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NASA NTRS1982

Photometry and polarimetry of Titan - Pioneer 11 observations and their implications for aerosol properties

Pioneer 11 measurements of Titan's limb darkening and polarization at red and blue wavelengths are refined, and the integrated disk brightness measurements at phase angles from 22 to 96 deg, over which Titan's phase coefficient averages about 0.014 magnitudes/deg in both colors, are reduced. Comparisons of the data with vertically homogeneous multiple scattering models indicate that the single-scattering phase functions of the aerosols in both colors are flat at scattering angles between 80 and 150 deg, with a small peak at larger scattering angles. Although a vertically homogeneous model in which particles are assumed to scatter as spheres cannot simultaneously match the polarization observations of both colors for any refractive index, observed polarizations are most sensitive to the particle properties near optical depth 1/2 in each color. Models based on single scattering by spheres can therefore be successful over a range of refractive indices, if the size of the particles increases with depth and if the cross section of the particles increases sufficiently rapidly with decreasing wavelength.

Tomasko, M. G.↗

NASA NTRS1982

Measurements of stratospheric aerosols over Mauna Loa, Hawaii and Boulder, Colorado

The direct solar radiation transmission record at Mauna Loa, dating from 1958 to the present, revealed with remarkable precision the presence of stratospheric aerosol from volcanic activity. This record can be used to quantify the intensity of the stratospheric volcanic aerosol perturbation following a significant eruption in reference to the Agung event in 1963. The Mount St. Helens' stratospheric cloud was first detected by lidar at 18 km over Mauna Loa on 17 July. The atmospheric transmission was seen to decrease slightly after that time, but only a few tenths of 1 percent. Although it is still fairly early to draw a definite conclusion on the ultimate magnitude of the Mount St. Helens stratospheric aerosol from the Mauna Loa results, it can be stated that the stratospheric aerosol optical depth presently observed is comparable with that observed from Fuego which erupted in 1974. At Boulder, Colorado, the atmospheric debris from Mount St. Helens was observed by lidar on a number of occasions. Also, observations of the diffuse, total and direct transmission of solar radiation were made on June 3 and 4. The latter set of observations is useful for deriving information on the scattering properties of the volcanic cloud. The lidar and solar radiation data are presented and some of their special features are discussed.

Deluisi, J. J.↗

NASA NTRS1982

The climatic effect of explosive volcanic activity: Analysis of the historical data

By using the most complete available records of direct beam radiation and volcanic eruptions, an historical analysis of the role of the latter in modulating the former was made. A very simple fallout and dispersion model was applied to the historical chronology of explosive eruptions. The resulting time series explains about 77 percent of the radiation variance, as well as suggests that tropical and subpolar eruptions are more important than mid-latitude eruptions in their impact on the stratospheric aerosol optical depth. The simpler climatic models indicate that past hemispheric temperature can be stimulated very well with volcanic and CO2 inputs and suggest that climate forecasting will also require volcano forecasting. There is some evidence that this is possible some years in advance.

Bryson, R. A.↗

NASA NTRS1983

First detection of the ground state JK = 1 sub 0 going to 0 sub 0 submillimeter transition of interstellar ammonia

The JK = 1 sub 0 approaching O sub 0 transition of ammonia at 572.5 GHz was detected in OMC-1 from NASA's Kuiper Airborne Observatory. The central velocity of the line (VLSR approximately = 9 km/s) indicates that it originates in the molecular cloud material, not the hot core. The derived filling factor of approximately 0.09 in a 2' beam implies a source diameter of approximately 35" if it is a single clump. This clump area is much larger than that derived from observations of the sub 1 inversion transition. The larger optical depth in the 1 sub 0 approaching 0 sub 0 transition (75-350) can account for the increased source area and linewidth as compared with those seen in the 1 sub 0 inversion transition.

