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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Upper limits on argon isotope abundances in the Venus thermosphere

On December 9, 1978 the neutral gas mass spectrometer aboard the NASA Pioneer Venus multiprobe bus has measured density, composition, and temperature of the Venus dayside thermosphere. There was no positive identification of argon down to the lowest measuring altitude of 130 km. For the altitude level of 135 km the following upper limits for the number densities of argon isotopes were derived: n(Ar-36) less than 1.3 times 10 to the 6th power per cu cm and n(Ar-40) less than 2.8 times 10 to the 6th power per cu cm. From our upper atmosphere observations we infer for the troposphere of Venus the following upper limits for the mixing ratios: n(Ar-36)/total number density less than 9 times 10 to the minus 6th power and n(Ar-40)/total number density less than 20 times 10 to the minus 6th power.

Mauersberger, K.↗
NASA NTRS1979

An upper limit on ultraviolet shot noise from Cygnus X-1

Rapid photometry of Cygnus X-1 through an ultraviolet filter centered on 0.35 micron has been obtained at 100-ms sampling intervals. The autocorrelation function of these data has been examined for shot noise analogous to the behavior of the X-ray light curve. The ultraviolet data are entirely consistent with white noise. Considering randomly occurring ultraviolet shots with the same duration (0.5 s) and average rate (1 per sec) as the X-ray shots, a 3-sigma upper limit on the ratio of optical to X-ray energies per shot is estimated to be 0.13, before the ultraviolet light is attenuated by interstellar dust. This limit is then generalized for shots of arbitrary duration and rate.

Duthie, J. G.↗
NASA NTRS1980

Voltage controller/current limiter for ac

Circuit protects ac power systems for overload failures, limits power surge and short-circuit currents to 150 percent of steady state level, regulates ac output voltage, and soft starts loads. Limiter generates dc error signal in response to line fluctuations and dumps power when overload is reached. Device is inserted between ac source and load.

Wu, T. T.↗
NASA NTRS1980

Recent research on V/STOL test limits at the University of Washington aeronautical laboratory

The occurence of flow breakdown during the wind tunnel testing of a powered V/STOL aircraft was studied. Flow breakdown is the low forward speed test limit in a solid wall wind tunnel and is characterized by a vortex which forms on the floor and walls of the wind tunnel thereby failing to simulate free air conditions. The flow is caused by the interaction of the model wake and tunnel boundary layer and affects the model's aerodynamic characteristics in such fashion as to negate their reliability as correctable wind tunnel data. The low speed test limit was examined using a model that possessed a discretely concentrated powered lift system using a pair of lift jets. The system design is discussed and the tests and results which show that flow breakdown occurs at a velocity ratio of approximately 0.20 are reported.

Shindo, S.↗
NASA NTRS1979

Prediction limits for foF2

Some of the factors limiting the prediction of foF2 are investigated using median and hourly foF2 values recorded at stations in the Australian ionosonde network. Different time and spatial scales together with different levels of solar and geomagnetic activity are considered. All data used in the analysis are displayed in a series of scatter diagrams which provide an overall view of the accuracy limits for foF2 predictions.

Wilkinson, P. J.↗
NASA NTRS1980

The Eddington limit and supercritical accretion. I - Time-independent calculations

Spherically symmetric, steady state accretion of an ionized hydrogen plasma onto a neutron star is considered for accretion rates which exceed a critical rate at which the Eddington limiting luminosity is produced. The coupled hydrodynamic and frequency integrated, radiative transfer equations are solved for accretion rates up to 10 times the nominal limit. Steady state solutions are presented that imply a multiplicity of different luminosity solutions for a single accretion rate in this 'supercritical' regime.

Burger, H. L.↗
NASA NTRS1980

Updating the limit efficiency of silicon solar cells

The limit efficiency of a silicon solar cell is investigated using an analytical approach. The analytical model is based on the solution of a transport equation for minority carriers derived from the Shockley equations. On the basis of the computations, a 'narrow-region' design approach is suggested for both the front and the back regions of the solar cell. The design relies on low effective surface-recombination velocities, a textured front surface, and an optical internally reflecting back surface. With this approach, the limit efficiency is near 25%, and the optimum cell is 50 to 150 microns thick.

