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NASA NTRS1982

Plasma clouds above the ionopause of Venus and their implications

The global distribution of the plasma clouds observed by the Pioneer Venus Orbiter suggest that they originate at the dayside ionopause as wavelike structures which may become detached and swept downstream in the ionosheath flow. Alternatively, the clouds may actually be attached streamers analogous to cometary structure. Estimates of the total ion escape rate from Venus by this process yields values up to 7 x 10 to the 26th ions/s. Preliminary analysis shows that such an escape flux could be supplied by the upward diffusion limited flow of O(+) from the entire dayside ionosphere. If such an escape flux were to continue over the entire lifetime of Venus, the effects upon the evolution of the primitive atmosphere may have been significant.

Brace, L. H.↗

NASA NTRS1985

Gamma rays of 0.3 to 30 MeV from PSR 0531+21

Pulsed gamma rays from the Crab Pulsar PSR 0531+21 are reported for energies of 0.3 to 30 MeV. The observations were carried out with the UCR gamma ray double Compton scatter telescope launched on a balloon from Palestine, Texas at 4.5 GV, at 2200 LT, September 29, 1978. Two 8 hr observations of the pulsar were made, the first starting at 0700 UT (0200 LT) September 30 just after reaching float altitude of 4.5 g/sq cm. Analysis of the total gamma ray flux from the Crab Nebula plus pulsar using telescope vertical cell pairs was published previously. The results presented supersede the preliminary ones. The double scatter mode of the UCR telescope measures the energy of each incident gamma ray from 1 to 30 MeV and its incident angle to a ring on the sky. The time of arrival is measured to 0.05 ms. The direction of the source is obtained from overlapping rings on the sky. The count rate of the first scatter above a threshold of 0.3 MeV is recorded every 5.12 ms. The Crab Pulsar parameters were determined from six topocentric arrival times of optical pulses.

White, R. S.↗

NASA NTRS1986

Capacitor-type micrometeoroid detectors

The metal oxide semiconductor (MOS) capacitor micrometeroid detector consists of a thin dielectric capacitor fabricated on a silicon wafer. In operation, the device is charged to a voltage level sufficiently near breakdown that micrometeoroid impacts will cause dielectric deformation or heating and subsequent arc-over at the point of impact. Each detector is capable of recording multiple impacts because of the self-healing characteristics of the device. Support instrumentation requirements consist of a voltage source and pulse counters that monitor the pulse of recharging current following every impact. An investigation has been conducted in which 0.5 to 5 micron diameter carbonized iron spheres traveling at velocities of 4 to 10 Km/sec were impacted on to detectors with either a dielectric thickness of 0.4 or 1.0 micron. This study demonstrated that an ion microprobe tuned to sufficiently high resolution can detect Fe remaining on the detector after the impact. Furthermore, it is also possible to resolve Fe ion images free of mass interferences from Si, for example, giving its spatial distribution after impact. Specifically this technique has shown that significant amounts of impacting particles remain in the crater and near it which can be analyzed for isotopic content. Further testing and calibration could lead to quantitive analysis. This study has shown that the capacitor type micrometeroid detector is capable of not only time and flux measurements but can also be used for isotopic analysis.

Wortman, J. J.↗

NASA NTRS1986

Magnetic braking in weakly ionized circumstellar disks

Recent observations of disk-like mass distributions around newly formed stars have provided evidence for rapid rotation on scales similar to less than 0.1pc with specific angular momenta much higher than typical stellar values. A likely mechanism for the extraction of angular momentum from these regions is magnetic braking by means of Alfven waves that propagate into the lower-density ambient medium. However, because of the relatively high particle densities and the correspondingly low implied ionization fractions in these apparent disks, their constituent ions and neutrals need not be well coupled to each other and could develop large relative drift velocities. For this reason, previous treatments of magnetic braking that assumed perfect coupling between ions and neutrals have to be modified in this case. In particular, one has to take into account both the azimuthal drift that develops because only the ions are directly coupled to the magnetic field and the radial drift (or ambipolar diffusion) which leads to a redistribution (and leakage) of the magnetic flux. The results of a preliminary analysis of these effects are described.

