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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
NASA NTRS1973

Twilight airglow. I - Photoelectrons and forbidden O I 5577-angstrom radiation.

A payload consisting of a number of experiments to study the earth's atmosphere was launched from White Sands on Feb. 8, 1971. The differential photoelectron flux spectrum was measured as a function of altitude. The energy distribution revealed the N2 vibrational structure appearing at 2.8 V, rising to a maximum at 4 eV, decreasing to an 8-volt-wide plateau at 20 V, and then further decreasing. The ion and electron density distributions were measured simultaneously. An optical measurement of forbidden O I 5577-A radiation was made. Both electron impact on atomic oxygen and dissociative recombination of O2(+) were found to produce this emission above 150 km. The recombination rate for the O(1 S) found from a reported nightglow profile is 2.5 plus or minus 1.5 x 10 to the minus 9th cu cm/sec. Between 140 and 120 km, photodissociation is a source of 5577 radiation. Chapman three-body recombination is dominant below 120 km.

Hays, P. B.↗
NASA NTRS1973

High voltage pulse ignition of mercury discharge hollow cathodes

A high voltage pulse generated by a capacitor discharge into a step-up transformer has been demonstrated capable of consistently igniting hollow cathode mercury discharges at propellant flows and heater power levels much below those required by conventional cathode starting. Results are presented for 3.2-mm diameter enclosed and open keeper cathodes. Starting characteristics are shown to depend on keeper voltage, mercury flow rate, heater power, keeper orifice size, emissive materials, and electrode to which the pulse is applied. This starting technique has been used to start a cathode over 10,000 times without any degradation of starting capability. The starting reliability, propellant and power savings offered by the high voltage pulse start should favorably impact performance of electron bombardment thrusters in missions requiring many on-off duty cycles.

Wintucky, E. G.↗
NASA NTRS1976

O2/1-Sigma sub g/+// relaxation in collisions - Temperature dependence of the interaction with HBr

Rate constants for the quenching of O2(1-Sigma sub g(+)) in the temperature range 170-400 K were obtained by observing the change in the rate of decay of this oxygen emission as a function of HBr concentration. At each temperature the decay was observed at eight different HBr concentrations ranging from 0 to 6 x 10 to the -6th moles/l. It is found that long range multipolar interactions make a major contribution to the quenching of the oxygen molecule by H2, with the quenching shown to be most effective at low temperatures. Short range repulsive interactions may also contribute to the quenching process, especially at high temperatures. The suggestion that the quenching process is dominated by the ability of the quencher to take up the energy of the oxygen molecule transition in vibrational and rotational motion is confirmed.

Braithwaite, M.↗
NASA NTRS1986

Rotationally resolved collisional transfer rates in OH

Fluorescence lidar measurements of the hydroxyl radical require detailed information concerning collision induced processes in order to deduce the radical number density from a lidar return. The Goddard SFC OH lidar currently utilizes a broadband detector which precludes the necessity of fully understanding collisional redistribution of rotational energy within the excited state. Numerous advantages result however from the inclusion of a detector with a bandpass only slightly larger that the Doppler width of a rotational line. This however places more stringent requirements on the spectroscopy. Measurements were accordingly made of rotationally resolved quenching rates for collisions with O2, N2, and H2O. Rotational transfer rates were also measured for the same colliders. Quenching rates were measured using a Nd-YAG pumped Rh6G dye laser doubled into the UV. The OH lifetimes were measured as a function of pressure of quenching gas at total pressures of between 50 and 250 microns. Rotational transfer rates were measured by recording the emission spectrum on an intensified diode array and integrating over 10.000 laser shots.

Mcgee, T. J.↗
NASA NTRS1986

Electron-cyclotron maser instability in relativistic plasmas

The electron-cyclotron maser instability is studied for the case of an anisotropic electron velocity distribution in the regime where the relativistic corrections to the wave dispersion are significant. Solution of the linear dispersion relation reveals that when the plasma frequency-gyrofrequency ratio is less than v(te)/c, the instability is localized just below k(perpendicular)c/Omega(e) = 1. The growth rate is then strongly peaked for emission at 90 deg to the magnetic field and is considerably larger than would be the case if the cold-plasma dispersion theory were valid. These features are confirmed by EM particle simulations.

Pritchett, P. L.↗
NASA NTRS1990

Stability constraints for line scanning cameras in orbit

Requirements, parameters, and constraints influencing the design of a line scanner camera system for imaging from orbit are described. The driving requirements are the area of the site to be imaged in each pass and the resolution as determined by the size of a pixel projected on the planet surface. Allowable jitter and distortion, required SNR, maximum data rate, and maximum variation of the emission angle during imaging are among the constraints. Simplified equations incorporating these requirements, constraints, and parameters are developed and presented.

