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At least 883 records · Page 49
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NASA NTRS1995

A 7 micron dust emission feature in oxygen-rich circumstellar shells

New 4 to 8 micron infrared spectroscopic observations of two oxygen-rich stars are presented and combined with IRAS low resolution spectrometer (LRS) data to span the 4 to 24 micron wavelength range. In the 4 to 8 micron range, we observe a 7.15 micron (1400 cm(exp -1)) emission feature. This new feature is not uniquely correlated with any of the sharply defined 10, 11, 13.1, and 19.7 micron emission features that are known to be present in this class of circumstellar shells, but it does appear to be correlated with the spectrally broad dust emission in the 10 to 20 micron spectral region. The feature has not been reported previously in any other astronomical environment. A reinterpretation of prior 4 to 8 micron spectroscopy of alpha Ori and R Cas reveals the presence of the 7.15 micron emission in alpha Ori and possibly in R Cas. The spectrally narrow 19.7 micron emission, that is distinctly different than the relatively broad silicate 18 micron emission feature in oxygen-rich dust shells, is also observed to be present in the LRS spectrum of SAO 197549. The implication of these observations is that a universal astronomical silicate does not exist in oxygen-rich circumstellar shells.

Goebel, J. H.↗
NASA NTRS1995

Aircraft Emission Inventories Projected in Year 2015 for a High Speed Civil Transport (HSCT) Universal Airline Network

This report describes the development of a three-dimensional database of aircraft fuel burn and emissions (fuel burned, NOx, CO, and hydrocarbons) from projected fleets of high speed civil transports (HSCT's) on a universal airline network.Inventories for 500 and 1000 HSCT fleets, as well as the concurrent subsonic fleets, were calculated. The objective of this work was to evaluate the changes in geographical distribution of the HSCT emissions as the fleet size grew from 500 to 1000 HSCT's. For this work, a new expanded HSCT network was used and flights projected using a market penetration analysis rather than assuming equal penetration as was done in the earlier studies. Emission inventories on this network were calculated for both Mach 2.0 and Mach 2.4 HSCT fleets with NOx cruise emission indices of approximately 5 and 15 grams NOx/kg fuel. These emissions inventories are available for use by atmospheric scientists conducting the Atmospheric Effects of Stratospheric Aircraft (AESA) modeling studies. Fuel burned and emissions of nitrogen oxides (NOx as NO2), carbon monoxide, and hydrocarbons have been calculated on a 1 degree latitude x 1 degree longitude x 1 kilometer attitude grid and delivered to NASA as electronic files.

Baughcum, Steven L.↗
NASA NTRS1996

Scheduled civil aircraft emission inventories for 1992: Database development and analysis

This report describes the development of a three-dimensional database of aircraft fuel burn and emissions (fuel burned, NOx, CO, and hydrocarbons) from scheduled commercial aircraft for each month of 1992. The seasonal variation in aircraft emissions was calculated for selected regions (global, North America, Europe, North Atlantic, and North Pacific). A series of parametric calculations were done to quantify the possible errors introduced from making approximations necessary to calculate the global emission inventory. The effects of wind, temperature, load factor, payload, and fuel tankering on fuel burn were evaluated to identify how they might affect the accuracy of aircraft emission inventories. These emissions inventories are available for use by atmospheric scientists conducting the Atmospheric Effects of Aviation Project (AEAP) modeling studies. Fuel burned and emissions of nitrogen oxides (NOx as N02), carbon monoxide, and hydrocarbons have been calculated on a 1 degree latitude x 1 degree longitude x 1 kilometer altitude grid and delivered to NASA as electronic files.

Baughcum, Steven L.↗
NASA NTRS1996

Electron Emission Observations from As-Grown and Vacuum-Coated Chemical Vapor Deposited Diamond

Field emission has been observed from chemical vapor deposited diamond grown on Mo and Si substrates. Emission was observed at fields as low as 20 kV/cm. The samples were tested in the as-grown form, and after coating with thin films of Au, CsI, and Ni. The emission current was typically maximum at the onset of the applied field, but was unstable, and decreased rapidly with time from the as-grown films. Thin Au layers, approximately 15 nm thick, vacuum deposited onto the diamond samples significantly improved the stability of the emission current at values approximately equal to those from uncoated samples at the onset of the applied field. Thin layers of CsI, approximately 5 nm thick, were also observed to improve the stability of the emission current but at values less than those from the uncoated samples at the onset of the applied field. While Au and CsI improved the stability of the emission, Ni was observed to have no effect.