Koenigl, A.↗

NASA NTRS1990

Procedures for minimizing the effects of high solar activity on satellite tracking and ephemeris generation

We are currently experiencing a period of high solar radiation combined with wide short-term fluctuations in the radiation. The short-term fluctuations, especially when combined with highly energetic solar flares, can adversely affect the mission of U.S. Space Command's Space Surveillance Center (SSC) which catalogs and tracks the satellites in orbit around the Earth. Rapidly increasing levels of solar electromagnetic and/or particle radiation (solar wind) causes atmospheric warming, which, in turn, causes the upper-most portions of the atmosphere to expand outward, into the regime of low altitude satellites. The increased drag on satellites from this expansion can cause large, unmodeled, in-track displacements, thus undermining the SSC's ability to track and predict satellite position. On 13 March 1989, high solar radiation levels, combined with a high-energy solar flare, caused an exceptional amount of short-term atmospheric warming. The SSC temporarily lost track of over 1300 low altitude satellites--nearly half of the low altitude satellite population. Observational data on satellites that became lost during the days following the 13 March 'solar event' was analyzed and compared with the satellites' last element set prior to the event (referred to as a geomagnetic storm because of the large increase in magnetic flux in the upper atmosphere). The analysis led to a set of procedures for reducing the impact of future geomagnetic storms. These procedures adjust selected software limit parameters in the differential correction of element sets and in the observation association process and must be manually initiated at the onset of a geomagnetic storm. Sensor tasking procedures must be adjusted to ensure that a minimum of four observations per day are received for low altitude satellites. These procedures have been implemented and, thus far, appear to be successful in minimizing the effect of subsequent geomagnetic storms on satellite tracking and ephemeris computation.

Bredvik, Gordon D.↗

NASA NTRS1991

IUE and ROSAT monitoring of the bright QSO H1821+643

The analysis is presented of IUE observations of the bright QSO H1821+643, obtained during the ROSAT All Sky Survey (the RIASS program). The objectives were: (1) to establish whether the UV and soft X ray radiation have the same physical origin; and (2) to determine if this physical origin is an accretion disk. Supporting ground based spectrophotometry was also obtained. The analysis shows that the shape and flux level of the UV continuum did not vary among the seven IUE observation spanning one month, to an upper limit of about 8 percent. So it is of great interest to determine whether the soft X ray flux varied during this period. Since X ray variability in AGNs is often more rapid and of higher amplitude than in the UV, detection of X ray variability in the ROSAT data could severely challenge the accretion disk model for the soft X ray excess.

Halpern, Jules↗

NASA NTRS1992

The interstellar gas experiment

The Interstellar Gas Experiment (IGE) exposed thin metallic foils to collect neutral interstellar gas particles. These particles penetrate the solar system due to their motion relative to the sun. Thus, it is possible to entrap them in the collecting foils along with precipitating magnetospheric and perhaps some ambient atmospheric particles. For the entire duration of the Long Duration Exposure Facility (LDEF) mission, seven of these foils collected particles arriving from seven different directions as seen from the spacecraft. In the mass spectroscopic analysis of the noble gas component of these particles, we have detected the isotopes of He-3, He-4, Ne-20, and Ne-22. In the foil analyses carried out so far, we find a distribution of particle arrival directions which shows that a significant part of the trapped particles are indeed interstellar atoms. The analysis needed to subtract the competing fluxes of magnetospheric and atmospheric particles is still in progress.

Lind, D. L.↗

NASA NTRS1992

Attitude stability of LDEF: Refinement of results from the silver pinhole camera

A measurement was previously described of the angular offset and attitude stability of the LDEF spacecraft using a simple pinhole camera device in the UAH experiment A0114. This device uses a silver surface to record the impact zone of atmospheric atomic oxygen passing through a small pinhole on the front surface of the satellite. The shape and size of this zone are well defined if the satellite velocity and gas temperature are known. The circular symmetry of the zone would be distorted by oscillation of the LDEF about its stable attitude, or by the effect of the co-rotation of the Earth's atmosphere for cases of satellites in nonequatorial orbits. The observed ellipticity of 1.05 with the major axis in the yaw direction is equivalent to an oscillation of + or - 0.2 deg about the stable attitude. The uncertainty in that measurement was estimated at about 0.1 deg. A refined analysis based on summing distributions of flux versus incidence angle for a large number of orbital positions, and including a co-rotating atmosphere, indicates that the combined distributions produce a result consistent with the measured exposed spot. It is suggested that, within the precision of the measurements, no actual oscillation of the LDEF is required to produce the results. Thus, the LDEF may have maintained a stable attitude to better than 0.1 deg, even under conditions of maximum aerodynamic perturbation when the silver oxygen record was made. This error is some two orders of magnitude lower than the predicted uncertainty in yaw oscillation and may indicate that the predict methodologies are too conservative.

