Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “extinction”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Interface extinction and subsurface peaking of the radiation pattern of a line source

The radiation pattern of a line source lying along the plane interface of two dielectric half-spaces is calculated. It is found that the pattern at the interface has a null (interface extinction); that the pattern in the upper half-space, whose index of refraction is taken to be less than that of the lower half-space, has a single lobe with a maximum normal to the interface; and that the pattern in the lower half-space (subsurface region) has two maxima (peaks) straddling symmetrically a minimum. Interpretation of these results in terms of ray optics, Oseen's extinction theorem, and the Cerenkov effect are given.

Engheta, N.↗
NASA NTRS1982

Impact mechanics of the Cretaceous-Tertiary extinction bolide

An examination of the mechanics of asteroidal, cometary, and meteor swarm impact on the earth determined if the enrichment of projectile material in the K-T layer is consistent with melts and impact breccias on the earth and moon, the size of the impacters, the distribution of the kinetic energy, and the sequence of impacts that could give rise to observed extinction phenomena. Flows resulting from spherical projectile impacts onto layers of air, water, and silicates were modeled and Eulerian finite difference algorithms were employed to solve conservation equations and equations of state. A range of speeds and impacter densities were considered, along with sizes from 0.17 km, which would be consumed in the atmosphere, to a 10 km object, which would have had a diameter greater than a reference 7.1 km atmosphere depth. It is concluded that an impact of the K-T bolide could result in global biotic extinction and worldwide material deposition.

Okeefe, J. D.↗
NASA NTRS1985

Phase function, backscatter, extinction, and absorption for standard radiation atmosphere and El Chichon aerosol models at visible and near-infrared wavelengths

Tabular values of phase function, Legendre polynominal coefficients, 180 deg backscatter, and extinction cross section are given for eight wavelengths in the atmospheric windows between 0.4 and 2.2 microns. Also included are single scattering albedo, asymmetry factor, and refractive indices. These values are based on Mie theory calculations for the standard rediation atmospheres (continental, maritime, urban, unperturbed stratospheric, volcanic, upper atmospheric, soot, oceanic, dust, and water-soluble) assest measured volcanic aerosols at several time intervals following the El Chichon eruption. Comparisons of extinction to 180 deg backscatter for different aerosol models are presented and related to lidar data.

Whitlock, C. H.↗
NASA NTRS1984

Star formation and extinct radioactivities

An assessment is made of the evidence for the existence of now-extinct radioactivities in primitive solar system material, giving attention to implications for the early stages of sun and solar system formation. The characteristics of possible disturbances in dense molecular clouds which can initiate the formation of cloud cores is discussed, with emphasis on these disturbances able to generate fresh radioactivities. A one-solar mass red giant star on the asymptotic giant branch appears to have been the best candidate to account for the short-lived extinct radioactivities in the early solar system.

Cameron, A. G. W.↗
NASA NTRS1985

Magnetic reversals and mass extinctions

The results of a study of reversals of the earth's magnetic field over the past 165 Myr are presented. A stationary periodicity of 30 Myr emerges which predicts pulses of increased reversal activity centered at 10, 40, 70, . . . Myr before the present. The correlation between the reversal intensity and biological extinctions is examined, and a nontrivial discrepancy is found between the magnetic and extinction periodicity.

Raup, D. M.↗
NASA NTRS1985

Extinction coefficient of H2CC(3B2) at 137 nm

In spite of the conduction of numerous studies regarding the vinylidene free radical, its role and importance as a reactive intermediate is not well characterized. Laufer (1980, 1983) has reported the absorption spectrum of metastable H2CC(3B2), the lowest excited state, in the vacuum ultraviolet and has measured several aspects of its quenching properties. The present study provides a measurement of the extinction coefficient of H2CC(3B2). Knowledge of the vinylidene concentration is required to convert readily available absorption data into an extinction coefficient or cross section. In the current work, the H2CC(3B2) concentration was determined in an investigation of the photodissociation of vinyl chloride.

Fahr, A.↗
NASA NTRS1985

Cretaceous extinctions - Evidence for wildfires and search for meteoritic material

The results of analyses of the contents of deposits in the Cretaceous-Ternary (K-T) transition at three sites worldwide are discussed. The study was undertaken to examine the composition of the object which may have struck the earth, causing widespread biotic extinction. The data indicate that most of the parent body was destroyed on impact, a condition which would also hold true for comets, suggesting that comets were not a source of prebiotic life. A four-orders-of-magnitude excess of carbon in the K-T layer is considered in terms of its source, which is suspected to be deposits from wildfires. The consequent extinctions of species are regarded as possibly making the current nuclear winter scenarios too optimistic.

