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At least 1,009 records · Page 56
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NASA NTRS2019

Directional limits on persistent gravitational waves using data from Advanced LIGO’s first two observing runs

We perform an unmodeled search for persistent, directional gravitational wave (GW) sources using data from the first and second observing runs of Advanced LIGO. We do not find evidence for any GW signals. We place limits on the broadband GW flux emitted at 25 Hz from point sources with a power law spectrum at F(α;Θ) < (0.05–25) × 10^(−8) erg/sq. cmsHz and the (normalized) energy density spectrum in GWs at 25 Hz from extended sources at Ω(α)(Θ) < (0.19–2.89) × 10^(−8) per sr where α is the spectral index of the energy density spectrum. These represent improvements of 2.5–3× over previous limits. We also consider point sources emitting GWs at a single frequency, targeting the directions of Sco X-1, SN 1987A, and the Galactic center. The best upper limits on the strain amplitude of a potential source in these three directions range from h(0) < (3.6–4.7) × 10^(−25), 1.5× better than previous limits set with the same analysis method. We also report on a marginally significant outlier at 36.06 Hz. This outlier is not consistent with a persistent gravitational-wave source as its significance diminishes when combining all of the available data.

B. P. Abbott↗
NASA NTRS1972

Electrostatic instability of ring current protons beyond the plasmapause during injection events

The stability of ring current protons with an injection spectrum modeled by an m = 2 mirror distribution function was examined for typical ring current parameters. It was found that the high frequency loss cone mode can be excited at wave numbers K lambda sub Di about = to 0.1 to 0.5, at frequencies omega about = to (0.2 to 0.6) omega sub pi and with growth rates up to gamma/omega about = to 0.03. These waves interact with the main body of the proton distribution and propagate nearly perpendicular to the local magnetic field. Cold particle partial densities tend to reduce the growth rate so that the waves are quenched at or near to the plasmapause boundary. Wave e-folding lengths are comparable to 0.1 R sub e, compared to the value of about 4 R sub e found for ion cyclotron waves at the same plasma conditions.

Coroniti, F. V.↗
NASA NTRS1974

Spectrometer

Ultraviolet spectrometer measures pure monochromatic wavelengths in predetermined narrow wave bands. Two stages are incorporated: stationary dispersed beam is intercepted by array of slits cut into plate at discrete wavelength locations; second stage is inverted spectrometer which recombines dispersed spectrum at single exit slit.

Menzel, D. H.↗
NASA NTRS1982

Temporal variability of ultraviolet cloud features in the Venus stratosphere

The observed temporal variability of ultraviolet cloud brightness in over two months of Pioneer Venus Orbiter Cloud Photopolarimeter imaging is considered. It is found that the longitudinal mean brightness record shows evidence of two planetary-scale waves. One appears to be centered on the equator but has significant amplitude at all latitudes. The other is centered at midlatitudes and is more confined latitudinally. Visually, the two waves correspond to the longitudinal motion of the dark equatorial band and the oscillation in latitude of the bright polar bands, respectively. The slope of the longitudinal cloud brightness power spectrum at intermediate wavenumbers fluctuates with a periodicity of approximately 2-3 weeks in southern midlatitudes. The polar regions in both hemispheres vary in brightness on a time scale of approximately two months. The disk-integrated brightness increased with time throughout the nominal mission.

Del Genio, A. D.↗
NASA NTRS1986

The ABC of Population III

The author discusses the circumstances in which Population III stars are expected to form and examines their cosmological consequences. Consideration of their production of light, metals, and remnants places a strong constraint on their formation epoch and mass spectrum. However, these constraints would still allow Population III stars to provide the dark matter in galactic halos, a helium abundance of around 25 percent, a detectable background of gravitational waves, a possible infrared background, and the generation of large-scale cosmological structure through explosions. All of these features could be achieved by VMOs in the range 100 to 100,000 solar masses. On the other hand, the Population III scenario would be most compatible with the standard Big Bang picture if the stars were SMOs with mass around a million solar masses. Such stars could still provide the dark matter and detectable gravitational waves.

Carr, B. J.↗
NASA NTRS2004

Exploring the Saturn System in the Thermal Infrared: The Composite Infrared Spectrometer

The Composite Inbred Spectrometer (CIRS) is a remote-sensing Fourier Transform Spectrometer on the Cassini orbiter that measures thermal radiation over two decades in wave number, from 10 to 1400 cm (1 mm to 7pm), with a spectral resolution that can be set from 0.5 to 20 cm. The far in portion of the spectrum (10 - 600 cm) is measured with a polarizing interferometer having thermopile detectors with a common 4-mrad field of view. The middle infrared portion is measured with a traditional Michelson interferometer having two focal planes (600 - 1100cm, 1100-1400 cm). Each focal plane is composed of a 1x10 array of HgCdTe detectors, each detector having a 0.3-mrad field of view. CIRS observations will provide three-dimensional maps of temperature, gas composition, and aerosols/condensates of the atmospheres of Titan and Saturn with good vertical and horizontal resolution, from deep in their tropospheres to high in their mesospheres. CIRS ability to observe atmospheres in the limb viewing mode (in addition to nadir) offers the opportunity to provide accurate and highly resolved vertical profiles of these atmospheric variables. The ability to observe with high-spectral resolution should facilitate the identification of new constituents. CIRS will also map the thermal and compositional properties of the surfaces of Saturn's icy satellites. It will similarly map Saturn's rings, characterizing their formation and evolution. The combination of broad spectral range, programmable spectral resolution, the small detector fields of view, and an orbiting spacecraft platform will allow CIRS to observe the Saturnian system in the thermal infrared at a level of detail not previously achieved.

