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At least 1,459 records · Page 81

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NASA NTRS1982

Characteristics of a microchannel plate intensifier

A telephoto optical system has been designed for imaging applications involving a use of the spectral region from 2000 to 7500 A. The optics images onto the photocathode of a multichannel plate photomultiplier (PMT). Interference filters are employed to provide spectral selectivity. The PMT consists of three microchannel plate intensifiers (MCP), arranged in a 'Z' configuration. MCP photomultiplier pulse height distributions are discussed along with aspects of spatial distribution, the electron gain pulse height distribution, and the charge loss due to edge effects. Attention is given to the electron gain pulse height distribution obtained from four anodes connected in parallel, taking into account a uniform illumination of the photocathode.

Sandie, W. G.↗

NASA NTRS1982

On the geometric form of volcanoes - Comment

The model of Lacey et al. (1981) accounting for the geometric regularity and approximate cone shape of volcanoes is discussed. It is pointed out that, contrary to the model, volcano eruptions do not occur randomly in elevation and azimuth, but are commonly restricted to summit vents and a few well defined flank zones, so that the form of a volcano is determined by its vent locations and styles of eruption. Other false predictions of the model include the constancy of lava volumes at all vent elevations, the increase in volcano radius as the square root of time, a critical height for volcano growth, the influence of planetary gravity on volcano height and the negligible influence of ash falls and flows and erosional deposition. It is noted that the model of Shteynberg and Solov'yev, in which cone shape is related to stresses due to increasing cone height, may provide a better understanding of volcano morphology.

Wood, C. A.↗

NASA NTRS1980

Charge distribution in lightning flashes

Four VHF receivers about 10 km apart were used to detect three-dimensional lightning VHF sources during active thunderstorms. A computer implemented algorithm was used to determine the difference in the time of arrival (DTOA) between the VHF pulses. The three-dimensional source locations determined from the DTOA were correlated with ground electric field measurements (0.1 Hz to 1.5 MHz). Four cloud to ground (CG) and three intracloud (IC) flashes, selected randomly, were studied. A point charge model was used to determine the charge being removed during the different phases of CG and IC flashes. It was determined that return strokes lowered between 24.1 and 3.6 coulombs of negative charge to ground from heights between 10.5 and 5.8 km above MSL. The interstroke process after the first return stroke in a CG flash lowered between 3.4 and 0.9 coulombs of negative charge between the heights of 14.2 (near the cloud tops) to heights of 5.8 km (near the beginning of the leader-return stroke process).

Rustan, P. L.↗

NASA NTRS1982

Seasat altimeter determination of ocean current variability

An experiment conducted in the Kuroshio Current, east of Japan, has confirmed the ability of radar altimeters of precision on the order of 10 cm, such as that of the Seasat satellite, to measure the small oceanic height variations associated with geostrophic ocean currents. Changes in surface dynamic height were inferred from data gathered by air-expendable barithermographs, which had been dropped to coincide with the Seasat subtrack, for the periods between the flights of September 25 and October 5 and 13, 1978. The changes registered generally agreed to within + or - 10 cm of the height changes observed in the altimeter data.

Bernstein, R. L.↗

NASA NTRS1982

Vertical flows of supergranular and mesogranular scale observed on the sun with OSO 8

A program of observations was carried out in order to study the penetration of supergranular flows over a broad range of heights in the solar atmosphere. Steady Doppler velocities are determined from observations of a Si II spectral line using the Ultraviolet Spectrometer on the Orbiting Solar Observatory 8 (OSO 8) satellite and Fe I and Mg I lines with the diode-array instrument on the vacuum telescope at Sacramento Peak Observatory (SPO). The heights of formation of these spectral lines span about 1400 km or nearly 11 density scale heights from the photosphere to the middle chromosphere. Steady vertical flows on spatial scales typical of supergranulation and mesogranulation have been detected in the middle chromosphere with OSO 8. The patterns of intensity and steady velocity of granular scale are reproducible in successive data sets. The patterns appear to evolve slowly over the 9 hr period spanned by six orbits.

November, L. J.↗

NASA NTRS1982

Lidar clear atmosphere multiple scattering dependence on receiver range

The degree of multiple scattering encountered by nadir-directed lidar in clear atmosphere conditions was assessed by the Monte Carlo method based on a model of the vertical distribution of aerosol scattering in the atmosphere. The lowest 3 km of the atmosphere were regarded as containing 90% of the aerosol optical thickness, with the unity normalized value for the forward peak of the aerosol phase function being 4.38 at a 694.3 nm laser wavelength. Results were obtained for two lidar receiver heights for three receiver field of view (FOV) angular halfwidths. An increase in receiver height was determined to cause a significant increase in the amount of angular scattering of the lidar signal. A factor of 20 change in receiver height produced an order of magnitude change in the single to multiple scattering ratio.

