroids. I - Presentation and data analysis considerations

Narrowband reflectance spectra have been acquired which contribute to the library of asteroid data in the visible and near-IR spectral regions. The spectra support the existence of aqueous alteration products on asteroids located in the outer part of the main asteroid belt out to at least 4 AU. No evidence for features similar to the spectral features of ordinary chondrite meteorites was found in the spectra of asteroids located near the 3:1 Kirkwood Gap chaotic zone.

Vilas, Faith↗
NASA NTRS1993

Long-term variations of interplanetary magnetic field spectra with implications for cosmic ray modulation

The paper calculates yearly averaged power spectra of interplanetary magnetic field turbulence at 1 AU over the period 1965-1988 for fluctuations in the frequency range 5.8 x 10 exp -6 to 4.6 x 10 exp -5 Hz, corresponding to periods of 6-48 hr. The amplitudes of the spectra vary with the sunspot cycle and are inversely correlated with the intensity of about 10-GeV cosmic rays. The observed spectra are used to calculate a lower limit to the cosmic ray scattering mean free path employing resonant magnetostatic quasi-linear theory for both 'slab' and isotropic geometries of the turbulence. The mean free paths thus obtained are typically about 0.1 AU in the slab model and about 0.3 AU in the isotropic model, but they are not significantly correlated with the modulated galactic cosmic ray intensity recorded by neutron monitors. It is inferred that the scattering processes described by resonant magnetostatic theory play, at best, a very minor role in the solar modulation of about 10-GeV cosmic rays.

Bieber, John W.↗
NASA NTRS1993

An atlas of ultraviolet spectra of star-forming galaxies

A systematic study is presented of the UV spectra of star-forming galaxies of different morphological type and activity class using a sample drawn from a uniformly reduced IUE data set. The spectra for a wide variety of galaxies, including normal spiral, LINER, starburst, blue compact, blue compact dwarf, and Seyfert 2 galaxies, are presented in the form of spectral energy distributions to demonstrate the overall characteristics according to morphology and activity class and in the form of absolute flux distributions to better show the absorption and emission features of individual objects. The data support the picture based on UV spectra of the Orbiting Astronomical Observatory and of the Astronautical Netherlands Satellite that spiral galaxies of later Hubble class have more flux at the shortest UV wavelengths than do spiral galaxies of earlier Hubble class.

Kinney, A. L.↗
NASA NTRS1990

Cosmic ray proton spectra at low rigidities

The cosmic ray proton rigidity spectra have been investigated with data collected in the Low Energy Antiproton (LEAP) balloon flight experiment flown from Prince Albert, Canada in 1987. The LEAP apparatus was designed to measure antiprotons using a superconducting magnet spectrometer with ancillary scintillator, time-of-flight, and liquid Cherenkov detectors. After reaching float altitude the balloon drifted south and west to higher geomagnetic cutoffs. The effect of the changing geomagnetic cutoff on the observed spectra was observed during analysis of the proton data along the balloon trajectory. This is the first measurement of the primary and splash albedo spectra over a wide rigidity range (few hundred MV to about 100 GV) with a single instrument.

Seo, E. S.↗
NASA NTRS1946

The infrared spectra of spiropentane methylenecyclobutane and 2-methyl-1-butene

The infrared spectra of spiropentane, methylenecyclobutane, and 2-methyl-1-butene were measured in the region from 3 to 14 microns with a rock salt prism spectrometer of medium dispersion. The pure samples were prepared at the NACA Cleveland Laboratory. The vapors of these three C5 hydrocarbons were investigated at room temperature and at pressures in the range from 80 to 300 millimeters of mercury absolute in a 10-centimeter cell. The spectra were compared with each other and with Ramon spectra for the same compounds.

Cleaves, Alden P↗
NASA NTRS1993

The Grand Banks ERS-1 SAR wave spectra validation experiment

As part of the ERS-1 validation program, the ERS-1 Synthetic Aperture Radar (SAR) wave spectra validation experiment was carried out over the Grand Banks of Newfoundland (Canada) in Nov. 1991. The principal objective of the experiment was to obtain complete sets of wind and wave data from a variety of calibrated instruments to validate SAR measurements of ocean wave spectra. The field program activities are described and the rather complex wind and wave conditions which were observed are summarized. Spectral comparisons with ERS-1 SAR image spectra are provided. The ERS-1 SAR is shown to have measured swell and range traveling wind seas, but did not measure azimuth traveling wind seas at any time during the experiment. Results of velocity bunching forward mapping and new measurements of the relationship between wind stress and sea state are also shown.

Vachon, P. W.↗
NASA NTRS1993

The effects of atmospheric pressure on infrared reflectance spectra of Martian analogs

The use of terrestrial samples as analogs of Mars soils are complicated by the Martian atmosphere. Spectral features due to the Martian atmosphere can be removed from telescopic spectra of Mars and ISM spectra of Mars, but this does not account for any spectral differences resulting from atmospheric pressure or any interactions between the atmosphere and the surface. We are examining the effects of atmospheric pressure on reflectance spectra of powdered samples in the laboratory. Contrary to a previous experiment with granite, no significant changes in albedo or the Christiansen feature were observed from 1 bar pressure down to a pressure of 8 micrometers Hg. However, reducing the atmospheric pressure does have a pronounced affect on the hydration features, even for samples retained in a dry environment for years.