Flasar, F. M.↗
NASA NTRS1977

Infrared and millimeter wave techniques for the Cosmic Background Explorer Satellite

The Cosmic Background Explorer Satellite will employ infrared and microwave techniques to gain information about the very early universe. Three instruments will cover the spectral range from 8 microns to 13 mm, determining both the spectrum and angular distribution of the large scale background radiation fields. A cryogenic polarizing Michelson interferometric spectrometer will measure the spectrum of the 3 deg K relic radiation from the big bang with precision. Four differential microwave radiometers will map the sky from 23 to 90 GHz in a search for anisotropy of the universe. A broadband cryogenic IR photometer will map zodiacal dust emission, galactic dust, and an extragalactic residual component.

Mather, J. C.↗
NASA NTRS2014

Single Mode Theory for Impedance Eduction in Large-Scale Ducts with Grazing Flow

An impedance eduction theory for a rigid wall duct containing an acoustic liner with an unknown impedance and uniform grazing flow is presented. The unique features of the theory are: 1) non-planar waves propagate in the hard wall sections of the duct, 2) input data consist solely of complex acoustic pressures acquired on a wall adjacent to the liner, and 3) multiple higher-order modes may exist in the direction perpendicular to the liner and the opposite rigid wall. The approach is to first measure the axial propagation constant of a dominant higher-order mode in the liner sample section. This axial propagation constant is then used in conjunction with a closed-form solution to a reduced form of the convected Helmholtz equation and the wall impedance boundary condition to educe the liner impedance. The theory is validated on a conventional liner whose impedance spectrum is educed in two flow ducts with different cross sections. For the frequencies and Mach numbers of interest, no higher-order modes propagate in the hard wall sections of the smaller duct. A benchmark method is used to educe the impedance spectrum in this duct. A dominant higher-order vertical mode propagates in the larger duct for similar test conditions, and the current theory is applied to educe the impedance spectrum. Results show that when the theory is applied to data acquired in the larger duct with a dominant higher-order vertical mode, the same impedance spectra is educed as that obtained in the small duct where only the plane wave mode is present and the benchmark method is used. This result holds for each higher-order vertical mode that is considered.

Watson, Willie R.↗
NASA NTRS1996

On the Two Components of Turbulent Mixing Noise from Supersonic Jets

It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.

Tam, Christopher K. W.↗
NASA NTRS1999

An Application of the Direct Coulomb Electron Pair Production Process to the Energy Measurement of the "VH-Group" in the "Knee" Region of the "All-Particle" Energy Spectrum

The "all-particle" cosmic ray energy spectrum appears to be exhibiting a significant change in the spectral index just above approximately 3000 TeV. This could indicate (1) a change in the propagation of the cosmic rays in the galactic medium, and/or (2) the upper limit of the supernova shock wave acceleration mechanism, and/or (3) a new source of high-energy cosmic rays. Air shower and JACEE data indicate the spectral change is associated with a composition change to a heavier element mixture whereas DICE does not indicate this. A detector concept will be presented that utilizes the energy dependence of the production of direct Coulomb electron-positron pairs by energetic heavy ions. Monte Carlo simulations of a direct electron pair detector consisting of Pb target foils interleaved with planes of 1-mm square scintillating optical fibers will be discussed. The goal is to design a large area, non-saturating instrument to measure the energy spectrum of the individual cosmic ray elements in the "VH-group" for energies greater than 10 TeV/nucleon.

Derrickson, J. H.↗
NASA NTRS1971

Radar observations of Mercury.

Pure monochromatic waves of a wavelength of 12.5 cm were beamed toward the planet. The echoes were no longer monochromatic because of the Doppler effect of the planet's rotation. A Fourier power spectrum was produced from the echo signals. On the surface of Mercury, the contours of constant Doppler shift are circles, concentric about a point on the limb near the equator.