Spinhirne, J. D.↗

NASA NTRS1982

Aerodynamic levitation of laser-heated solids in gas jets

The aerodynamic levitation technique is developed for studies of high-temperature material properties and gas/condensed-phase reaction kinetics. Stable levitation is demonstrated in a supersonic jet from a 0.081 cm nozzle with 0.03-0.20 g 0.24-0.47 cm diameter solid spheres at a height between 0.7-2.0 cm above the nozzle and ambient pressures between 1.1-18 Torr. A model of supersonic jet levitation is developed which accurately predicts the values of height vs pressure over the full range of conditions investigated. It is found that the efficiency with which jet momentum is converted into levitation force decreases with the jet/specimen diameter ratio and the jet Reynolds number, and the rate of jet spreading with distance from the nozzle is found to agree with that measured by pitot tube traverses of the jet. In addition, laser heating is shown to reduce the jet momentum required for levitation at a given height and to increase levitation stability. Measurements of sphere levitation in subsonic gas jets show that the required jet momentum flow rate exceeds the specimen weight by about 2/the specimen drag coefficient at its terminal free-fall speed under ambient conditions.

Nordine, P. C.↗

NASA NTRS1982

Microwave emissions from soils with rough surfaces

The effect of surface roughness on the thermal microwave emission of soils has been studied with the Kirchoff approach. A model is presented that includes both the coherent and incoherent reflectivities of the rough surface. It is demonstrated that both the coherent and incoherent terms must be included in order for theoretical computations to provide good agreement with the experimental data, especially for wet soil, where surface roughness causes a dramatic increase in brightness temperature. In addition, the model including both the coherent and incoherent terms allows one to use the physically measured surface height deviations of the rough surface in the model. Previous models have required that an effective height, not equal to the actual surface height measurements, be used in order for theoretical computations to match measurements.

Tsang, L.↗

NASA NTRS1982

Verification and transfer of thermal pollution model. Volume 3: Verification of 3-dimensional rigid-lid model

The six-volume report: describes the theory of a three dimensional (3-D) mathematical thermal discharge model and a related one dimensional (1-D) model, includes model verification at two sites, and provides a separate user's manual for each model. The 3-D model has two forms: free surface and rigid lid. The former, verified at Anclote Anchorage (FL), allows a free air/water interface and is suited for significant surface wave heights compared to mean water depth; e.g., estuaries and coastal regions. The latter, verified at Lake Keowee (SC), is suited for small surface wave heights compared to depth (e.g., natural or man-made inland lakes) because surface elevation has been removed as a parameter. These models allow computation of time-dependent velocity and temperature fields for given initial conditions and time-varying boundary conditions. The free-surface model also provides surface height variations with time.

Lee, S. S.↗

NASA NTRS1982

Verification and transfer of thermal pollution model. Volume 4: User's manual for three-dimensional rigid-lid model

The theory of a three dimensional (3-D) mathematical thermal discharge model and a related one dimensional (1-D) model are described. Model verification at two sites, a separate user's manual for each model are included. The 3-D model has two forms: free surface and rigid lid. The former allows a free air/water interface and is suited for significant surface wave heights compared to mean water depth, estuaries and coastal regions. The latter is suited for small surface wave heights compared to depth because surface elevation was removed as a parameter. These models allow computation of time dependent velocity and temperature fields for given initial conditions and time-varying boundary conditions. The free surface model also provides surface height variations with time.

Lee, S. S.↗

NASA NTRS1983

Correlation of 3 hour radiosonde data with coincident Nimbus-7 total ozone measurements

A special network exercise supporting the VAS demonstration is reported. The second near-real time total ozone mapping spectrometer (TOMS) activity involving out of sequence processing of Nimbus-7 data over the United States and Europe is presented. The ozone data maps were used by airlines to search for regions of clear air turbulence and by an aircraft data gathering program, for synoptic upper air and tropopause height analyses. The TOMS total ozone levels are correlated with radiosonde tropopause height for testing the ability of TOMS data to give independent satellite measurements of tropopause height.

Gatlin, J. A.↗

NASA NTRS1982

The Venus ionosphere at grazing incidence of solar radiation - Transport of plasma to the night ionosphere

A quasi-two-dimensional model of the Venusian ionosphere is used to calculate the ion number densities and horizontal ion bulk velocities expected for a range of solar zenith angles near the terminator (80-100 deg). These results are compared with data from the Pioneer Venus Orbiter retarding potential analyzer. It is shown that antisunward horizontal plasma fluxes produced by solar EUV-induced pressure gradients are sufficient to maintain the nighttime ionosphere. While photoionization is the dominant source of ionospheric plasma for solar zenith angles less than 92 deg, plasma transport from the dayside is the dominant plasma source for solar zenith angles greater than 95 deg. It is also shown that the main nightside plasma peak at a height of 140 km is of the F2 type; its height and shape are therefore quite insensitive to the height of the ion source.

Whitten, R. C.↗

NASA NTRS1983

Influence of the E region dynamo on equatorial spread F

The integrated E region Pedersen conductivity can be an important parameter in determining whether the bottomside of the equatorial F layer will be stable against the Rayleigh-Taylor gravitational instability. The F layer is observed to become unstable when it rises to great heights after sunset. One effect of this height rise is to decrease the stabilizing influence of ion-neutral collisions at F region heights. It is shown here that the same eastward electric field that raises the F layer also decreases the Pedersen conductivity of the E region, which further destabilizes convective overturning. Because the conductivity of magnetic tubes that penetrate the main F layer is large compared to the E layer contribution, these effects are important only for the bottomside of the equatorial F layer.

