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NASA NTRS1983

Gravitational lens effects of a cosmological density of compact objects

Amplification of quasar light by a cosmological density of compact objects causes significant effects on many quasars in magnitude-limited samples. For lens masses solar mass less than 100,000 solar mass the continuum would be amplified by a magnitude or more but the line emission would not. Examination of the UV selected sample of Marshall et al. (1983) gives limits to more than 90 percent statistical confidence of Omega(c) less than 0.1 for a mass between 200 and 100,000 solar mass, where Omega(c) is the mean density of objects of mass M relative to the closure density. Preliminary results from an X-ray selected sample may probe to more than 0.1 solar mass and give a value for Omega(c) of less than one. These limits indicate that the remnants of an early population of massive stars cannot make a cosmologically significant contribution to the mass density of the universe. On a separate topic, recent work on the enhanced surface density of quasars near galaxies due to lensing by stars in the galaxy halos is reviewed.

Canizares, C. R.↗

NASA NTRS1984

Exact differential equation for the density and ionization energy of a many-particle system

The present investigation is concerned with relations studied by Hohenberg and Kohn (1964) and Kohn and Sham (1965). The properties of a ground-state many-electron system are determined by the electron density. The correct differential equation for the density, as dictated by density-functional theory, is presented. It is found that the ground-state density n of a many-electron system obeys a Schroedinger-like differential equation which may be solved by standard Kohn-Sham programs. Results are connected to the traditional exact Kohn-Sham theory. It is pointed out that the results of the current investigations are readily extended to spin-density functional theory.

Levy, M.↗

NASA NTRS1985

Analysis of atomic thermospheric nitrogen density

A NASA grant provided support for a research project at Augsbury College, Minneapolis, Minnesota for the analysis of atomic nitrogen density data obtained by the Neutral Atmospheric Composition Spectrometer (NACS) on board the Dynamics Explorer-2 satellite. Initial funding was for an exploratory study of the feasibility of obtaining ambient densities of N from source densities of NO. Funding was continued under the Dynamics Explorer Guest Investigator Program when initial studies indicated probable success in obtaining such ambient densities. The major scientific focus of the later work was to be to characterize the behavior of N densities at high latitudes.

Engebretson, M. J.↗

NASA NTRS1986

Solar flare track densities in interplanetary dust particles The determination of an asteroidal versus cometary source of the zodiacal dust cloud

The possibility is explored whether an IDP (interplanetary dust particle) is cometary or asteroidal from measurements of the solar flare track density within its constituent mineral grains. Dust particles that are larger than 1 micron, when injected into the Solar System from comets and asteroids, will spiral into the sun due to the Poynting-Robertson effect. During the process of spiraling in, such dust particles accumulate solar flare tracks. The accumulated track density for a given dust grain is a function of the duration of its space exposure and its distance from the sun. Using a computer model, it was determined that the expected track density distributions from grains produced by comets are very different from those produced by asteroids. Individual asteroids produce populations of particles that arrive at 1 AU with scaled track density distributions containing 'spikes,' while comets supply particles with a flatter and wider distribution of track densities.

Sandford, Scott A.↗

NASA NTRS1988

Electron density depletions in the nightside auroral zone

Dynamics Explorer 1 measurements are used to investigate regions of low electron density in the nightside auroral zone. Sharply defined regions of low electron density are found in auroral zone crossings from the predusk hours until the early morning hours at all radial distances up to at least 4.6 earth radii. Densities in the auroral cavity are shown to fall to values below 0.3/cu cm. Within the auroral cavity, electron-density-profile variations of a factor of 2 or more on spatial scales of tens of kilometers are found, and the electron plasma frequency to electron cyclotron frequency ratios are 0.02-0.4. The results suggest associations between the density depletions in the nightside auroral zone and auroral acceleration processes.

Persoon, A. M.↗

NASA NTRS1988

The hydrogen density of the local interstellar medium and an upper limit to the galactic glow determined from Pioneer 10 ultraviolet photometer observations

Pioneer 10 Lyman-alpha data obtained beyond the heliocentric distance of 30 AU are used here to determine the local ISM hydrogen density and downwind Galactic glow. The resulting asymptotic interstellar hydrogen density obtained using a conventional interplanetary model is 0.06/cu cm, and the Galactic Lyman-alpha glow is found to be negligible in the downwind direction. Discrepancies are found between observations and the conventional theoretical predictions of the glow dependence on radial distance. Based solely on data obtained between 30 and 39 AU, a better fit is obtained with a constant density distribution beyond 30 AU. In this case, a density of 0.05/cu cm and Galactic glow of 1.5 R is obtained. These results suggest that a complex density distribution may be more appropriate than either the conventional exponential distribution or the constant distribution.