Bernard, Douglas E.↗
NASA NTRS1991

Infrared observations of comets

Selected comets are observed in the near infrared (1 to 2.2 micron) and thermal infrared (3.5 to 20 micron) with the NASA Infrared Telescope Facility (IRTF) and other telescopes as appropriate, in order to characterize the physical properties of the dust grains; their composition, size distribution, emissivity, and albedo. Systematic variations in these properties among comets are looked for, in order to understand the heterogeneity of comet nuclei. Spectrophotometry of the 10 micron silicate emission feature is particularly emphasized. The rate of dust production from the nucleus and its temporal variability are also determined. Knowledge of the dust environment is essential to S/C design and mission planning for NASA's CRAF mission.

Hanner, Martha S.↗
NASA NTRS1992

An infrared search for formaldehyde in several comets

Models which assume either a parent or a distributed source for H2CO are presently used to retrieve 3-sigma upper limits to that species' production rate from a 3.55-3.64 micron search for cometary IR emission from the nu1 and nu5 bands of formaldehyde. A parent-source H2CO production rate relative to water of 0.15-0.80 percent is obtained, together with a distributed source production rate of 1.7-5.8 percent.

Reuter, Dennis C.↗
NASA NTRS1992

158 micrometers (CII) mapping of NGC 6946: Probing the atomic medium

The strong 158 micrometers (C2) cooling line of the NGC 6946 galaxy is investigated. The data was acquired with a far infrared imaging Fabry-Perot interferometer with 55 minutes resolution. About 1 percent of the total far infrared luminosity of the galaxy is analyzed. The (C2) emission comes from a mizture of components of interstellar gas. The brightest emission is associated with the nucleus, a second component traces the spiral arms and the largest star forming/H2 regions contained within them, and a third extended component of low brightness can be detected at least 12 kpc from the nucleus. The nuclear and spiral arm components are most likely associated with dense photon dominated regions at molecular cloud surfaces that are exposed to ultraviolet radiation produced by young massive stars. The (C2) emission is analyzed and the cooling rate in the atomic medium is consistent with photoelectric heating by diffuse ultraviolet radiation.

Madden, S. C.↗
NASA NTRS1994

10 micron detection of the hard X-ray transient GRO J0422+32: Free-free emission from an X-ray-driven accretion disk wind?

We report the detection of 10 micrometer emission from the transient low-mass X-ray binary (LMXB) and optical nova GRO J0422+32 near the maximum of its outburst. We discuss this result in terms of (1) a 'standard' model according to which low-energy radiation of LMXB is caused by reprocessing of X-rays in an accretion disk; (2) emission from a cool secondary star; (3) emission from dust grains heated by the transient X-rays, and (4) free-free emission from an X-ray-driven wind from the accretion disk. Only the fourth alternative provides a viable explanation for the observed 10 micrometer emission, with a mass-loss rate in the disk wind that may be substantially higher than the rate of accretion onto the compact star. The presence of such a wind may have a profound effect on the evolution of the binary, and contribute to the solution of the 'birthrate problem' of millisecond ratio pulsars.

Paradijs, Van J.↗
NASA NTRS1994

GHRS Spectroscopy of individual stars in R136a

The installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR) Instrument on the Hubble Space Telescope (HST) makes it possible to observe stars in very crowded regions with high spatial and spectral purity. To demonstrate this capability, we have used the Goddard High Resolution Spectrograph (GHRS) to obtain spectra of two stars in the dense center of the 30 Doradus ionizing cluster: R136a5, and its nearest neighbor, R136a2, only 0.17 sec away. R136a5 is shown to ben an O3f/WN star, while R136a2 is a WN4-w star. From both Wide Field/Planetary Camera (WFPC) photometry and GHRS, spectroscopy we estimate the following properties of R136a5: T(sub eff) = 42,500 K, R = 16.4 solar radius, L(sub bol) = 8 x 10(exp 5) solar luminosity, and M approx. equals 50 solar mass -- all indicating that, despite its spectral type, R136a5, as indicated by the strength of He II lambda 1640 emission. The observed mass-loss rate, dot-M = 1.8 x 10(exp -5) solar mass/yr, is an order of magnitude higher than is assumed by current stellar evolutionary models. We argue that this high rate of mass loss will alter drastically the evolutionary path of R136a5. If so, evolutionary models for massive stars require substantial revision.

