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NASA NTRS1981

Photon-counting array detectors for space and ground-based studies at ultraviolet and vacuum ultraviolet /VUV/ wavelengths

The Multi-Anode Microchannel Arrays (MAMAs) are a family of photoelectric photon-counting array detectors, with formats as large as (256 x 1024)-pixels that can be operated in a windowless configuration at vacuum ultraviolet (VUV) and soft X-ray wavelengths or in a sealed configuration at ultraviolet and visible wavelengths. This paper describes the construction and modes of operation of (1 x 1024)-pixel and (24 x 1024)-pixel MAMA detector systems that are being built and qualified for use in sounding-rocket spectrometers for solar and stellar observations at wavelengths below 1300 A. The performance characteristics of the MAMA detectors at ultraviolet and VUV wavelengths are also described.

Timothy, J. G.↗

NASA NTRS1982

Dual-wavelength correlation measurements with an airborne pulsed carbon dioxide lidar system

Dual-wavelength correlation data are presented for ground-level and airborne measurements of topographic and ocean backscatter near the 9.5-micron wavelength. Data were acquired with a pulsed carbon dioxide, dual-laser, dual-wavelength lidar system. The best system performance gave a dual-wavelength uncorrelated noise component of 2.8% and 9% standard deviation for ground-based and airborne data, respectively. This sets one limit on lidar measurement accuracy with a single pulse pair.

Bufton, J. L.↗

NASA NTRS1985

Wavelength division multiplexing

Wavelength division multiplexing (WDM) represents an approach for expanding the communication capacity and for implementing special data techniques in a fiber optics system. This technology is implemented by adding optical sources of different wavelengths at optical transmitting locations. The present paper is concerned with some of the current efforts in WDM. WDM applications are related to long haul communications, local area data networks, spacecraft and aircraft data systems, fault tolerant computer networks, special sensor devices, high speed data processors, closed circuit and cable television, and submarine cable systems. Attention is given to the current state of wavelength division multiplexing applications, the availability and status of WDM components semiconductor lasers/transmitters, availability and status of fiber optic detectors/receivers, optical fibers/cables/connectors/taps/star couplers, wavelength multiplexers/demultiplexers, and future WDM for local area networks.

Hendricks, H. D.↗

NASA NTRS1986

Measurement of wavelengths and lamb shifts for inner-shell transitions in Fe XVIII-XXIV

The wavelengths of inner-shell 1s-2p transitions in the ions Fe XVIII-XXIV have been measured in solar flare spectra recorded by the Naval Research Laboratory crystal spectrometer (SOLFLEX) on the Air Force P78-1 spacecraft. The measurements are compared with previous measurements and with recently calculated wavelengths. It is found that the measured wavelengths are systematically larger than the wavelengths calculated using the Z-expansion method by up to 0.65 mA. For the more highly charged ions, these differences can be attributed to the QED contributions to the transition energies that are not included in the Z-expansion calculations.

Seely, J. F.↗

NASA NTRS1986

Cometary grain scattering versus wavelength, or 'What color is comet dust'?

Optical and near-infrared observations of comets are combined in a systematic study of the wavelength dependence of the scattering from cometary grains. The normalized rate of change of the reflectivity of cometary grains with respect to the wavelength of observations decreases as the wavelength increases. The observed wavelength dependence of the reflectivity gradient is consistent with an origin by scattering from micron-sized or larger, slightly absorbing spheres. The optically important comet grains are about an order of magnitude larger than the optically important interstellar grains. This size difference is a probable result of grain growth in the cloud from which the comets condensed. Grain properties differ so widely among the comets that any phase angle or heliocentric distance dependences of the continuum color are hidden.