Lamouri, A.↗
NASA NTRS1996

Hard Gamma Ray Emission from the Starburst Galaxy NGC 253

We have completed the study to search for hard gamma ray emission from the starburst galaxy NGC 253. Since supernovae are thought to provide the hard gamma ray emission from the Milky Way, starburst galaxies, with their extraordinarily high supernova rates, are prime targets to search for hard gamma ray emission. We conducted a careful search for hard gamma ray emission from NGC 253 using the archival data from the EGRET experiment aboard the CGRO. Because this starburst galaxy happens to lie near the South Galactic Pole, the Galactic gamma ray background is minimal. We found no significant hard gamma ray signal toward NGC 253, although a marginal signal of about 1.5 sigma was found. Because of the low Galactic background, we obtained a very sensitive upper limit to the emission of greater than 100 MeV gamma-rays of 8 x 10(exp -8) photons/sq cm s. Since we expected to detect hard gamma ray emission, we investigated the theory of gamma ray production in a dense molecular medium. We used a leaky-box model to simulate diffusive transport in a starburst region. Since starburst galaxies have high infrared radiation fields, we included the effects of self-Compton scattering, which are usually ignored. By modelling the expected gamma-ray and synchrotron spectra from NGC 253, we find that roughly 5 - 15% of the energy from supernovae is transferred to cosmic rays in the starburst. This result is consistent with supernova acceleration models, and is somewhat larger than the value derived for the Galaxy (3 - 10%). Our calculations match the EGRET and radio data very well with a supernova rate of 0.08/ yr, a magnetic field B approx. greater than 5 x 10(exp -5) G, a density n approx. less than 100/sq cm, a photon density U(sub ph) approx. 200 eV/sq cm, and an escape time scale tau(sub 0) approx. less than 10 Myr. The models also suggest that NGC 253 should be detectable with only a factor of 2 - 3 improvement in sensitivity. Our results are consistent with the standard picture of gamma-ray acceleration by supernovae.

Jackson, James M.↗
NASA NTRS1998

Emissions Measurements from a Lobed Fuel Injector/Burner

The present experimental study examines NO(x) and CO emissions associated with alternative fuel injector geometries. These injectors mix fuel and air to differing extents and thus create different local equivalence ratios upstream of flame ignition and stabilization. Two of the devices studied are lobed fuel injectors, in which mixing of reactants is associated with stream wise vorticity generation and straining of fuel-air interfaces, while the third is a non-lobed fuel injector which creates relatively little fuel-air mixing prior to ignition.Results show that one lobed injector geometry appears to produce locally lean premixed flame structures, resulting in low NO. emissions when compared with non-lobed injector emissions. The other lobed injector geometry appears to produce a local fuel-air mixture which is closer to stoichiometric conditions, with NO(x) emissions that are actually higher than for the non-lobed injector. For both lobed injector geometries examined here, CO emissions become high for over-all lean operating conditions, consistent with premixed combustion behavior. The present study demonstrates the importance of control of the local equivalence ratio in minimizing burner emissions.

Mitchell, M. G.↗
NASA NTRS1998

Burner Emissions Associated with Lobed and Non-Lobed Fuel Injectors

The present experimental study examines NO(sub x), and CO emissions associated with three alternative fuel-injector geometries. These injectors mix fuel and air and strain their interfaces to differing extents and thus create different local equivalence ratios within flow regions upstream of flame ignition and stabilization. Two of the devices studied are lobed fuel injectors, in which molecular mixing of reactants is associated with streamwise vorticity generation, while the third one is a non-lobed fuel injector. Results show that rapid mixing allowed both lobed injector geometries to produce very lean premixed flame structures, with a lower achievable turn-down or fuel/air mass flux ratio than for the analogous non-lobed injector, which largely, produced distinct diffusion flames. All three injectors exhibited some level of sooting near walls and in the far-field region, with the non-lobed injector sooting to the greatest extent. At low fuel flow rates, in which the lobed injectors created locally very lean premixed conditions, there resulted lower NO(sub x) emissions as compared with non-lobed injector emissions. Yet at higher fuel-air mass flux ratios, NO(sub x) emissions from the lobed injectors were actually higher than for the nonlobed injector, likely due to reduced sooting and hence reduced radiative heat losses associated with enhanced mixing. For both lobed injector geometries examined here, CO emissions became high for low values of the fuel/air mass flux ratio, again consistent with locally premixed combustion behavior, despite the fact that fuel was injected separately from air directly into the burner test section. The present study demonstrates that, for fuel-air mixing enhancement devices, control of the local equivalence ratio is critical in order to optimize burner emissions.