Wolbach, W. S.↗
NASA NTRS1985

Extinction of visible and infrared radiation in rain Comparison of theory and experiment

A critical review is given of the experimental and theoretical results concerning the measurement of rainfall using optical extinction, i.e., the attenuation of radiation with wavelength less than or equal to that of the infrared band. It is shown that rainfall rates found from an empirical relation involving optical extinction generally display average deviations without regard for sign of only 25 percent when compared with those measured by raingages directly beneath the optical beam. It is also shown that the differences between experimental and theoretical results can be explained in terms of variations of the shape of the raindrop size distribution, i.e., deviations from exponentiality.

Ulbrich, C. W.↗
NASA NTRS1986

The extinction law in the open cluster NGC 457 and the intrinsic energy distribution of Phi Cassiopeiae (F0 Ia)

Five early B-type stars near the main-sequence turnoff in NGC 457 have been observed at low dispersion with the short-wavelength prime and the long-wavelength redundant cameras of the IUE satellite. The equivalent widths of spectral features that are particularly strong and sensitive to temperature and luminosity were computed in the cluster stars and in 20 lightly reddened stars of types O9-B3 and luminosity classes III-V. The comparison of the equivalent widths provides a reliable method for finding matching pairs. Having identified the best comparison star for each program star, binned fluxes were used to determine the mean extinction curve. In order to cover the visible region, monochromatic fluxes of Phi Cas were derived from observations with the intensified Reticon scanner mounted on the No. 2 0.9 m telescope of KPNO, and they were dereddened with the mean extinction curve of Savage and Mathis. Thus, the intrinsic energy distribution of Phi Cas were determined from 1500 to 5800 A for use in a detailed model-atmosphere analysis.

Rosenzweig, P.↗
NASA NTRS1986

Lidar method of measurement of atmospheric extinction and ozone profiles

A description of a method of measurement of atmospheric extinction and of ozone profiles by use of the backscatter signal from a monostatic lidar is given. The central feature of the procedure involves a measurement of the ratio of the Raman backscatter returns of both the oxygen and nitrogen atmospheric content. Because the ratio of the number density of both species is known to high accuracy, the measurement itself becomes a measure of the ratio of two transmissions to altitude along with a ratio of the two system constants. The calibration measurement for determining the value of the ratio of the two system constants or electro-optical conversion constants is accomplished by a lidar measurement of identical atmospheric targets while at the same time interchanging the two optical filters in the two optical channels of the receiver. More details of the procedure are discussed. Factoring this calibrated value into the measured O2/N2 profile ratio provides a measured value of the ratio of the two transmissions. Or equivalently, it provides a measurement of the difference of the two extinction coefficients at the O2 and N2 Raman wavelengths as a function of the height.

Cooney, J. A.↗
NASA NTRS1986

Antarctic springtime measurements of ozone, nitrogen dioxide, and aerosol extinction by SAM II, SAGE, and SAGE II

Simultaneous vertical profiles of O3, NO2, and aerosol extinction obtained with the Stratospheric Aerosol Measurement II, Stratospheric Aerosol and Gas Experiment (SAGE), and SAGE II satellite instruments across the southern polar vortex show that significant differences exist at all altitudes. Both gaseous species display lower concentrations within the vortex over measurement altitudes ranging from the tropopause to 60 km and 20 to 40 km for O3 and NO2, respectively. Aerosol extinction above 15-18 km and total aerosol stratospheric column are also lower inside the vortex than outside. Total column amounts of O3 and NO2 are found to be strongly coupled to spatial location within the vortex, with minimum total values located around the vortex center. Vertical profiles selected to emphasize the observed difference across the circumpolar vortex are presented for October 13, 1981, and October 13, 1985, near 70 and 68 deg S latitude, respectively.

Mccormick, M. P.↗
NASA NTRS1986

Wavelength dependence of aerosol extinction coefficient for stratospheric aerosols

A simple empirical formula for the wavelength dependence of the aerosol extinction coefficient is proposed. The relationship between the constants in the formula and the variable parameter in the aerosol size distribution is explicitly expressed. Good agreement is found between the extinction coefficients calculated from the proposed formula and that calculated from Mie theory. The proposed expression is shown to be better than the Angstroem formula commonly used by atmospheric scientists.