Wolf, M.↗
NASA NTRS1980

A lower limit on the magnitude of the companion to HDE 226868 /Cygnus X-1/

High-precision digital spectra of the HDE 226868/Cygnus X-1 system have been obtained at a variety of orbital phases, using a Digicon photon counting array at the coude focus of the Lick Shane reflector. The spectra have been cross-correlated to search for evidence for the existence of a luminous stellar companion to HDE 226868, as has been suggested by several models where the secondary star is not a black hole. We find no evidence for such a star and place a lower limit on the difference between HDE 226868 and its companion of 4 magnitudes. This limit rules out all of the models for the Cygnus X-1 system calculated by Avni and Bahcall which include main-sequence stars as secondaries rather than black holes. A merit function generally useful for cross-correlation of digital spectra is described.

Shafter, A. W.↗
NASA NTRS1980

Limits on the streaming and escape of electrons in thermal models for solar hard X-ray emission

Upper limits on the number of fast electrons streaming through and escaping from a plasma whose electrons have been heated to approximately 100 billion K and confined by a collisionless ion-acoustic thermal conduction front are determined. It is shown that such a front is fairly transparent to fast electrons with velocities much larger than the thermal velocity because the anisotropic ion-acoustic waves cannot scatter them, making them collisionless on a scale much larger than the thickness of the front. The collisionless analog of the collisional thermoelectric field is derived self-consistently and shown to offer a significant impediment to fast electrons because they must climb over a larger potential barrier than in the collisional case. The only factors limiting the escape of electrons able to surmount this barrier are their rate of production and the requirement that they carry less heat flux than the maximum heat flux allowable. The rate of production is determined for the case of a Maxwellian whose tail is being filled collisionally.

Smith, D. F.↗
NASA NTRS1981

Limit on galactic 6.13 MeV gamma-ray line

The University of New Hampshire large gamma-ray detector observed the galactic center region during a balloon flight from Alice Springs, Australia on 1977 November 21-22. The absence of any observable line at 6.13 MeV or its escape-peak energy makes it possible to place an upper limit of 8.1 x 10 to the -4th photons/(sq cm s) at the 99% confidence level on the 0-16 de-excitation line at this energy from the galactic disk in the direction of the center. This limit restricts the interpretation given by Willett et al. (1979) of the line at 6.13 MeV which they observed while viewing the galactic anticenter. The present results indicate that it is highly unlikely that the line which these authors report is due to dark nebulae or the quiet sun. Possible explanations for their observation are atmospheric background, local production in the detector, a localized cosmic source in the direction of the galactic anticenter, or a statistical fluctuation.

Dunphy, P. P.↗
NASA NTRS1981

Further observational support for the limited-latitude magnetodisc model of the outer Jovian magnetosphere

A distinction is made between the solar-wind-influenced limited-latitude magnetodisk and magnetic anomaly models of the outer Jovian magnetosphere, and an observational comparison of the two models is presented based on Pioneer and Voyager measurements. Predictions of the two models concerning the location of the current sheet as a function of Jovigraphic latitude, System III longitude and radial distance are contrasted, and it is shown that both models can satisfactorily explain the merging of the current sheep crossings by Voyager 1 and 2. Variations in the energetic particle intensities observed on the outbound pass of Voyager 1 and 2 are observed to correspond to scale heights for energetic particle latitudinal confinement consistent with MHD calculations and Pioneer 10 and Voyager magnetic observations only when the scale heights are calculated on the basis of the limited-latitude magnetodisk model. It is thus suggested that the solar wind must have a greater influence on magnetosphere structure than internal longitudinal plasma asymmetry.

Thomsen, M. F.↗
NASA NTRS1953

Analytical Investigation of Icing Limit for Diamond-Shaped Airfoil in Transonic and Supersonic Flow

Calculations have been made for the icing limit of a diamond airfoil at zero angle of attack in terms of the stream Mach number, stream temperature, and pressure altitude. The icing limit is defined as a wetted-surface temperature of 320 F and is related to the stream conditions by the method of Hardy. The results show that the point most likely to ice on the airfoil lies immediately behind the shoulder and is subject to possible icing at Mach numbers as high as 1.4.