Keene, J.↗

NASA NTRS1982

The dynamics of planetary rings

The physical processes that occur in planetary rings are discussed. The theoretical arguments leading to the conclusion that Saturn's rings are solid particles in nonuniform rotation are summarized, and the optical depth, thickness, and particle size of the rings are discussed. The influence of nearby satellites on the rings is analyzed, and asymmetries in the rings are briefly discussed. What is known of the rings of Uranus and of Jupiter's ring is summarized. Some of the dynamical processes and influences that are expected to be incorporated in more advanced theories of planetary rings are reviewed in detail, including radiation drag, plasma drag, interparticle collisions and viscosity, resonances with external satellites, shepherd satellites and moonlets. Finally, the orbital evolution of the shepherd satellites caused by the rings is estimated.

Goldreich, P.↗

NASA NTRS1982

The nucleus of M81 - Simultaneous 2.3 and 8.3 GHz Mark III VLBI observations

Mark III very long baseline interferometry (VLBI) observations have been made of the nucleus in the normal galaxy M81 (NGC 3031) simultaneously at 2.3 and 8.3 GHz, and it was found that nearly 100% of the flux density of the nuclear region originates in an elongated radio core with linear dimensions 1000-4000 AU, dependent on frequency. This galactic nucleus is the most compact observed in any extragalactic source. The position coincides with that of the nucleus at optical and X-ray wavelengths within the larger uncertainties of the latter two. The position angle of the major axis of the M81 nucleus is within 3 deg of 75 deg at 2.3 GHz and within 6 deg of 50 deg at 8.3 GHz and is frequency dependent at the 4 sigma level. These values straddle the 62 deg position angle of the projection on the sky of the rotation axis of the galaxy. The spectrum of the core is slightly inverted, and the length of its major axis decreases with frequency. These results are consistent with the emission's being incoherent electron synchrotron radiation from an inhomogeneous region with an optical depth varying along the rotation axis of the galaxy.

Bartel, N.↗

NASA NTRS1982

Marine sediment tolerances for remote sensing of atmospheric aerosols over water

In surveying the literature, it is pointed out that there is a need to quantify the turbidity below which reflectance from the water column is negligible in comparison with atmospheric effects to allow the monitoring of aerosol optical depth over water bodies. Data that partially satisfy this need are presented. Laboratory measurements of reflectance upwelled from the water column are given for mixtures with various types of sediment at wavelengths between 400 and 1600 nm. The results of the study described here are a quantitative endorsement of the recommendations of Morell and Gordon (1980).

Whitlock, C. H.↗

NASA NTRS1982

The photochemistry and dynamics of a dusty cometary atmosphere

The solving of a simultaneous set of differential equations representing conservation of number density, momentum, and energy together with solar radiation transfer in the streams which result in photolytic processes and the heating of the nucleus yields a self-consistent solution of the dynamical and thermal structure of an H2O-dominated two-phase dusty gas cometary atmosphere. Two models are considered for the transfer of solar radiation through the circumnuclear dust halo. The first considers only the direct extinction by the dust, and in the second the diffuse radiation field due to multiple scattering by the dust halo, which compensates for radiation removed by direct absorption when the optical depth is near unity, is approximated by neglecting the attenuation of radiation given off by the dust. It is shown that while dust attenuation has a strong effect on the H2O production rate, it also increases the electron density in the inner coma over the unattenuated case.

Marconi, M. L.↗

NASA NTRS1982

Maps of the rings of Uranus at a wavelength of 2.2 microns

Maps of the ring system of Uranus, as seen in reflected sunlight at a wavelength of 2.2 microns, are presented for May 1978 and July 1979. Large azimuthal brightness variations revealed by these maps are consistent with the variable width established for the epsilon ring from occultation studies and support the existing elliptical model for this ring. An upper limit of approximately 0.010 is placed on the normal optical depth of any broad (approximately 5000 km wide) axisymmetric ring component. There exists, however, an unexplained east-west asymmetry in the May 1978 map. In addition, multiaperture broadband infrared photometry of Uranus is reported, from which the geometric albedo of the planet at wavelengths of 1.25, 1.65, and 2.2 microns and the geometric albedo of the rings at 2.2 microns are derived.