Wu, F. M.↗

NASA NTRS1989

Modelling ionospheric density structures

Large-scale density structures are a common feature in the high-latitude ionsphere. The structures were observed in the dayside cusp, polar cap, and nocturnal auroral region over a range of altitudes, including the E-region, F-region and topside ionosphere. The origins, lifetimes and transport characteristics of large-scale density structures were studied with the aid of a three-dimensional, time-dependent ionospheric model. Blob creation due to particle precipitation, the effect that structured electric fields have on the ionosphere, and the lifetimes and transport characteristics of density structures for different seasonal, solar cycle, and interplanetary magnetic field (IMF) conditions were studied. The main conclusions drawn are: (1) the observed precipitation energy fluxes are sufficient for blob creation if the plasma is exposed to the precipitation for 5 to 10 minutes; (2) structured electric fields produce structured electron densities, ion temperatures, and ion composition; (3) the lifetime of an F-region density structure depends on several factors, including the initial location where it was formed, the magnitude of the perturbation, season, solar cycle and IMF; and (4) depending on the IMF, horizontal plasma convection can cause an initial structure to break up into multiple structures of various sizes, remain as a single distorted structure, or become stretched into elongated segments.

Schunk, R. W.↗

NASA NTRS1989

Density of intercalated graphite fibers

The density of Amoco P-55, P-75, P-100, and P-120 pitch-based graphite fibers and their intercalation compounds with bromine, iodine monochloride, and copper (II) chloride have been measured using a density gradient column. The distribution of densities within a fiber type is found to be a sensitive indicator of the quality of the intercalation reaction. In all cases the density was found to increase, indicating that the mass added to the graphite is dominant over fiber expansion. Density increases are small (less than 10 percent) giving credence to a model of the intercalated graphite fibers which have regions which are intercalated and regions which are not.

Gaier, James R.↗

NASA NTRS1990

A higher density VLBI catalog for navigating Magellan and Galileo

The density of radio sources near the ecliptic in the astrometric JPL Very Long Baseline Interferometry (VLBI) catalog has been increased by over 50 percent since 1985. This density increase has been driven by the need for more sources for the VLBI navigation of the Magellan and Galileo spacecraft, but the sources also will be usable for Mars Observer and other future missions. Since the last catalog, including observations made through 1985, was published in 1988, a total of 21 radio sources has been added that fulfill the following criteria: (1) they lie within 10 deg of the ecliptic plane; (2) their correlated flux densities are above 0.2 Jy on at least one of the Deep Space Network intercontinental baselines at both 2.3 and 8.4 GHz; and (3) the source positions are known to better than 5 milliarcseconds (25 nanoradians). The density of such sources in the catalog has been increased from 15.6 per steradian to 25.2 per steradian. Ten more sources have been added that fulfill the last two criteria given above and lie between 10 deg and 20 deg from the ecliptic plane. Analysis shows that there may be approx. 70 more sources with correlated flux densities above 0.2Jy that are within approx. 20 deg of the ecliptic. However, VLBI navigation observations of the new and prospective sources with the 250-kHz bandwidth of the current operational system will require the use of two 70-m antennas in most cases. Including both old and new sources, if two 34-m antennas are used, there will be usable navigation sources within 10 deg of a spacecraft in only 30 percent of the ecliptic, and sources within 20 deg of a spacecraft over 70 percent of the ecliptic.

Ulvestad, J. S.↗

NASA NTRS1990

A method for measuring temperatures and densities in hypersonic wind tunnel air flows using laser-induced O2 fluorescence

Laser-induced fluorescence in oxygen, in combination with Raman scattering, is shown to be an accurate means by which temperature, density, and their fluctuations due to turbulence can be measured in air flows associated with high-speed wind tunnels. For temperatures above 60 K and densities above 0.01 amagat, the uncertainty in the temperature and density measurements can be less than 2 and 3 percent, respectively, if the signal uncertainties are dominated by photon-statistical noise. The measurements are unaffected by collisional quenching and can be achieved with laser fluences for which nonlinear effects are insignificant. Temperature measurements using laser-induced fluorescence alone have been demonstrated at known densities in the range of low temperatures and densities which are expected in a hypersonic wind tunnel.

Laufer, Gabriel↗

NASA NTRS1990

Evidence for an inner scale to the density turbulence in the interstellar medium

The form of the density power spectrum is studied. The relationship between the visibility and the properties of density turbulence is discussed. Observational determinations of the power-law index of the interstellar density power spectrum are presented. An inner scale is derived, and the effect of an inner scale in the interstellar plasma turbulence is examined. It is concluded that the host plasma for the density irregularities must be similar to the warm ionized medium in the Mckee-Ostriker model (1979) of the interstellar medium or the low-density envelopes of H II regions.

