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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Statistical model of meteor streams. IV - A study of radio streams from the synoptic year

The paper gives a detailed description of a search procedure for radio meteor streams and reports the detection of 275 streams in a synoptic-year sample of 19,698 radio meteors observed by the radar system of the Radio Meteor Project at Havana, Illinois. The orbital elements and related parameters of the detected streams are presented, the two parameters of the D-distribution of the streams are determined, and their mean radiants are plotted. Possible associations between the streams and possible parent objects are identified; it is found that streams may be associated with seven periodic comets, five other comets, at least nine asteroids (especially Adonis), and several fireballs. The mean space density in the streams is found to be much lower than the sporadic density, although the central density may be considerably greater than the sporadic density. It is shown that the derived absolute stream-density values are in agreement with the order of magnitude of the space densities estimated from cometary production rates for solid material of comparable particle size.

Sekanina, Z.↗
NASA NTRS1980

A hill-sliding strategy for initialization of Gaussian clusters in the multidimensional space

A hill sliding technique was devised to extract Gaussian clusters from the multivariate probability density estimate of sample data for the first step of iterative unsupervised classification. Each cluster was assumed to posses a unimodal normal distribution. A clustering function proposed distinguished elements of a cluster under formation from the rest in the feature space. Initial clusters were extracted one by one according to the hill sliding tactics. A dimensionless cluster compactness parameter was proposed as a universal measure of cluster goodness and used satisfactorily in test runs with LANDSAT multispectral scanner data. The normalized divergence, defined by the cluster divergence divided by the entropy of the entire sample data, was utilized as a general separability measure between clusters. An overall clustering objective function was set forth in terms of cluster covariance matrices, from which the cluster compactness measure could be deduced. Minimal improvement of initial data partitioning was evaluated by this objective function in eliminating scattered sparse data points. The hill sliding clustering technique developed herein has the potential applicability to decomposition any multivariate mixture distribution into a number of unimodal distributions when an appropriate distribution function to the data set is employed.

Park, J. K.↗
NASA NTRS1979

Chemical stratification of the mantle

A possible scenario for the chemical stratification of the earth's mantle is presented. Differentiation of the mantle by either the production of basaltic magmas or partial melting by the upper mantle is proposed to lead to a thick basalt layer, the lower part of which is converted to eclogite as the earth cools. Density estimates indicate that the eclogite formed would not be able to sink to below 670 km. The eclogite layer is thus demonstrated to be trapped as a result of whole-mantle convection and possible irreversible differentiation of the mantle into eclogite and overlying residual peridotite layers.

Anderson, D. L.↗
NASA NTRS1980

Observations of SO in dark and molecular clouds

The 1(0)-0(1) transition of SO at 30 GHz has been observed in several sources, including the first detection of sulfur monoxide in cold dark clouds without apparent internal energy sources. The SO transition appears to be an excellent tracer of structure in dark clouds, and the data support suggestions that self-absorption is important in determining emission profiles in such regions for large line-strength transitions. Column densities estimated from a comparison of the results for the two isotopic species indicate a high fractional abundance of SO in dark clouds.

Rydbeck, O. E. H.↗
NASA NTRS1982

The Crab Nebula. I - Spectrophotometry of the filaments

Relative line intensities in the 3700-7400 A region are obtained from new spectrophotometric measurements of ten positions in the Crab Nebula. O III forbidden line intensities imply electron temperatures of 11,000-18,300 K, while generally lower temperatures are indicated by the O II, N II and S II forbidden lines. The 6717 and 6731 A forbidden S II line ratio yields electron density estimates of 550-3500/cu cm, with a typical filament value of 1300 cu cm. While Balmer decrements are generally consistent with pure recombination, some filaments show steeper decrements. He line intensities are found to have significant variation. The presence of a moderately strong line at 7378 A, which may be due to the Ni II forbidden line is confirmed. Some filaments are found to exhibit a wide range of ionization states.

Fesen, R. A.↗
NASA NTRS1982

Cross-validation for densities and regressions

Virtually all nonparametric (infinite dimensional) problems require the choice of a smoothing parameter. It is natural to attempt to use the data to guide the choice of this parameter. The most widely applicable data driven technique is cross validation. The method of cross validation is introduced and results are presented which establish consistency for certain cross validated density estimators and consistency as well as asymptotic normality for rigid regression.