Hanson, W. B.↗

NASA NTRS1982

Satellite-in situ measurement intercomparisons at Wallops Island, Virginia

A field program was carried out at Wallops Island, Virginia in which a number of in situ soundings were made during February to May 1980 in conjunction with near overpasses of NOAA-6. A comparison of NOAA-6 measured and derived quantities with those of in situ instruments was made. Positive temperature biases (satellite warmer) were found to appear in the lower troposphere and tropopause regions; negative differences generally were found to appear in the middle and upper stratosphere. Average geopotential height differences were relatively small at all levels, but increasing rms errors showed decreasing reliability with height of satellite geopotential heights. Satellite-derived geostrophic winds were found to have less shear than in situ determined winds. Bias in the directional differences was found to range from less than 5 deg to about 20 deg, with rms differences apparently decreasing near the level of wind speed.

Nestler, M. S.↗

NASA NTRS1983

High altitude Venus haze from Pioneer Venus limb scans

High-resolution limb scans of Venus made with the Orbiter Cloud Photopolarimeter aboard Pioneer Venus have been used to observe the high altitude Venus haze and its variation over almost one half a Venusian year. A model for a spherical atmosphere with tangential viewing geometry was used to analyze the observations and derive the haze properties. Values are found for the particle size and refractive index, particle number density and scale height, and eddy diffusion coefficient. Single scattering computations show good agreement with observed intensities for particles smaller than 0.3 micron radius and refractive index less than 1.7, consistent with, but not limited to, concentrated sulfuric acid. Particle scale height in the 0.5 to 2 mbar pressure regions varies between 1 and 3 km over the season, latitude, and local time ranges of the observations. Detached layers of haze are sometimes present. An average particle scale height of 2.2 km at 84 km altitude yields an eddy diffusion coefficient of 130,000 sq cm/sec.

Lane, W. A.↗

NASA NTRS1983

Analysis of the error associated with grid representation of point sources

The steady-state, three-dimensional transport equation for inert pollutant dispersion in the atmosphere is solved analytically to obtain expressions for pollutant concentrations from both a point source and a rectangular area source. The results of this analysis have application to numerical grid models in which the smallest resolvable source is the size of a single grid. The concentrations at ground level along the plume centerline from the two types of sources are in close agreement with each other at distances greater than 10 km downwind when the horizontal dimension of the rectangular source is not less than 100 m. The dimension of the source in the vertical direction is held constant at 50 m. Similar results are obtained for concentrations along the plume centerline at the effective stack height. When the size of the source in the crosswind direction is increased to 1 km, good agreement between the two concentrations is obtained for downwind distances over 60 km for the ground level case and over 140 km for the concentration at stack height level. For the horizontal dimension equal to 10 km, good agreement between the two cases is only obtained at extremely large downwind distances at both the stack height and ground levels.

Karamchandani, P.↗

NASA NTRS1983

Cumulus cloud transport of transient tracers

A theoretical framework is developed for including cumulus cloud transport, rainout of water soluble gases, and aqueous phase chemistry into gas phase photochemical models. Cloud populations are represented as ensemble distributions of individual clouds of various heights. An individual cloud is represented as a one-dimensional, steady state plume with height-independent radius entraining air from the boundary of the cloud. The model is applied to several hypothetical atmospheric tracers to show how clouds may affect the tropospheric distributions of ozone, NO(y), SO2, peroxyacetylnitrate, hydrocarbons, and other gases. Although the numerical experiments were based on cloud mass fluxes from diagnostic studies in the tropics and thus are not representative of the entire globe, it was found that an increasing mixing ratio with height in the free troposphere can be produced for some gases with only a surface source when clouds are present. This suggests that some reactive tropospheric species with primarily surface sources may play a somewhat more important role in tropospheric chemistry than is presently believed depending on the global distribution of cloud mass fluxes. Deficiencies in existing photochemical models due to the way clouds are typically treated are discussed.

Gidel, L. T.↗

NASA NTRS1983

Ozone and water vapor monitoring using a ground-based lidar system

Measurements of atmospheric ozone and water vapor profiles obtained using a differential absorption lidar (DIAL) technique at Haute Provence Observatory are reported. The DIAL technique is based on the comparison of differences in attenuation in backscattered signals at wavelengths centered on strong absorptions or absorption lines of the species in question, and wavelengths corresponding to weaker or no absorption. The system in use is based on three lasers - a Nd:YAG Q-switched laser frequency doubled to emit at 532 nm, and two tunable narrowline dye lasers used to convert this emission into the IR or UV spectral regions. Ozone concentration profiles have been recorded up to a height of 30 km, and reveal large day-to-day variations in number density which were correlated with a decrease in tropopause height. Measurements of water vapor concentrations show a variability of the mixing ratio at heights up to 2.5 km representative of dynamic processes, and a variability above 2.5 km that is within the measurement error. Day-to-day variations in tropospheric mixing ratio were also observed.

Megie, G.↗