Bishop, Janice L.↗
NASA NTRS1993

The calculation of theoretical chromospheric models and the interpretation of solar spectra from rockets and spacecraft

Since the early 1970s we have been developing the extensive computer programs needed to construct models of the solar atmosphere and to calculate detailed spectra for use in the interpretation of solar observations. This research involves two major related efforts: work by Avrett and Loeser on the Pandora computer program for non-LTE modeling of the solar atmosphere including a wide range of physical processes, and work by Kurucz on the detailed synthesis of the solar spectrum based on opacity data for over 58 million atomic and molecular lines. Our goals are to determine models of the various features observed on the Sun (sunspots, different components of quiet and active regions, and flares) by means of physically realistic models, and to calculate detailed spectra at all wavelengths that match observations of those features. These two goals are interrelated: discrepancies between calculated and observed spectra are used to determine improvements in the structure of the models, and in the detailed physical processes used in both the model calculations and the spectrum calculations. The atmospheric models obtained in this way provide not only the depth variation of various atmospheric parameters, but also a description of the internal physical processes that are responsible for non-radiative heating, and for solar activity in general.

Avrett, Eugene H.↗
NASA NTRS1994

Ferromagnetic resonance spectra of H2-reduced minerals and glasses

In an earlier paper, we reported that H2 reduction of basaltic glass, olivine, pyroxene, and plagioclase resulted in the formation of metallic iron, in the darkening and reddening of the reflectance spectra, and the masking of individual spectral features in the visible and near-IR. In this work, we report FMR spectra for H2-reduced minerals and glasses that include the samples studied in the earlier paper. The FMR spectra were reduced at room temperature at a nominal frequency of 9.5 GHz. Sample saturation magnetization reported as F3(0) was measured with a vibrating sample magnetometer.

Morris, Richard V.↗
NASA NTRS1994

Simulation of gamma-ray spectra for a variety of user-specified detector designs

The gamma-ray spectrum simulation program BSIMUL was designed to allow the operator to follow the path of a gamma-ray through a detector, shield and collimator whose dimensions are entered by the operator. It can also be used to simulate spectra that would be generated by a detector. Several improvements have been made to the program within the last few months. The detector, shield and collimator dimensions can now be entered through an interactive menu whose options are discussed below. In addition, spectra containing more than one gamma-ray energy can now be generated with the menu - for isotopes listed in the program. Adding isotopes to the main routine is also quite easy. Subroutines have been added to enable the operator to specify the material and dimensions of a collimator. This report details the progress made in simulating gamma-ray spectra for a variety of user-specified detector designs. In addition, a short discussion of work done in the related areas of pulse shape analysis and the spectral analysis is included. The pulse shape analysis and spectral analysis work is being performed pursuant to the requirements of contract F-94-C-0006, for the Advanced Research Projects Agency and the U.S. Air Force.

Rester, A. C., Jr.↗
NASA NTRS1994

Pressure and higher-order spectra for homogeneous isotropic turbulence

The spectra of the pressure, and other higher-order quantities including the dissipation, the enstrophy, and the square of the longitudinal velocity derivative are computed using data obtained from direct numerical simulation of homogeneous isotropic turbulence at Taylor-Reynolds numbers R(sub lambda) in the range 38 - 170. For the pressure spectra we find reasonable collapse in the dissipation range (of the velocity spectrum) when scaled in Kolmogorov variables and some evidence, which is not conclusive, for the existence of a k(exp -7/3) inertial range where k = absolute value of K, is the modulus of the wavenumber. The power spectra of the dissipation, the enstrophy, and the square of the longitudinal velocity derivative separate in the dissipation range, but appear to converge together in the short inertial range of the simulations. A least-squares curve-fit in the dissipation range for one value of R(sub lambda) = 96 gives a form for the spectrum of the dissipation as k(exp 0)exp(-Ck eta), for k(eta) greater than 0.2, where eta is the Kolmogorov length and C is approximately equal to 2.5.

Pullin, D. I.↗
NASA NTRS1995

Summary of LET spectra and dose measurements on ten STS missions

A comparison of linear energy transfer (LET) spectra measurements made with plastic nuclear track detectors (PNTD's) from area passive dosimeters (APD's), was made for ten different STS missions under similar shielding. The results show that integral flux, dose rate and equivalent dose rate values follow a general increase with respect to increasing orbital inclination and altitude but that there are large variations from a simple relationship. This is to be expected since it has been shown that Shuttle attitude variations, combined with the anisotropic particle flux at the South Atlantic Anomaly (SAA), can result in differences of a factor of 2 in dose rate inside the Shuttle (Badhwar et al., 1995). Solar cycle and shielding differences also result in variations in radiation dose between STS missions. Spaceflight dosimeters from the STS missions are also being used in the development of a method for increasing LET spectra measurement accuracy by extending LET measurements to particle tracks of ranges 10-80 microns. Refinements in processing and measurement techniques for the flight PNTD's have yielded increased detection efficiencies for the short tracks when LET spectra determined by using the standard and refined methods were intercompared.

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