Geman, S.↗
NASA NTRS1982

A bootstrap approach to bump hunting

An important question in cluster analysis and pattern recognition is the determination of the number of clusters into which a given population should be divided. Frequently, particularly when certain specific clustering methods are being used, the number of clusters is taken to be equal to the number of modes, or local maxima, in the probability density function underlying the given data set. The use of kernal density estimates in mode estimation is discussed. The test statistic to be used is defined and a bootstrap technique for assessing significance is given. An illustrative application is followed by an examination of the asymptotic behavior of the test statistic.

Silverman, B. W.↗
NASA NTRS1983

Determinations of SIII, OIV and NeV abundances in planetary nebulae from IR lines

Airborne observations of the infrared forbidden lines (SIII) 18.71 microns, (NeV) 24.28 microns and (OIV) 25.87 microns were made for twelve planetary nebulae. One or more of the lines was detected in seven of these nebulae and ionic abundances were calculated. These results are insensitive to nebula temperatures, in contrast to the case for optical or UV lines. However, density estimates from optical and UV forbidden lines were required to obtain abundances. The NeV infrared line flux from NGC 7662 was combined with the 3426A flux to obtain a NeV electron temperature of 11,200 (+2000, - 1100) K, which overlaps OIII temperature measurements. Since the ionization potential of NeIV is much greater than that of OII, T sub e (NeV) would be expected to be much greater than T sub e (OIII). In fact, numerical models predict T sub e (NeV) (16-20) x 1000 K. This discrepancy may indicate inaccuracies in currently available atomic parameters for NeV.

Shure, M. A.↗
NASA NTRS1983

Covariance hypotheses for LANDSAT data

Two covariance hypotheses are considered for LANDSAT data acquired by sampling fields, one an autoregressive covariance structure and the other the hypothesis of exchangeability. A minimum entropy approximation of the first structure by the second is derived and shown to have desirable properties for incorporation into a mixture density estimation procedure. Results of a rough test of the exchangeability hypothesis are presented.

Decell, H. P.↗
NASA NTRS1982

Observations of the infrared fine-structure lines of S III at 18.71 and 33.47 microns in four H II regions

Infrared fine-structure lines provide a particularly useful probe of ionized nebulae. The present investigation is concerned with measurements of the forbidden S III lines at 18.71 and 33.47 micrometers for four H II regions, S158A, S158G, G75.84+0.4, and W3 IRS 1. These lines are used to estimate densities, and comparisons are made with rms densities determined from radio observations to evaluate the importance of clumping. For the case of the optical H II region S158A, comparisons are made with both optical and forbidden O III line determinations of the density. The reported observations were made using a dual-grating, liquid-helium-cooled spectrometer containing a three-element Si:Sb detector array and a three-element Ge:Ga detector array. It is found that clumping is important in the cases of G75.84+0.4, W3 IRS 1, and M42. These three H II regions have filling factors of 0.024, 0.09, and 0.03, respectively.

Herter, T.↗
NASA NTRS1983

Determinations of S III, O IV, and Ne V abundances in planetary nebulae from infrared lines

Twelve planetary nebulae have been subjected to airborne observations of the S III (18.71 microns), Ne V (24.28 microns) and O IV (25.87 microns) IR forbidden lines, of which one or more lines were detected in seven of the nebulae. These results are insensitive to nebula temperatures, in contrast to optical or UV lines, although density estimates from the latter were required to obtain abundances. The Ne V electron temperature of 11,200 + 2000 or - 1100 K temperature obtained overlaps O III temperature measurements. Since the ionization potential of Ne IV is much greater than that of O II, the electron temperature of Ne V should be much greater than that of O III. This discrepancy may indicate inaccuracies in currently available atomic parameters for Ne V.

Shure, M. A.↗
NASA NTRS1984

Composition, Structure and Evolution of Uranian and Neptunian Satellites

Large uncertainties in the current estimated densities of all of these satellites prevent detailed modeling or predictions. Nevertheless, current evidence suggests that at least Titania and Oberon might have anomalously high densities of 2-39 cm(-3), possibly requiring almost ice-free hydrated silicates or formation in a CO-rich environment, implying presence of CO-clathrate and a small ice/rock ratio. Trition and the four largest satellites of Uranus are massive enough to have undergone significant accretional heating and early differentiation; NH3-H2O volcanism; partial outgassing of CO, N2, Ch4; formation of dark surficial deposits of carbon-rich material obtained by UV irradiation of outgassed material; and, at least in the cases of Ariel and Triton, a possibility of weak ongoing icy volcanic activity. Triton may be the largest captured body in the solar system, with an unusual history and composition, including the possibility of substantial liquid or solid nitrogen obtained from either primordial NH3 photolysis or clathrate decomposition.