Heap, Sara R.↗
NASA NTRS1994

IR excitation of contaminant water by oxygen for the space shuttle at low Earth orbit altitude

As the water outgas of a space shuttle passes through the rarefied atmosphere at orbital altitude, collisions occur between the gases with sufficient energy to excite infrared-active water molecules to various vibrational and rotational states. An infrared contaminant model (IR model) has been developed to study the shuttle-induced excitation and emission of water molecules outgassed from the space shuttle. The focus of the first application of the model is translation-to-vibration (T-V) energy transfer since estimates suggest that this process should dominate the production of vibrationally excited H2O under typical low Earth orbit conditions. Using the velocity and position distribution functions of interacting neutral gases obtained from a neutral gases interaction model, the spatial distributions of excitation and IR radiation from contaminant water are computed, and typical results are presented. Infrared spectral data (450 - 2500/cm), measured by the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS-1A) sensor on STS-39 (April 28 to May 6, 1991) at an altitude near 265 km, are used to test model predictions. The dependence of the radiant emission structure and brightness on outgassing rates and altitudes is discussed. The time history of the contaminant water outgassing rate is inferred for STS-39, and it is compared with the mass-spectrometer-based results for STS-4 (June 26 to July 4, 1982). Also, estimates of H2O column density at mission elapsed time (MET) 50 hours are compared for missions STS-2, STS-3, STS-4, and STS-39.

Zhou, D. K.↗
NASA NTRS1998

International Space Station Cathode Life Testing Status

To demonstrate adequate lifetime and performance capabilities of a hollow cathode for use on the International Space Station (ISS) plasma contactor system, life tests of multiple hollow cathode assemblies (HCAs) were initiated at operating conditions simulating on-orbit operation. Three HCAs are presently being tested. These HCAs are operated with a continuous 6 sccm xenon flow rate and 3 A anode current. Emission current requirements are simulated with a square waveform consisting of 50 minutes at a 2.5 A emission current and 40 minutes with no emission current. As of July 1998, these HCAs have accumulated between 1 1,700 and 14,200 hours. While there have been changes in operatin, behavior the three HCAs continue to operate stably within ISS specifications and are expected to demonstrate the required lifetime.

Sarver-Verhey, Timothy R.↗
NASA NTRS2002

X-Ray Properties of Lyman Break Galaxies in the Hubble Deep Field North Region

We describe the X-ray properties of a large sample of z approximately 3 Lyman Break Galaxies (LBGs) in the region of the Hubble Deep Field North, derived from the 1 Ms public Chandra observation. Of our sample of 148 LBGs, four are detected individually. This immediately gives a measure of the bright AGN (active galactic nuclei) fraction in these galaxies of approximately 3 per cent, which is in agreement with that derived from the UV (ultraviolet) spectra. The X-ray color of the detected sources indicates that they are probably moderately obscured. Stacking of the remainder shows a significant detection (6 sigma) with an average luminosity of 3.5 x 10(exp 41) erg/s per galaxy in the rest frame 2-10 keV band. We have also studied a comparison sample of 95 z approximately 1 "Balmer Break" galaxies. Eight of these are detected directly, with at least two clear AGN based on their high X-ray luminosity and very hard X-ray spectra respectively. The remainder are of relatively low luminosity (< 10(exp 42) erg/s, and the X-rays could arise from either AGN or rapid star-formation. The X-ray colors and evidence from other wavebands favor the latter interpretation. Excluding the clear AGN, we deduce a mean X-ray luminosity of 6.6 x 10(exp 40) erg/s, a factor approximately 5 lower than the LBGs. The average ratio of the UV and X-ray luminosities of these star forming galaxies L(sub UV)/L (sub X), however, is approximately the same at z = 1 as it is at z = 3. This scaling implies that the X-ray emission follows the current star formation rate, as measured by the UV luminosity. We use our results to constrain the star formation rate at z approximately 3 from an X-ray perspective. Assuming the locally established correlation between X-ray and far-IR (infrared) luminosity, the average inferred star formation rate in each Lyman break galaxy is found to be approximately 60 solar mass/yr, in excellent agreement with the extinction-corrected UV estimates. This provides an external check on the UV estimates of the star formation rates, and on the use of X-ray luminosities to infer these rates in rapidly starforming galaxies at high redshift.

Nandra, K.↗
NASA NTRS2002

Bn and Si-Doped Bn Coatings on Woven Fabrics

A computer controlled, pulsed chemical vapor infiltration (CVI) system has been developed to deposit BN from a liquid borazine (B3N3H6) source, as well as silicon doped BN coatings using borazine and a silicon source, into 2-D woven ceramic fabric preforms. The coating process was evaluated as a function of deposition temperature, pressure, and precursor flow rate. Coatings were characterized by field emission scanning electron microscopy, electron dispersive spectroscopy and Auger spectroscopy. By controlling the reactant feed ratios, Si incorporation could be controlled over the range of 6-24 atomic percent.