Peters, P. N.↗

NASA NTRS1992

The interstellar medium of M31. III - Narrow-band imagery in H alpha and (SII)

Deep CCD imagery in H alpha and (SII) is presented of the major spiral arms of M31 with particular attention given to the data reduction and the analysis of the (SII)/H alpha flux ratios. A diffuse ionized gas noted in the images is analyzed which shows higher (SII)/H alpha ratios, and 967 discrete nebulae are listed with gray-scale images, finding charts, and absolute fluxes. The differential H-alpha luminosity function is found to have a slope of -0.95 for brighter objects and flattens out below a critical level. The curve is shown to correspond to the point at which single-star ionization accounts for the H alpha luminosities and is consistent with previous observations. The catalog of objects and fluxes is the largest existing sample of this type, and the unresolved objects in the sample are considered to be planetary nebulae.

Walterbos, R. A. M.↗

NASA NTRS1990

The all-sky extragalactic X-ray foreground

It is argued that the local gravitational dipole implied by the 50 brightest X-ray clusters of galaxies at z greater than 0.013 considered by Lahav et al. (1989) is relatively small compared with that inferred from the only three clusters at lower redshifts. Recent dipole analysis of the X-ray flux from bright AGN observed with HEAO-1A2 indicates that they are indeed strong tracers of this matter. The implications of this for the very pronounced large-scale foreground anisotropies to be measured via low-redshift AGN resolved in more sensitive all-sky surveys are explored. For the total extragalactic X-ray sky, i.e., including the relatively large Cosmic X-ray Background as well as the contribution of the resolved sources, the dipole moment is small compared to the monopole. Because of the large peculiar velocity of the Local Group, it is found that the currently estimated value for this total dipole moment can be used to set severe constraints on the volume emissivity arising from all X-ray sources within the present epoch.

Boldt, Elihu↗

NASA NTRS1993

On the origin of the soft X-ray background

The angular autocorrelation function and spectrum of the soft X-ray background is studied below a discrete source detection limit, using two deep images from the Rosat X-ray satellite. The average spectral shape of pointlike sources, which account for 40 to 60 percent of the background intensity, is determined by using the autocorrelation function. The background spectrum, in the 0.5-0.9 keV band (M band), is decomposed into a pointlike source component characterized by a power law and a diffuse component represented by a two-temperature plasma. These pointlike sources cannot contribute more than 60 percent of the X-ray background intensity in the M band without exceeding the total observed flux in the R7 band. Spectral analysis has shown that the local soft diffuse component, although dominating the background intensity at energies not greater than 0.3 keV, contributes only a small fraction of the M band background intensity. The diffuse component may represent an important constituent of the interstellar or intergalactic medium.

Wang, Q. D.↗

NASA NTRS1993

Experimental technique to investigate the interstellar gas - Preliminary analysis

The Interstellar Gas Experiment (IGE) exposed thin metallic foils to collect neutral interstellar gas particles. These particles penetrate the solar system due to their motion relative to the sun. Thus, it was possible to entrap them in the collecting foils along with precipitating magnetospheric and perhaps some ambient atmospheric particles. For the entire duration of the Long Duration Exposure Facility mission, seven of these foils collected particles arriving from seven different directions as seen from the spacecraft. In the mass spectrometric analysis of the noble gas component of these particles, we have detected the isotopes of He-3, He-4, Ne-20, and Ne-22. In the foil analyses carried out so far, we find a distribution of particle arrival directions which shows that a significant part of the trapped particles are indeed interstellar atoms. The analysis needed to subtract the competing fluxes of magnetospheric and atmospheric particles is still in progress.

Lind, D. L.↗

NASA NTRS1993

A three-dimensional pressure flux-split RNS application to sub/supersonic flow in inlets and ducts

The reduced Navier-Stokes (RNS) formulation is combined with a pressure based flux split procedure for the computational analysis of three dimensional flow. Only lowest order diffusion, that is required to satisfy the no-slip boundary condition is retained in this approximation. The governing equations are appropriately differenced such that the physical boundary conditions are closely coupled and combined to form a closed discrete system. No numerical or characteristic type boundary conditions are required. A global pressure relaxation procedure, i.e. multi-sweep PNS, is considered. A sparse matrix direct solver is applied to the crossplane solution. At the outflow boundary only the pressure or pressure gradient is prescribed. The resulting formulation has been applied to the solution of three-dimensional subsonic/supersonic internal flows in inlets and ducts.