Yue, Glenn K.↗
NASA NTRS1987

Periodic extinction of families and genera

Eight major episodes of biological extinction of marine families over the past 250 million years stand significantly above local background (P less than 0.05). These events are more pronounced when analyzed at the level of genus, and generic data exhibit additional apparent extinction events in the Aptian (Cretaceous) and Pliocene (Tertiary) Stages. Time-series analysis of these records strongly suggests a 26-million-year periodicity. This conclusion is robust even when adjusted for simultaneous testing of many trial periods. When the time series is limited to the four best-dated events (Cenomanian, Maestrichtian, upper Eocene, and middle Miocene), the hypothesis of randomness is also rejected for the 26-million-year period (P less than 0.0002).

Raup, David M.↗
NASA NTRS1987

Comet showers as a cause of mass extinction

Three independent pieces of evidence supporting a connection between comet showers and clustering in terrestrial cratering and mass extinctions are presented. The temporal profile of a comet shower triggered by a star passing through the Oort cloud is calculated. Four weak peaks are found in the age of distribution of impact craters over the past 100 Myr, as well as two compact clusters of ages of impact glass broadly coincident with crater-age peaks. Recent paleontological observations are reviewed that indicate a stepwise character for some well-documented mass extinctions in the past 100 Myr which roughly coincide with three of the four peaks in crater ages and which have a duration compatible with comet shower predictions.

Hut, Piet↗
NASA NTRS1987

Extinction and abundance properties of Alpha Scorpii circumstellar grains

The UV extinction and element depletions in the circumstellar envelope Alpha Sco are investigated using IUE, Copernicus, and AS data of Alpha Sco B which can be seen in absorption through the cool-star envelope. The circumstellar reddening, depletions, and spectral extinction toward Alpha Sco B are estimated. Evidence is presented which suggests that the circumstellar grains are large and siliceous.

Snow, Theodore P., Jr.↗
NASA NTRS1988

The use of a helicopter mounted ranging scatterometer for estimation of extinction and scattering properties of forest canopies-II: Experimental results for high-density aspen

An analytic expression is derived that describes the backscatter power from a semi-infinite plane parallel homogeneous canopy as a function of distance from an airborne radar. This model is fitted to observed data for a high-density aspen canopy by a modification of a technique developed by Tyapkin (1960). This inversion of the model provides unbiased estimates of the canopy extinction and backscattering parameters. An active radar calibrator located underneath the canopy provides an independent method of determining the volume extinction coefficient. The results reported indicate that the coefficients change throughout the year. A comparison of these coefficients with Eom and Funk's (1984) disk model, using measured canopy properties, shows that at C-band frequency, only a part of the scattering and absorption can be attributed to the canopy leaves.

Pitts, David E.↗
NASA NTRS1988

Iridium abundance maxima in the Upper Cenomanian extinction interval

Two iridium abundance peaks, both 0.11 ppb (whole-rock basis) over a local background of 0.017 ppb, have been found in Middle Cretaceous marine rocks near Pueblo, Colorado. They occur just below the 92-million-year-old Cenomanian-Turonian (C-T) stage boundary. No other peaks were found in 45 meters of strata (about 2.5 million years of deposition) above and below the boundary interval. The broad lower peak straddles the first in a series of extinctions of benthic and nektonic macrobiota which comprise the C-T extinction event. The sharp upper peak occurs stratigraphically about 1.2 meters above the lower peak. The excess Ir might be from meteoroid impacts.

Orth, C. J.↗
NASA NTRS1988

Testing for periodicity of extinction

The statistical techniques used by Raup and Sepkoski (1984 and 1986) to identify a 26-Myr periodicity in the biological extinction record for the past 250 Myr are reexamined, responding in detail to the criticisms of Stigler and Wagner (1987). It is argued that evaluation of a much larger set of extinction data using a time scale with 51 sampling intervals supports the finding of periodicity. In a reply by Sigler and Wagner, the preference for a 26-Myr period is attributed to a numerical quirk in the Harland et al. (1982) time scale, in which the subinterval boundaries are not linear interpolations between the stage boundaries but have 25-Myr periodicity. It is stressed that the results of the stringent statistical tests imposed do not disprove periodicity but rather indicate that the evidence and analyses presented so far are inadequate.

Raup, David M.↗