Jewitt, David↗

NASA NTRS1988

Wavelength dependence of eddy dissipation and Coriolis force in the dynamics of gravity wave driven fluctuations in the OH nightglow

This paper examines the effect of inclusion of Coriolis force and eddy dissipation in the gravity wave dynamics theory of Walterscheid et al. (1987). It was found that the values of the ratio 'eta' (where eta is a complex quantity describing the ralationship between the intensity oscillation about the time-averaged intensity, and the temperature oscillation about the time-averaged temperature) strongly depend on the wave period and the horizontal wavelength; thus, if comparisons are to be made between observations and theory, horizontal wavelengths will need to be measured in conjunction with the OH nightglow measurements. For the waves with horizontal wavelengths up to 1000 km, the eddy dissipation was found to dominate over the Coriolis force in the gravity wave dynamics and also in the associated values of eta. However, for waves with horizontal wavelengths of 10,000 km or more, the Coriolis force cannot be neglected; it has to be taken into account along with the eddy dissipation.

Hickey, M. P.↗

NASA NTRS1988

Smoke optical depths - Magnitude, variability, and wavelength dependence

An airborne autotracking sun-photometer has been used to measure magnitudes, temporal/spatial variabilities, and the wavelength dependence of optical depths in the near-ultraviolet to near-infrared spectrum of smoke from two forest fires and one jet fuel fire and of background air. Jet fuel smoke optical depths were found to be generally less wavelength dependent than background aerosol optical depths. Forest fire smoke optical depths, however, showed a wide range of wavelength depedences, such as incidents of wavelength-independent extinction.

Pueschel, R. F.↗

NASA NTRS1989

Making Displaced Holograms At Two Wavelengths

Two-wavelength holographic system augmented with pair of prisms to introduce small separation between holograms formed simultaneously at two wavelengths on holographic plate. Principal use in study of flows. Gradients in index of refraction of fluid caused by variations in temperature, concentration, or both. Holography at one wavelength cannot be used to distinguish between two types of variations. Difference between spacings of fringes in photographs reconstructed from holograms taken simultaneously at two different wavelengths manipulated mathematically to determine type of variation.

Witherow, William K.↗

NASA NTRS1989

Light scattering characteristics of various aerosol types derived from multiple wavelength lidar observations

The present study demonstrates the potential of a multiple-wavelength lidar for discriminating between several aerosol types on the basis of the wavelength dependence of the aerosol backscatter coefficient. The two-component lidar equation was solved under the assumption of similarity in the derived profiles of backscatter coefficients for each wavelength. It is shown that a three-wavelength lidar system operating at 300, 600, and 1064nm can provide unique information for discriminating between various aerosol types (continental, maritime, Saharan-dust, stratospheric aerosols in a tropopause fold event, and tropical forest aerosols). Mie calculations were made using in situ aerosol data and aerosol models to compare with the lidar results. The disagreement between the theoretical and empirical results in some cases was substantial. These differences may be partly due to uncertainties in the lidar data analysis and aerosol characteristics and also due to the conventional assumption of aerosol sphericity for the aerosol Mie calculations.

Sasano, Yasuhiro↗

NASA NTRS1989

The measurement of wavelength in space plasmas

A plasma wave can be described by its frequency and wave vector, while a nonlinear plasma structure such as a solitary wave can be described by its temporal signature and scale size. Time series or frequency measurements are relatively easy to perform from spacecraft, but wavelength or scale size measurements are difficult. Nevertheless, direct measurements of wavelength have contributed to understanding plasma waves and fluctuations; for example, such measurements have established the widespread existence of spatial irregularities in the magnetosphere, determined the characteristics of waves upstream of the earth's bow shock, and allowed the size and motion of double-layer-like structures along auroral field lines to be inferred. This paper reviews progress in making measurements of wavelength and scale sizes. The different techniques for inferring wavelength are described, and then a series of case studies is considered to demonstrate applications.

Labelle, J.↗

NASA NTRS1990

First high spatial resolution interferometric observations of solar flares at millimeter wavelengths

The first high spatial resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array are presented. The observations were made at 3.3 mm wavelength during the very active periods of March 1989, using one or three baselines with fringe spacings of 2-5 arcsec. The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. It appears that millimeter burst sources are not much smaller than microwave sources. The most intense bursts imply brightness temperatures of over 10 to the 6th K and are due to nonthermal gyrosynchrotron emission or possibly thermal free-free emission. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, thermal gyrosynchrotron models can be ruled out for the radio emission because the flux at millimeter wavelengths is too high.