Matthews, K.↗

NASA NTRS1981

Conjectures regarding the structure of a sunspot penumbra

A study of simple radiative transfer models for some of the dark filaments on the outer edge of penumbras, which observations suggest may lie several hundred km above the base of the quiet photosphere, has determined that elevated dark filaments probably have higher temperatures and densities than the surrounding atmosphere. The possibility of a connection between the dark filaments, the photospheric Evershed flow, and umbral dots, is discussed. An important observational test of the present model would involve an attempt to separate temperature and optical depth effects in the dark filaments. This could be accomplished by either continuum photometry at a few, widely separated wavelengths, or by spectroscopic studies.

Cram, L. E.↗

NASA NTRS1982

On the interpretation of gamma-ray burst continua and possible cyclotron absorption lines

It is pointed out that most gamma-ray bursts have spectra that are fit by an optically thin thermal bremsstrahlung shape. The small emitting volume implied by the observation of spectral features identified as cyclotron absorption lines from a 10 to the 12th Gauss field is shown to prevent thermal bremsstrahlung from providing sufficient luminosities to account for the observed fluxes unless the optical depth to Compton scattering is large. It is also demonstrated that complicated detector response functions can introduce artifacts that mimic absorption features below 60 keV unless the absolute gain of the detector is established and correctly applied in analysis. Comparing the continuum as observed by the Berkeley/Los Alamos ISEE-3 gamma-ray detector with the continuum observed by the Konus (Mazets et al. 1981) experiments, a lack of agreement is found, suggesting that the gain of one of these experiments may be incorrect.

Fenimore, E. E.↗

NASA NTRS1982

The global distribution of water vapor in the middle atmosphere of Venus

Near-IR measurements are presented of the mean vertical and horizontal distribution of water vapor in the Venus clouds as measured by the Pioneer Venus Orbiter IR radiometer, and comparisons are made with previous data. Six thermal channels were used to generate several hundred thousand readings for determination of the mean mixing ratio. Averaging was performed as a function of the solar zenith angle, with profiles retrieved with a relaxation method applied to radiance data at 45 microns. Consideration was given to mean cloud models and temperature profiles obtained from the five temperature sounding channels scanning from 11.5-15 microns. Laboratory tests were effected to validate the transmission functions. The results included a maximum column abundances above the cloud optical depth in the early afternoon in the equatorial regions. Mixing ratio enhancement was highest on the dayside and at high altitudes, with a mean ratio of 0.0001 at a 40% uncertainty level. Day-to-day fluctuations in the pressure level at 11.5 microns was larger than 10%, far below the factors of 2-3 determined by other investigators.

Schofield, J. T.↗

NASA NTRS1983

Spectrophotometry of the broad absorption-line QSO PHL 5200

Spectrophotometric observations of PHL 5200 conducted from 1974 to 1979 are presented. The semiforbidden C III 1909 and Mg II 2799 emission lines give an emission redshift of 1.981 + or - 0.002 using the peak of the profile. The semiforbidden C III 1909/C IV 1549 emission line ratio, after correction for absorption, is more than the average for normal quasars. In addition, the spectrum shows more structure between C IV 1549 emission and semiforbidden C III 1909 emission than in the case of normal quasars. The absorption profiles are investigated by using symmetrical logarithmic profiles to model the emission lines. The residual intensities at different wavelengths in the absorption troughs are employed to map optical depth as a function of velocity, assuming that the absorption region covers a small part of the sky as seen from the central source. Spectra from five observing seasons are compared, and no apparent changes in the C IV and Si IV absorption troughs are found.

Junkkarinen, V. T.↗

NASA NTRS1983

A non-spherically symmetric model for absorption regions near quasars

An approximate analytic solution of the radiative-transfer equation is used to estimate upper limits on the fraction of sky covered (Omega/4pi) by absorption regions near broad absorption-line quasars. The resonance scattering of line radiation tends to 'fill in' absorption troughs, and regions with large Omega/4pi produce troughs that are severely 'filled in'. PHL 5200 and several other broad absorption line quasars have absorption troughs that are flat-bottomed and deep. It is noted that these quasars probably have small Omega/4pi. Simple solutions are obtained for the case of large optical depth and a point central source using a two-stream model for the radiation field. It is shown that the scattered radiation can produce flat-bottomed absorption troughs if the logarithmic velocity derivative in the outflow is constant. For a large class of models, Omega/4pi is approximately less than the residual intensity (the fraction left after absorption) in a deep flat-bottomed absorption trough.