Mitchell, M. G.↗
NASA NTRS1998

Monitoring Ly-Alpha Emission From the Blazar 3C 279

The blazar 3C 279 is well studied and shows frequent large continuum flares from radio to gamma-ray wavelengths. There have been a number of multiwavelength observations of 3C 279, and hence there are extensive ultraviolet data for this object available in the UV archives. In this paper we present Ly-alpha emission line measurements for 3C 279 using all the archival IUE SWP spectra from 1988 to 1996 and all archival Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) G190H spectra from 1992 to 1996. Individual archival IUE spectra of 3C 279 show weak Ly-alpha emission at approximately 1868 A (z = 0.536), which is easily seen in the co-added data. The Ly-alpha emission is observed in all the HST/FOS spectra. The strength of Ly-alpha is nearly constant (approximately 5 x 10(exp -14) erg/sq cm.s), while the 1750 A continuum varies by a factor of approximately 50, from approximately 0.6 to 31.6 x 10(exp -15) ergs/sq cm.s.A. The behavior of the Ly-alpha emission line flux and continuum flux is similar to that of the only other well observed blazar, 3C 273, which shows constant line flux while the continuum varies by a factor of approximately 3. This near-constancy of emission-line flux in the two best-studied blazars suggests that the highly variable beamed continuum is not a significant source of photoionization for the gas. Some other source, such as thermal emission from an accretion disk, must be providing a significant fraction of the photoionizing flux in these objects. The large amplitude variability seen at gamma-ray energies must be due to changes in the energetic electrons in the jet rather than changes in the external photon field.

Koratkar, Anuradha↗
NASA NTRS1999

Thermal Emission Spectroscopy of 1 Ceres: Evidence for Olivine

Thermal emission spectra of the largest asteroid 1 Ceres obtained from the Kuiper Airborne Observatory display features that may provide information on its surface mineralogy. A plot of the Ceres spectrum (calibrated using alpha Boo as a standard) divided by a standard thermal model (STM) is shown. Also shown is the emissivity spectrum deduced from reflectivity measurements for olivine grains <5 microns in diameter. The general shape of the Ceres and the olivine curves agree in essential details, such as the maxima from 8 to 12 microns, the minimum between 12 and 14 microns, the broad peak near 17.5 micron, and the slope beyond 22 micron. (Use of the 10 to 15-micron grain reflectivities provides a better match to the 12- to 14-micron dip. We used a value of unity for beta, the beaming factor associated with small-scale surface roughness in our STM. Adjustment of beta to a lower value raises the long-wavelength side of the Ceres spectrum, providing an even better match to the olivine curve.) The emissivity behavior roughly matches the emission coefficients which were calculated for olivine particles with a particle radius of 3 microns. Their calculations show not only the negative slope from 23 to 25 pm, but a continued decrease past 30 micron. The Ceres emissivity is thus similar to that of small olivine grains from 8 to 30 micron, but olivine's emissivity is lower from 5 to 8 pm.

Witteborn, F. C.↗
NASA NTRS1998

Galileo Direction Finding of Jovian Radio Emissions

The Galileo spacecraft, in orbit about Jupiter, has observed distinct spin modulation of plasma wave emissions near the Ganymede (G1 and G2) encounters in the frequency range from about 100 kHz to approximately 6 MHz. Assuming circularly polarized, transverse electromagnetic radiation, we have used the spin modulation of the sweep-frequency receivers of the electric dipole antenna over many spins to estimate the source location in the spin plane of the spacecraft. Hectometric (HOM) and decametric (DAM) emission is observed by Galileo as a general and continuous background with frequent bursts that last tens of minutes and can be separated by minutes or hours. We have analyzed HOM and DAM emissions observed near Jupiter just after the GI and G2 encounters, including two HOM/DAM "arc" signatures observed after the G2 encounter. These latter appear to be low-frequency extensions of DAM arcs, with source regions along either the Io or the Ganymede flux tube. While the uncertainties associated with the data analysis do not allow a precise source location, the HOM/DAM emission observed near the G1 and G2 encounters is consistent with a gyroresonant source region, but it is necessary to require refraction due to the Io torus to understand the results. To explain emission from apparent source regions above a gyroresonant source region, wave refraction from asymmetries in the Io plasma torus that extend along magnetic field lines is postulated. Alternatively, if such torus density asymmetries do not exist, emission with sources above a gyroresonant source region would require another free-energy source such as energetic plasma beams in the presence of density gradients or temperature anisotropies.