Stevenson, D. J.↗
NASA NTRS1985

A new approach to evaluate gamma-ray measurements

Misunderstandings about the term random samples its implications may easily arise. Conditions under which the phases, obtained from arrival times, do not form a random sample and the dangers involved are discussed. Watson's U sup 2 test for uniformity is recommended for light curves with duty cycles larger than 10%. Under certain conditions, non-parametric density estimation may be used to determine estimates of the true light curve and its parameters.

Dejager, O. C.↗
NASA NTRS1987

Driving mechanism of unsteady separation shock motion in hypersonic interactive flow

Wall pressure fluctuations were measured under the steady separation shock waves in Mach 5 turbulent interactions induced by unswept circular cylinders on a flat plate. The wall temperature was adiabatic. A conditional sampling algorithm was developed to examine the statistics of the shock wave motion. The same algorithm was used to examine data taken in earlier studies in the Princeton University Mach 3 blowdown tunnel. In these earlier studies, hemicylindrically blunted fins of different leading-edge diameters were tested in boundary layers which developed on the tunnel floor and on a flat plate. A description of the algorithm, the reasons why it was developed and the sensitivity of the results to the threshold settings, are discussed. The results from the algorithm, together with cross correlations and power spectral density estimates suggests that the shock motion is driven by the low-frequency unsteadiness of the downstream separated, vortical flow.

Dolling, D. S.↗
NASA NTRS1987

A kinematic model for the evolution of the Gorda Plate

The magnetic lineation pattern produced by the Gorda Rise reflects a complex tectonic history marked by propagating ridges and nonrigid deformation of the Gorda plate. The Pacific-side lineation pattern is matched using a kinematic model which incorporates northward and southward propagating ridge segments and ridge rotation. By superimposing flexural-slip style deformation on the resulting Gorda-side lineations, and allowing convergent motion at the Mendocino Fracture Zone, the Gorda magnetic pattern is also successfully predicted. A similar model, but one which does not allow convergence at the Mendocino Fracture Zone, fails to produce the observed lineation pattern. Model predictions of the amount of material that would be 'obducted' at the Mendocino due to convergence between the Gorda and Pacific agree with bathymetric and density estimates of the amount of material located along the Mendocino Ridge.

Stoddard, Paul R.↗
NASA NTRS1988

Magnetic reversal spurts: Rain gauges for comet showers

Abrupt increases in the rate of magnetic reversals (magnetic reversal spurts) were first studied by many others. They hypothesized that spurts result from increased turbulence in the earth's core dynamo during episodes of intense bolide bombardment of the earth. Mechanisms for creating episodes of intense bombardment of the earth involve gravitational perturbation of the Oort cloud of comets, either by a hidden planet, a solar companion, or massive matter in the galactic plane. Herein, the time variation in reversal rate is analyzed using methods of statistical density estimation. A smooth, continuous estimate of reversal rate is obtained using an adaptive kernel method, in which the kernel width is adjusted as a function of reversal rate. The estimates near the ends of the data series (at 165 my ago and the present) are obtained by extending the data by reflection. The results show that the reversal spurts are not associated demonstrably with extinctions or well-dated impacts. If the spurts do record episodes of intense bombardment of the earth, then the mass extinctions do not, in general, occur at times of impacts. Furthermore, the large impact craters seen are not obviously related to the spurts, suggesting that the craters may have been caused by bolides of a different nature and with a different temporal pattern. However, the most simple explanation seems to be that the spurts do not record comet showers, either because the recording mechanism suggested by Muller and Morris is not effective or because comet showers are not triggered in the ways considered by Hut et al.

Lutz, T. M.↗
NASA NTRS1989

Hot ion plasmas from the cloud of neutral gases surrounding the Space Shuttle

Large intensities of hot positive ions are observed out to distances of several hundred meters from the Space Shuttle with a plasma analyzer on a small free-flying satellite, the Plasma Diagnostics Package. This ion plasma is inferred to be generated by the charge exchange of ionospheric O+ ions with a large cloud of water molecules from the Space Shuttle. The measured ion density ranges from about 30 to 10,000 H2O+ ions/cu cm. A model for the water vapor cloud provides the basis for density estimates as high as 10 to the 9th H2O molecules/cu cm at a distance of 50 m from the Space Shuttle. Thus, the Space Shuttle possesses a substantial coorbiting atmosphere.

Paterson, W. R.↗