Spangler, Steven R.↗

NASA NTRS1990

Infrared and radio measurements of the density structure of compact H II regions

The density structure of compact H II regions is studied by comparing the ratio of the S III forbidden 33.47-micron and 18.71-micron emission lines and 5-GHz continuum maps. Electron densities are computed for DR 22, G29.9 - 0.0, G75.84 + 0.4, M8, M42, S158, W3, and W33. For DR 22, M42, and W3, the electron densities determined from the IR data are consistent with those derived from radio observations. The densities for S158 and M8 are quite low, implying that the S III emission is dominated by the diffuse ionized gas observed in the radio continuum. For G29.9 - 0.0, G75.84 + 0.4, and W33, the S III data give higher densities than predicted from radio observations. This implies that either these nebulae are filled with dense clumps of ionized gas which are not resolved in the radio maps, or that the S III zone is confined to the small, dense cores found in the radio maps of all three nebulae.

Megeath, S. T.↗

NASA NTRS1990

Density of intercalated graphite fibers

Gaier, James R.↗

NASA NTRS1990

Observations of the density profile in the magnetosheath near the stagnation streamline

An enhancement in the plasma density occurs in more than half of the ISEE-1 and 2 magnetosheath passes near the stagnation streamline just in front of the magnetopause. On average, this structure is about 0.4 R(e) thick and clearly separated from the magnetopause. The average density enhancement is about 44 percent above the ambient magnetosheath density. The anticorrelation of the magnetic field and density is that expected for the MHD slow mode. Strong fluctuations in the density within the structure are usually accompanied by fluctuations in velocity.

Song, P.↗

NASA NTRS1990

Method for measuring temperatures and densities in hypersonic wind tunnel air flows using laser-induced O2 fluorescence

Laser-induced fluorescence in oxygen, in combination with Raman scattering, is shown to be an accurate means by which temperature, density, and their fluctuations owing to turbulence can be measured in air flows associated with high-speed wind tunnels. For temperatures above 60 K and densities above 0.01 amagat, the uncertainties in the temperature and density measurements can be less than 2 percent, if the signal uncertainties are dominated by photon statistical noise. The measurements are unaffected by collisional quenching and can be achieved with laser fluences for which nonlinear effects are insignificant. Temperature measurements using laser-induced fluorescence alone have been demonstrated at known densities in the range of low temperatures and densities which are expected in a hypersonic wind tunnel.

Laufer, Gabriel↗

NASA NTRS1991

Densities and abundances of hot cometary ions in the coma of P/Halley

The densities of the hot component in the ion population observed in the P/Halley coma by the ion mass spectrometer aboard the Giotto spacecraft has been recomputed, and the results are reported. Except for protons and H2(+), all densities are found to be at least an order of magnitude higher than predicted by the physicochemical model of Schmidt et al. (1988). These densities require ionization mechanisms in addition to, or stronger than, plasma compression. The change in the density of solar wind ions across a magnetic discontinuity located 135,000 km from the comet is barely distinguishable in the density profiles of hot cometary ions. This result is consistent with the interpretation that the magnetic pileup boundary detected by Giotto is due to a discontinuity in the solar wind.

Neugebauer, M.↗

NASA NTRS1992

Correlated flux densities from VLBI observations with the DSN

Correlated flux densities of extragalactic radio sources in the very long baseline interferometry (VLBI) astrometric catalog are required for the VLBI tracking of Galileo, Mars Observer, and future missions. A system to produce correlated and total flux density catalogs was developed to meet these requirements. A correlated flux density catalog of 274 sources, accurate to about 20 percent, was derived from more than 5000 DSN VLBI observations at 2.3 GHz (S-band) and 8.4 GHz (X-band) using 43 VLBI radio reference frame experiments during the period 1989-1992. Various consistency checks were carried out to ensure the accuracy of the correlated flux densities. All observations were made on the California-Spain and California-Australia DSN baselines using the Mark 3 wideband data acquisition system. A total flux density catalog, accurate to about 20 percent, with data on 150 sources, was also created. Together, these catalogs can be used to predict source strengths to assist in the scheduling of VLBI tracking passes. In addition, for those sources with sufficient observations, a rough estimate of source structure parameters can be made.

Coker, R. F.↗

NASA NTRS1993

The critical density for star formation in HII galaxies

The star formation rate (SFR) in galaxies is believed to obey a power law relation with local gas density, first proposed by Schmidt (1959). Kennicutt (1989) has shown that there is a threshold density above which star formation occurs, and for densities at or near the threshold density, the DFR is highly non-linear, leading to bursts of star formation. Skillman (1987) empirically determined this threshold for dwarf galaxies to be approximately 1 x 10(exp 21) cm(exp -2), at a linear resolution of 500pc. During the course of our survey for HI companion clouds to HII galaxies, we obtained high resolution HI observations of five nearby HII galaxies. HII galaxies are low surface brightness, rich in HI, and contain one or a few high surface brightness knots whose optical spectra resemble those of HII regions. These knots are currently experiencing a burst of star formation. After Kennicutt (1989) we determine the critical density for star formation in the galaxies, and compare the predictions with radio and optical data.

Taylor, Christopher L.↗