Kundu, M. R.↗

NASA NTRS1991

Separation of cloud and aerosol in two-wavelength satellite occultation data

The separation of cloud and aerosol extinction in satellite solar occultation data is discussed in the context of a simple model for the wavelength dependence of extinction. The model is shown graphically in terms of a scatter plot of 0.525-micron extinction versus 1.02-micron extinction. This plot contains two regions: one arising from aerosol and the other from an aerosol/cloud mixture. Examples of two-wavelength data obtained by the SAGE II instrument are presented and found to agree well with the model. The single wavelength criteria used previously to separate cloud and aerosol are examined in the context of the two-wavelength model.

Kent, G. S.↗

NASA NTRS1991

The wavelength dependence of Triton's light curve

Using Voyager observations, it is demonstrated that Triton's orbital light curve is strongly wavelength-dependent, a characteristic which readily explains some of the apparent discrepancies among pre-Voyager telescopic measurements. Specifically, a light curve amplitude (peak to peak) is found that decreases systematically with increasing wavelength from about 0.08 magnitude (peak to peak) near 200 nm to less than 0.02 magnitude near 1000 nm. Peak brightness occurs near 90 deg orbital longitude (leading hemisphere). The brightness variation across this hemisphere is close to sinusoidal; the variation across the darker hemisphere is more complex. The decrease in light curve amplitude with increasing wavelength appears to be due to a decrease in contrast among surface markings, rather than to atmospheric obscuration. The model also explains the observed decrease in the amplitude of Triton's light curve at visible wavelengths over the past decade, a decrease related to the current migration of the subsolar latitude toward the south pole; it is predicted that this trend will continue into the 1990s.

Hillier, J.↗

NASA NTRS1993

Long-wavelength PtSi infrared detectors fabricated by incorporating a p(+) doping spike grown by molecular beam epitaxy

By incorporating a 1-nm-thick p(+) doping spike at the PtSi/Si interface, we have successfully demonstrated extended cutoff wavelengths of PtSi Schottky infrared detectors in the long wavelength infrared (LWIR) regime for the first time. The extended cutoff wavelengths resulted from the combined effects of an increased electric field near the silicide/Si interface due to the p(+) doping spike and the Schottky image force. The p(+) doping spikes were grown by molecular beam epitaxy at 450 C, using elemental boron as the dopant source, with doping concentrations ranging from 5 x 10 exp 19 to 2 x 10 exp 20/cu cm. Transmission electron microscopy indicated good crystalline quality of the doping spikes. The cutoff wavelengths were shown to increase with increasing doping concentrations of the p(+) spikes. Thermionic emission dark current characteristics were observed and photoresponses in the LWIR regime were demonstrated.

Lin, T. L.↗

NASA NTRS1994

Wavelength calibration techniques and subtle surface and atmospheric absorption features in the Mariner 6, 7 IRS reflectance data

1994 marks the 25th anniversary of the Mariner 6 and 7 flyby missions to Mars. Despite its age, the Mariner 6,7 Infrared Spectrometer (IRS) data are a unique set of measurements that can provide important information about the Martian surface, atmospheric, and atmospheric aerosol composition. For certain mid-IR wavelengths, the IRS spectra are the only such spacecraft data obtained for Mars. At other wavelengths, IRS measured surface regions different from those measured by Mariner 9 or Phobos 2 and under different dust opacity conditions. We are interested in examining the IRS reflectance data in the 1.8 to 3.0 micron region because there are numerous diagnostic absorption features at these wavelengths that could be indicative of hydrated silicate minerals or of carbonate- or sulfate-bearing minerals. Groundbased telescopic data and recent Phobos ISM measurements have provided controversial and somewhat contradictory evidence for the existence of mineralogic absorption features at these wavelengths. Our goal is to determine whether any such features can be seen in the IRS data and to use their presence or absence to re-assess the quality and interpretations of previous telescopic and spacecraft measurements.