Khosla, P. K.↗

NASA NTRS1992

Integration of space and in-situ observations to study atmosphere, ocean and land processes

A research investigation was conducted into the possibility of using atmospheric observations made in the past from both terrestrial and space-based platforms to create a global, coherent four dimensional analysis for the purpose of studying atmospheric, oceanic, and land surface processes relevant to climate simulation, monitoring, and change. This investigation consisted of the following tasks: (1) a mature global data assimilation system was obtained from the National Meteorological Center and modified for use on a Cray X-MP computer system; (2) atmospheric observations for the period 20 Nov. 1982 through 1 Mar. 1983, including rawinsonde soundings, aircraft-based measurements, pilot balloons, and temperature soundings from polar orbiting satellites were obtained from several sources; and (3) the global data assimilation system was used to reassimilate the atmospheric observations to produce a new atmospheric analysis which was then compared with the contemporaneous analysis. The global hydrologic cycle, including fluxes between the atmosphere and both the land and ocean surfaces, was estimated. The flux of water from the ocean surface into the atmosphere, its transport in the form of latent heat to remote regions, and its return to the surface in the form of precipitation were estimated globally. In addition, several regional budgets for selected tropical oceanic and extratropical continental areas were also done.

Shukla, J.↗

NASA NTRS1993

Examination of the observed synoptic scale cirrus cloud environment: The December 3-6 FIRE cirrus case study

Recently, Sassen provided evidence for supercooled water droplets in cirrus uncinus cell heads at temperatures between 40 and -50 C. Chemistry related to volcanic aerosol of stratospheric origin was evoked as an explanation for this phenomenon. Sassen speculated that injections of sulfuric acid droplets into the upper troposphere were accomplished by tropopause folds associated with subtropical jet streams. He also postulated global climatic perturbations due to the effect of these cirrus microphysical perturbations on radiative fluxes. Using data processing and objective analysis techniques described by Mace and Ackerman, the synoptic scale environment was examined for evidence of tropopause folds that may have served as a source mechanism of stratospheric aerosol in the upper troposphere.

Mace, Gerald G.↗

NASA NTRS1960

An Analysis of Nuclear-Rocket Nozzle Cooling

A nuclear-rocket regenerative-cooling analysis was conducted over a range of reactor power of 46 to 1600 megawatts and is summarized herein. Although the propellant (hydrogen) is characterized by a large heat-sink capacity, an analysis of the local heat-flux capability of the coolant at the nozzle throat indicated that, for conventional values of system pressure drop, the cooling capability was inadequate to maintain a selected wall temperature of 1440 R. Several techniques for improving the cooling capability were discussed, for example, high pressure drop, high wall temperature, refractory wall coatings, thin highly conductive walls, and film cooling. In any specific design a combination of methods will probably be utilized to achieve successful cooling.

Robbins, William H.↗

NASA NTRS2004

"Chiron": A Proposed Remote Sensing Prompt Gamma Ray Activation Analysis Instrument for a Nuclear Powered Prometheus Mission

Prompt Gamma Ray Activation Analysis (PGAA) from neutron capture is an important experimental method that yields information on the elemental abundance of target materials. Gamma ray analysis has been used in planetary exploration missions by taking advantage of the production of neutrons as a result of Galactic Cosmic Ray interaction within the planetary surfaces. The .gamma ray signal that can be obtained from the GCR production of neutrons is very low, so we seek a superior neutron source. NASA s Project Prometheus and the Dept. of Energy aim to develop a nuclear power system for planetary exploration. This provides us with a tremendous opportunity to harness the reactor as a source of neutrons that can be used for PGAA. We envision a narrow stream of neutrons from the reactor directed toward the surface of an asteroid or comet producing the prompt gamma ray signal for analysis. Under ideal conditions of neutron flux and spacecraft orbit, both the signal strength and the spatial resolution will improved by several orders of magnitude over previously missions.

Floyd, Samuel R.↗

NASA NTRS2000

NASA Stennis Space Center Test Technology Branch Activities

This paper provides a short history of NASA Stennis Space Center's Test Technology Laboratory and briefly describes the variety of engine test technology activities and developmental project initiatives. Theoretical rocket exhaust plume modeling, acoustic monitoring and analysis, hand held fire imaging, heat flux radiometry, thermal imaging and exhaust plume spectroscopy are all examples of current and past test activities that are briefly described. In addition, recent efforts and visions focused on accomodating second, third, and fourth generation flight vehicle engine test requirements are discussed.

Solano, Wanda M.↗