Hurwitz, Frances I.↗
NASA NTRS2007

A Comparison of Major and Minor Species in Io's Exosphere

The atmosphere of the volcanically active moon Io is continuously stripped away through interactions with Jupiter's magnetic field and replenished by volcanically supplied gas. Io's exosphere, the low density region consisting of bound and escaping, non-interacting neutrals, is dominated by oxygen and sulfur atoms formed from the dissociation of SO2. The radial distributions of oxygen and sulfur emissions exhibit an asymmetry between the intensity over Io's leading and trailing hemispheres (Wolven et al. 2001). Sodium, a minor but well studied exospheric component, shows a different asymmetry: the sub-Jupiter exosphere is denser than the anti-Jupiter exosphere (Burger et al. 2001). In addition, the sodium density decreases much faster with distance than the intensities of the oxygen and sulfur emissions. We explore possible sources for these. One possibility is the dependence of the observed emissions lines on plasma flowing through the exosphere. Variations in the electrons affect the excitation rates of the transitions, while leaving the resonantly scattered sodium emission unaffected. The observations may also point to differences in the loss mechanisms from Io's atmosphere: sodium is believed to be sputtered from the atmosphere and surface by high energy ions in the magnetosphere (reviewed Johnson et al 2004; Thomas et al. 2004). It is unclear whether oxygen and sulfur are lost in the same manner, or instead by the dissociation of SO2 near or above the exobase, which would produce a significantly different energy distribution than sputtering, producing the observed differences.

Burger, M. H.↗
NASA NTRS2008

The Radio Luminosity Function and Galaxy Evolution in the Coma Cluster

We investigate the radio luminosity function and radio source population for two fields within the Coma cluster of galaxies, with the fields centered on the cluster core and southwest infall region and each covering about half a square degree. Using VLA data with a typical rms sensitivity of 28 (mu)Jy per 4.4" beam, we identify 249 radio sources with optical counterparts brighter than r = 22 (equivalent to M(sub r) = -13 for cluster member galaxies). Comprehensive optical spectroscopy identifies 38 of these as members of the Coma cluster, evenly split between sources powered by an active nucleus and sources powered by active star formation. The radio-detected star-forming galaxies are restricted to radio luminosities between about 10(exp 21) and 10(exp 22) W/Hz, an interesting result given that star formation dominates field radio luminosity functions below about 10(exp 23) W/Hz. The majority of the radio-detected star-forming galaxies have characteristics of starbursts, including high specific star formation rates and optical spectra with strong emission lines. In conjunction with prior studies on post-starburst galaxies within the Coma cluster, this is consistent with a picture in which late-type galaxies entering Coma undergo a starburst prior to a rapid cessation of star formation. Optically bright elliptical galaxies (Mr less than or equals -20.5) make the largest contribution to the radio luminosity function at both the high (> approx. 3x10(exp 22) W/Hz) and low (< approx. 10(exp 21) W/Hz) ends. Through a stacking analysis of these optically-bright ellipticals we find that they continue to harbor radio sources down to luminosities as faint as 3x10(exp 19) W/Hz. However, contrary to published results for the Virgo cluster we find no evidence for the existence of a population of optically faint (M(sub r) approx. equals -14) dwarf ellipticals hosting strong radio AGN.

Miller, Neal A.↗
NASA NTRS2014

Assessment of Reduced-Kinetics Mechanisms for Combustion of Jet Fuel in CFD Applications

A computational effort was undertaken to analyze the details of fluid flow in Lean-Direct Injection (LDI) combustors for next-generation LDI design. The National Combustor Code (NCC) was used to perform reacting flow computations on single-element LDI injector configurations. The feasibility of using a reduced chemical-kinetics approach, which optimizes the reaction rates and species to model the emissions characteristics typical of lean-burning gas-turbine combustors, was assessed. The assessments were performed with Reynolds- Averaged Navier-Stokes (RANS) and Time-Filtered Navier Stokes (TFNS) time-integration, with a Lagrangian spray model with the NCC code. The NCC predictions for EINOx and combustor exit temperature were compared with experimental data for two different single-element LDI injector configurations, with 60deg and 45deg axially swept swirler vanes. The effects of turbulence-chemistry interaction on the predicted flow in a typical LDI combustor were studied with detailed comparisons of NCC TFNS with experimental data.

Ajmani, Kumud↗