Callaghan, Edmund E.↗
NASA NTRS1982

Measurement accuracies in band-limited extrapolation

The problem of numerical instability associated with extrapolation algorithms is addressed. An attempt is made to estimate the bounds for the acceptable errors and to place a ceiling on the measurement accuracy and computational accuracy needed for the extrapolation. It is shown that in band limited (or visible angle limited) extrapolation the larger effective aperture L' that can be realized from a finite aperture L by over sampling is a function of the accuracy of measurements. It is shown that for sampling in the interval L/b absolute value of xL, b1 the signal must be known within an error e sub N given by e sub N squared approximately = 1/4(2kL') cubed (e/8b L/L')(2kL') where L is the physical aperture, L' is the extrapolated aperture, and k = 2pi lambda.

Kritikos, H. N.↗
NASA NTRS1981

Superheavy elements - An early solar system upper limit for elements 107 to 110

The abundance of samarium-152 in the Santa Clara iron meteorite is found to be 108 x 10 to the 7th atoms per gram. This quantity, if attributed to fission of a superheavy element with atomic number 107 to 109, limits the amount of superheavy elements in the early solar system to 0.000017 times the abundance of uranium-238. For element 110, the limit is 0.000034.

Nozette, S.↗
NASA NTRS1981

Coherence limits for very-long-baseline interferometry

The quality of the frequency standards used in very-long-baseline interferometry (VBLI) limits the coherent integration time and the accuracy of geodetic experiments except in special cases when clock instabilities can be made to cancel out by using differential interferometry. Formulas are derived for estimating the coherence of these interferometers from the Allan variances of the frequency standards. Experiments using extremely high-quality frequency standards, such as hydrogen masers, may be limited by the phase noise that results from atmospheric and ionospheric fluctuations.

Rogers, A. E. E.↗
NASA NTRS1982

A note on the limits of Rossby wave amplitudes

Quasi-geostrophic dynamics, as usually formulated, centers on the conservation of pseudopotential vorticity (PV) on isobaric surfaces. The present study has the objective to investigate some simple constraints which conservation of PV imposes on internal Rossby waves. Attention is given to some of the limitations that conservation of potential vorticity imposes on the dynamics of the stratosphere. Another possible constraint imposed by hydrodynamic stability is also discussed. The quantitative limitations these constraints put on the amplitudes of internal Rossby waves are assessed and compared with observations of the lower stratosphere.

Lindzen, R. S.↗
NASA NTRS1982

Photon bunching and the photon-noise-limited performance of infrared detectors

The photon-noise-limited performance of a radiation detector exposed to a thermal background of temperature T is analyzed by calculating the resulting specific detectivity. Both ideal photon detectors of arbitrary quantum efficiency eta and ideal thermal detectors of arbitrary emissivity eta are considered; and the effects of both shot noise and excess noise are taken into account. The relative contributions of these two sources depend on the quantum efficiency or emissivity of the system. For frequencies nu such that hnu/kT is much less than eta, excess noise can make an appreciable contribution to the total system noise. For the case of the detection of the narrow-band radiation, the specific detectivity is independent of eta in the limit hnu/kT is much less than eta.

Boyd, R. W.↗
NASA NTRS1982

Ecological optimality in water-limited natural soil-vegetation systems. I - Theory and hypothesis

The solution space of an approximate statistical-dynamic model of the average annual water balance is explored with respect to the hydrologic parameters of both soil and vegetation. Within the accuracy of this model it is shown that water-limited natural vegetation systems are in stable equilibrium with their climatic and pedologic environments when the canopy density and species act to minimize average water demand stress. Theory shows a climatic limit to this equilibrium above which it is hypothesized that ecological pressure is toward maximization of biomass productivity. It is further hypothesized that natural soil-vegetation systems will develop gradually and synergistically, through vegetation-induced changes in soil structure, toward a set of hydraulic soil properties for which the minimum stress canopy density of a given species is maximum in a given climate. Using these hypotheses, only the soil effective porosity need be known to determine the optimum soil and vegetation parameters in a given climate.

Eagleson, P. S.↗