Menietti, J. D.↗
NASA NTRS1999

A Highly Doppler Blueshifted Fe-K Emission Line in the High-Redshift QSO PKS 2149-306

We report the results from an ASCA observation of the high-luminosity, radioloud quasar PKS 2149-306 (redshift 2.345), covering the approximately 1.7 - 30 keV band in the quasar-frame. We find the source to have a luminosity of approximately 6 x 10(exp 47) ergs/s in the 2 - 10 keV band (quasar frame). We detect an emission line centered at approximately 17 keV in the quasar frame. Line emission at this energy has not been observed in any other active galaxy or quasar to date. We present evidence rejecting the possibility that this line is the result of instrumental artifacts, or a serendipitous source. The most likely explanation is blueshifted Fe-K emission (the equivalent width, is EW approximately 300 +/- 200 eV, quasar frame). Bulk velocities of the order of 0.75c are implied by the data. We show that Fe-K line photons originating in an accretion disk and Compton-scattering off a leptonic jet aligned along the disk axis can account for the emission line. Curiously. if the emission-line feature recently discovered in another quasar (PKS 0637-752, z = 0.654) at 1.6 keV in the quasar frame, is due to blueshifted OVII emission, the Doppler blueshifting factor in both quasars is similar (approximately 2.7 - 2.8).

Yaqoob, Tahir↗
NASA NTRS2001

Discovery of Soft X-Ray Emission From Io, Europa and the Io Plasma Torus

We report the discovery of soft (0.25 - 2 keV) x-ray emission from the moons Io and Europa, probably Ganymede, and from the Io Plasma Torus (IPT). Bombardment by energetic (greater than 10 keV) H, O, and S ions from the region of the IPT seems the likely source of the x-ray emission from the Galilean moons. According to our estimates, fluorescent x-ray emission excited by solar x-rays, even during flares from the active Sun, charge-exchange processes, previously invoked to explain Jupiter's x-ray aurora and cometary x-ray emission, and ion stripping by dust grains fall to account for the observed emission. On the other hand, bremsstrahlung emission of soft X-rays from non-thermal electrons in the few hundred to few thousand eV range may account for a substantial fraction of the observed x-ray flux from the IPT.

Elsner, R. F.↗
NASA NTRS2001

Quantitative Surface Emissivity and Temperature Measurements of a Burning Solid Fuel Accompanied by Soot Formation

Surface radiometry is an established technique for noncontact temperature measurement of solids. We adapt this technique to the study of solid surface combustion where the solid fuel undergoes physical and chemical changes as pyrolysis proceeds, and additionally may produce soot. The physical and chemical changes alter the fuel surface emissivity, and soot contributes to the infrared signature in the same spectral band as the signal of interest. We have developed a measurement that isolates the fuel's surface emissions in the presence of soot, and determine the surface emissivity as a function of temperature. A commercially available infrared camera images the two-dimensional surface of ashless filter paper burning in concurrent flow. The camera is sensitive in the 2 to 5 gm band, but spectrally filtered to reduce the interference from hot gas phase combustion products. Results show a strong functional dependence of emissivity on temperature, attributed to the combined effects of thermal and oxidative processes. Using the measured emissivity, radiance measurements from several burning samples were corrected for the presence of soot and for changes in emissivity, to yield quantitative surface temperature measurements. Ultimately the results will be used to develop a full-field, non-contact temperature measurement that will be used in spacebased combustion investigations.

Piltch, Nancy D.↗
NASA NTRS1999

Effectiveness of Near-Grazing Incidence Reflection in Creating the Rotationally Modulated Lanes in the Jovian Hectometric Radio Emission Spectrum

The Galileo plasma wave instrument has identified a narrow (in frequency) attenuation band in the hectometric emission that varies in frequency with system 3 longitude. It is possible to model this emission band assuming a high-latitude cyclotron source region with emission that is efficiently attenuated when the ray path is nearly tangent to an L shell that is close to the Io flux tube. The data suggest that the mechanism for attenuating the emission is very efficient, with the ratio of attenuated to unattenuated emission I/I(sub o) < 0.02, and not a strong function of frequency. In this paper we demonstrate that incoherent scattering alone cannot explain the attenuation lane, which does not preclude coherent scattering by uncertain processes. We find rather that the source of attenuation is consistent with near-grazing incidence reflection of emission from an L shell that is near the Io flux tube (a caustic surface).