Junkkarinen, V. T.↗

NASA NTRS1983

On the presence of O I lambda 1302 emission in Be stars

The absence of UV O I emission from Be stars is investigated in terms of a model for a circumstellar disk. Observations of the O I 8446 A transition are shown to produce the UV O I triplet 1302.17, 1304.86, and 1306.02 A, which has not been seen. It is demonstrated that the total scattering optical depth in the circumstellar gas is sufficient to ensure that most photons arising from the O I 8446 A transition will be destroyed in an envelope with a density of 10 to the 12th electrons/cu cm, such as has been projected for Be stars. It is also mentioned that a flattened Be star envelope will, if viewed equator-on, will scatter more 1302 A photons from the line-of-sight, implying that Be stars observed pole-on would exhibit an enhanced 1302 A emission because the photons are scattered perpendicularly from the flattened, equatorial disk.

Oegerle, W. R.↗

NASA NTRS1983

Spatially resolved methane band photometry of Saturn. II - Cloud structure models at four latitudes

Saturn's cloud vertical structures in the Equatorial Zone, South Equatorial Belt, and North and South Temperate Regions near + or - 30 deg latitudes are determined by means of an analysis of spatially resolved reflectivity measurements in the 6190, 7250, and 8996 A methane bands. Radiative transfer models are computed for a structure whose parameters are the methane column abundance in an aerosol-free layer at the top of the atmosphere, and the specific abundance of methane in a semiinfinite homogeneous gas-and-cloud mixture deep in the atmosphere. The structure for the South Equatorial Belt resembles that for the North Temperate Region. The level where unit cloud optical depth occurs in the South Temperate Region is deeper than the corresponding level at other latitudes. The differences between model parameters derived by means of different absorption bands are discussed.

West, R. A.↗

NASA NTRS1982

Space observations of aerosols and ozone; Proceedings of the Topical Meeting, Ottawa, Canada, May 16-June 2, 1982

The measurement of aerosols from space is discussed, taking into account the role of aerosols in climate, instrumentation and further measurement systems, retrieval procedures, measurements and observations, ground truth measurements, and effects on remote sensing and on climate. Aspects of ozone variability in the middle atmosphere are explored, giving attention to the quasi-biennial oscillation in equatorial stratospheric temperatures and total ozone, global pictures on the ozone field from high altitudes from DE-1, measurements of atmospheric ozone from aircraft and from balloons, a mesospheric ozone profile at sunset, periodic and aperiodic ozone variations in the middle and upper stratosphere, solar eclipse induced variations in mesospheric ozone concentrations, and solar UV and ozone balloon measurements. The determination of aerosol optical depth is considered along with a method for estimating cross radiance.

Mccormick, M. P.↗

NASA NTRS1983

The Delta band dissociation of nitric oxide - A potential mechanism for coupling thermospheric variations to the mesosphere and stratosphere

Depletion of solar radiation at discrete wavelengths by nitric oxide has a significant impact on the dissociation rate of this gas in the delta(0, 0) and delta(1, 0) bands. Inclusion of the opacity provided by a typical NO profile reduces the dissociation rate in the upper stratosphere to 50-75 pct of that predicted when the optical depth calculation omits this contribution. A substantial fraction of the NO column abundance as measured near the stratopause resides in the lower thermosphere where correlations of NO with solar and magnetic activity are well documented. Variations in the thermospheric NO abundance therefore modulate the radiation field at the precise wavelengths absorbed by this molecule in the mesosphere and upper stratosphere. Predicted changes in the dissociation rate arising from a varying thermospheric opacity exceed 8 pct throughout the mesosphere and reach 15 pct between 65 km and 95 km.

Frederick, J. E.↗