Goldstein, R. M.↗
NASA NTRS1973

Lunar Surface Gravimeter Experiment

The lunar surface gravimeter which was emplaced on the moon by the Apollo 17 flight is described and a schematic diagram of the sensor is provided. The objective of the lunar surface gravimeter is to use the moon as an instrumented antenna to detect gravitational waves. Another objective is to measure tidal deformation of the moon. Samples of signals received during lunar sunrise activity and during quiet periods are presented in graph form based on power spectrum analysis

Giganti, J. J.↗
NASA NTRS1976

Space Shuttle communications experiments

Experiments which are under consideration for possible flight on the Space Shuttle in the 1980's to aid in the development of new communications techniques are outlined. A millimeter wave communications experiment for the study of propagation above 10 GHz and an electromagnetic environment experiment for the characterization of fields in orbit are planned to provide information needed for effective spectrum conservation. An antenna range experiment is being studied to provide an in-orbit far-field RF signal source for measuring ground antenna patterns and gain over a wide frequency range. Additional experiments will be intended to test spacecraft technology developments, including such communications system components as large phased array antennas and solid state transmitters and receivers.

Wolff, E. A.↗
NASA NTRS1978

Simulation of turbulent wall pressure

A Monte Carlo procedure was developed to simulate turbulent boundary layer wall pressure fluctuations. The approach utilizes much of the newly available conditional sampling information to construct the required distribution functions. Various disturbance wave forms were examined, as well as the effect of frequency-dependent decay. Good agreement between the simulation and experimental data was achieved for root mean square pressure level, power spectrum, and space time correlation.

Ash, R. L.↗
NASA NTRS1987

Quasi-stationary zonally asymmetric circulations in the equatorial lower mesosphere

Data from the Limb Infrared Monitor of the Stratosphere (LIMS) are used to identify a new type of planetary scale disturbance in the equatorial lower mesosphere during northern winter 1978/1979. The disturbances consist of two or three vertically stacked temperature extrema of alternating sign. They persist for as long as two weeks and do not propagate. Their occurrence is confined to regions of very weak or negative inertial stability, and their meridional-to-vertical aspect ratio, meridional structure, and zonal spectrum are consistent with disturbances predicted by inertial instability theory. However, they are found only when there is strong forcing of the subtropical mesosphere by zonal wavenumber one and two Rossby waves. This fact, together with the absence of zonal propagation, suggests that stationary Rossby waves determine their occurrence and longitudinal structure. These structures can significantly modify the zonal mean flow and should be taken into account in dynamical models of the equatorial mesosphere.

Hitchman, Matthew↗
NASA NTRS1989

Evolution of cosmic string networks

A discussion of the evolution and observable consequences of a network of cosmic strings is given. A simple model for the evolution of the string network is presented, and related to the statistical mechanics of string networks. The model predicts the long string density throughout the history of the universe from a single parameter, which researchers calculate in radiation era simulations. The statistical mechanics arguments indicate a particular thermal form for the spectrum of loops chopped off the network. Detailed numerical simulations of string networks in expanding backgrounds are performed to test the model. Consequences for large scale structure, the microwave and gravity wave backgrounds, nucleosynthesis and gravitational lensing are calculated.

Albrecht, Andreas↗
NASA NTRS2002

Acousto-ultrasonics to Assess Material and Structural Properties

This report was created to serve as a manual for applying the Acousto-Ultrasonic NDE method, as practiced at NASA Glenn, to the study of materials and structures for a wide range of applications. Three state of the art acousto-ultrasonic (A-U) analysis parameters, ultrasonic decay (UD) rate, mean time (or skewing factor, "s"), and the centroid of the power spectrum, "f(sub c)," have been studied and applied at GRC for NDE interrogation of various materials and structures of aerospace interest. In addition to this, a unique application of Lamb wave analysis is shown. An appendix gives a brief overview of Lamb Wave analysis. This paper presents the analysis employed to calculate these parameters and the development and reasoning behind their use. It also discusses the planning of A-U measurements for materials and structures to be studied. Types of transducer coupling are discussed including contact and non-contact via laser and air. Experimental planning includes matching transducer frequency range to material and geometry of the specimen to be studied. The effect on results of initially zeroing the DC component of the ultrasonic waveform is compared with not doing so. A wide range of interrogation problems are addressed via the application of these analysis parameters to real specimens is shown for five cases: Case 1: Differences in density in [0] SiC/RBSN ceramic matrix composite. Case 2: Effect of tensile fatigue cycling in [+/-45] SiC/SiC ceramic matrix composite. Case 3: Detecting creep life, and failure, in Udimet 520 Nickel-Based Super Alloy. Case 4: Detecting Surface Layer Formation in T-650-35/PMR-15 Polymer Matrix Composites Panels due to Thermal Aging. Case 5: Detecting Spin Test Degradation in PMC Flywheels. Among these cases a wide range of materials and geometries are studied.

Kautz, Harold E.↗
NASA NTRS1994

Infrared laser spectroscopy of the linear C13 carbon cluster

The infrared absorption spectrum of a linear, 13-atom carbon cluster (C13) has been observed by using a supersonic cluster beam-diode laser spectrometer. Seventy-six rovibrational transitions were measured near 1809 wave numbers and assigned to an antisymmetric stretching fundamental in the 1 sigma g+ ground state of C13. This definitive structural characterization of a carbon cluster in the intermediate size range between C10 and C20 is in apparent conflict with theoretical calculations, which predict that clusters of this size should exist as planar monocyclic rings.

Non-NASA Center↗