Menietti, J. D.↗
NASA NTRS1996

Modeling Global Biogenic Emission of Isoprene: Exploration of Model Drivers

Vegetation provides the major source of isoprene emission to the atmosphere. We present a modeling approach to estimate global biogenic isoprene emission. The isoprene flux model is linked to a process-based computer simulation model of biogenic trace-gas fluxes that operates on scales that link regional and global data sets and ecosystem nutrient transformations Isoprene emission estimates are determined from estimates of ecosystem specific biomass, emission factors, and algorithms based on light and temperature. Our approach differs from an existing modeling framework by including the process-based global model for terrestrial ecosystem production, satellite derived ecosystem classification, and isoprene emission measurements from a tropical deciduous forest. We explore the sensitivity of model estimates to input parameters. The resulting emission products from the global 1 degree x 1 degree coverage provided by the satellite datasets and the process model allow flux estimations across large spatial scales and enable direct linkage to atmospheric models of trace-gas transport and transformation.

Alexander, Susan E.↗
NASA NTRS2002

Control of Jupiter's Radio Emission and Aurorae by the Solar Wind

Radio emissions from Jupiter provided the first evidence that this giant planet has a strong magnetic field and a large magnetosphere. Jupiter also has polar aurorae, which are similar in many respects to Earth's aurorae. The radio emissions are believed to be generated along the high-latitude magnetic field lines by the same electrons that produce the aurorae, and both the radio emission in the hectometric frequency range and the aurorae vary considerably. The origin of the variability, however, has been poorly understood. Here we report simultaneous observations using the Cassini and Galileo spacecraft of hectometric radio emissions and extreme ultraviolet auroral emissions from Jupiter. Our results show that both of these emissions are triggered by interplanetary shocks propagating outward from the Sun. When such a shock arrives at Jupiter, it seems to cause a major compression and reconfiguration of the magnetosphere, which produces strong electric fields and therefore electron acceleration along the auroral field lines, similar to the processes that occur during geomagnetic storms at the Earth.

Gurnett, D. A.↗
NASA NTRS2002

Disentangling X-Ray Emission Processes In Vela-Like Pulsars

This grant is to support analysis of data from the X-ray Multi-mirror Mission (XMM). Specifically, we have been awarded time to observe two young neutron stars, B1823-13 and B1046-58, whose X-ray emission is expected to be a complicated combination of emission from an associated supernova remnant, from a wind-powered synchrotron nebula, from magnetospheric pulsations, and from the surface of the neutron star itself. It is only with XMM's unique combination of spectral, temporal and angular resolution that all these different processes can be separated and studied. Observations of B1823-13 have been conducted and analyzed. We interpret the data as follows: The unpulsed extended non-thermal nature of the central core argues that the extended source of emission corresponds to synchrotron emission from a nebula powered by the pulsar. The temperature of the diffuse component is too high to be interpreted as thermal emission; we rather argue that this extended component is non-thermal emission from a surrounding supernova remnant shell.

Gaensler, Bryan↗
NASA NTRS2002

The Soft X-Ray Emission in a Large Sample of Galaxy Clusters with ROSAT PSPC

The study of soft X-ray emission of 38 X-ray selected galaxy clusters observed by ROSAT PSPC indicates that the soft excess phenomenon may be a common occurrence in galaxy clusters. Excess soft X-ray radiation, above the contribution from the hot intra-cluster medium, is evident in a large fraction of sources, and is clearly detected with large statistical significance in the deepest observations. The investigation relies on new, high resolution 21 cm HI observations. The sample selection also features analysis of infrared images, to further ensure reliability of results with respect to the characteristics of Galactic absorption. The possibility of background or calibration effects as cause of the excess emission is likewise investigated; a detailed analysis of the distribution of the excess emission with respect to detector position and Galactic HI column density shows that the excess emission is a genuine celestial phenomenon. We find evidence for a preferential distribution of the soft excess emission at distances larger than approx. 150-200 kpc from the centers of clusters; this behavior may be naturally explained in the context of a non-thermal Inverse-Compton scenario. Alternatively, we propose that the phenomenon maybe caused by thermal emission of very large-scale 'warm' filaments seen in recent hydrodynamic simulations. This new interpretation relieves the very demanding requirements of either the traditional intra-cluster 'warm' gas and the non-thermal scenarios. We also investigate the possibility of the soft excess originating from unresolved, X-ray faint cluster galaxies.

Bonamente, Massimiliano↗