Bell, James F., III↗

NASA NTRS1994

Multi-wavelength profiles of aerosol backscatter over Lauder, New Zealand, 24 November 1992

Simultaneous profiles of aerosol backscatter ratio were measured over Lauder, New Zealand (45 deg S, 170 deg E) on the night of November 24, 1992. Instrumentation comprised two complementary lidar systems and a backscattersonde, to give measurements at wavelengths 351, 490, 532, and 940 nm. The data from the lidars and the backscattersonde were self-consistent, enabling the wavelength dependence of aerosol backscatter to be determined as a function of altitude. This wavelength-dependence is a useful parameter in radiative transfer calculations. In the stratosphere, the average wavelength exponent between 351 and 940 nm was -1.23 +/- 0.1, which was in good agreement with values derived from measured physical properties of aerosols.

Mckenzie, R. L.↗

NASA NTRS1995

A Snapshot of the Continuous Emission of the Active Galactic Nucleus in NGC 3783 from Gamma-Ray to Radio Wavelengths

To better understand the physical processes that produce the continuous emission in active galactic nuclei (AGN), a snapshot of the overall continuous energy distribution of NGC 3783, from gamma ray to radio wavelengths, has been obtained within the framework of the World Astronomy Days. The data collected in this campaign are from GRO, ROSAT, Voyager 2, IUE, HST, CTIO, SAAO, and the VLA. Great care has been taken in disentangling the genuine AGN continusous emission from other contributions; depending on the waveband, the latter might be (1) unrelated contaminating sources in cases where the instrument field of view is large (2) components within which the AGN is embedded, such as the stellar bulge population which accounts for a significant fraction of the optical continuum, and free-bound and FE2 blends wich contribute to the ultraviolet flux. After correction for these other contributins, the continuous emission of the isolated AGN appears to be rather flat (i.e., approximately equal energy per unit logarithmic frequency) from soft gamma ray to infrared wavelengths. At high energies (0.1 MeV to 0.1 keV), the AGN continuum can be fitted by a power law F nu approaches Nu(exp -a) with a spectral index of alpha approximately 1. At longer wavelengths, two excesses above this power law ('bumps') appear: in the ultraviolet, the classical big blue bump, which can be interpreted as thermal emission from the accretion disc surrounding a massive black hole, and in the infrared, a second bump which can be ascribed to thermal emission from dust in the vicinity of the AGN, heated by ultraviolet radiation from the central source. By fitting accretion-disk models to the observed AGN spectral energy distribution, we find values for the accretion disk innermost temperature, accretion rate, and black hole mass, with some differences that depend on whether or not we extrapolate the high energy power law up to infrared wavelengths. A fit to the IR bump above the extended alpha equals 1 power law suggests the presence of a dust component covering the region from a distance rho approximately equals 80 light days (hot grains at a temperature of approximately equals 1500 K) to rho approximately equals 60 light years (cool grains at T approximately equals 200 K). The total mass of dust is around 60 solar masses.

Alloin, D.↗

NASA NTRS1997

GOME wavelength calibration using solar and atmospheric spectra

Spectral information in the global ozone monitoring experiment (GOME) solar irradiance spectra and GOME earth radiance spectra are used in conjunction with the GOME solar reference spectrum to provide absolute vacuum internal wavelength calibration for GOME. Two methods for wavelength calibration of GOME data are investigated. The first employs chi-square minimization of a merit function involving wavelength and the GOME slit function. It is quite robust and requires little GOME data in the processing (calibration window regions from 15 to 40 pixels). The second employs cross correlation of GOME data and the solar reference spectrum in the Fourier transform domain, using a procedure in the image reduction and analysis facility (IRAF) software system developed for the determination of galaxy redshifts. It also requires small amounts of GOME data (calibration window regions with from 10 to 15 pixels). Both methods provide absolute wavelength calibration accurate to a small fraction of a GOME pixel across the entire GOME spectrum, and to 0.001 nm over much of the range.

Caspar, C.↗