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At least 199 records · Page 11
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NASA NTRS1987

Electromagnetic cascades in the magnetosphere of a very young pulsar - A model for the positron production near the Galactic center

A detailed model for positron production by a young pulsar is presented. It is shown that electromagnetic cascades can develop in a young pulsar's magnetosphere, and the model results are applied to the pulsar which is hypothesized to lie near the Galactic center. It is found that such a pulsar would be expected to produce relatively low energy electron-positron pairs with an efficiency rating high enough to explain the observed luminosity of the Galactic center annihilation line. Virtually all of the gamma ray continuum radiation produced in the cascades would be beamed along the magnetic poles of the neutron star, and therefore probably would not be observed from earth. Some observational predictions generated by the proposed model for the Galactic center positron source are given.

Mastichiadis, Apostolos↗
NASA NTRS1987

Hydrogenated amorphous carbon (a:C-H) in the planetary nebula NGC 7027

A spectroscopic identification of the infrared continuum radiation is proposed for the planetary nebula NGC 7027. Hydrogenated amorphous carbon (a:C-H) is shown to account for the undulating spectrum between 5 and 15 microns. The unidentified infrared emission bands lie at the peaks in the a:C-H spectrum, pointing to their association with a carbon polymorph, possibly a:C-H or polycyclic aromatic hydrocarbon molecules. Except for atomic emission lines, all the infrared emission from NGC 7027 comes from one or another polymorph of carbon.

Goebel, John H.↗
NASA NTRS1988

Polarization images of Comet Halley

Observations from two dates 5 Jan. and 7 Jan. 1986 are presented. The images were obtained with the Durham imaging polarimeter on the 1-m telescope of the Wise Observatory, Israel. The observations were made through a broadband filter centered at 0.67 microns with a 0.17 micron effective bandwidth. This wavelength region should be dominated by continuum radiation. The intensity images cover 46 x 78 arcsecs. The intensity images are shown in logorithmic form and they are analyzed.

Eaton, N.↗
NASA NTRS1988

Ray tracing planetary radio emissions

Planetary ray tracing calculations of free escaping electromagnetic waves are presented, with special attention given to calculations of the earth's auroral kilometric and continuum radiations and of the Jovian decametric and kilometric radiation. The technique is used to study the composition and propagation effects causing multiion resonances and shadow zones. Although results obtained for Jovian broadband kilometric radiation have been used to estimate the location of the source region, no unique solutions are obtained.

Green, James L.↗
NASA NTRS1989

A progress report on using bolometers cooled by adiabatic demagnetization refrigeration

For sensitive detection of astronomical continuum radiation in the 200 micron to 3 mm wavelength range, bolometers are presently the detectors of choice. In order to approach the limits imposed by photon noise in a cryogenically cooled telescope in space, bolometers must be operated at temperatures near 0.1 K. Researchers report progress in building and using bolometers that operate at these temperatures. The most sensitive bolometer had an estimated noise equivalent power (NEP) of 7 x 10(exp 017) W Hz(exp -1/2). Researchers also briefly discuss the durability of paramagnetic salts used to cool the bolometers.

Lesyna, L.↗
NASA NTRS1989

Magnetic fields in the central engines of active galactic nuclei

Important physical processes which may occur in the central engines of active galactic nuclei and which rely on the presence of a strong magnetic field are discussed. These processes include those involved in the plasma physics of hot tenuous accretion flows, the production of nonthermal continuum radiation, and the radiative manifestation of hydromagnetic jet production. The main arguments which support the hypothesis that supermassive black holes are the prime movers in the central engines are reviewed, and some major deduction regarding the physical state of the accreting gas are pointed out.

Begelman, Mitchell C.↗
NASA NTRS1990

Beam-generated upper hybrid noise in Jupiter's outer magnetosphere

A model for generation of upper hybrid waves in Jupiter's outer magnetosphere is presented. Energetic electrons accelerated on high-latitude auroral field lines at 1 R(J) altitudes are assumed to stream outwards to the distant magnetosphere. The large decrease in the local magnetic field strength results in a highly collimated, field-aligned electron beam due to conservation of the particle's magnetic moment. As the beam travels outward it passes through the plasma sheet boundary layer region located on the edges of the plasma sheet. Because of the large field-aligned anisotropy of the beam, the distribution is unstable to the excitation of the upper hybrid waves which amplify and then undergo mode conversion at the f(p) layer to generate the Jovian continuum radiation. A novel feature of the model is the beam-anisotropic heat flux instability which drives UH waves unstable without the need for any positive slope on the electron distribution function.

Barbosa, D. D.↗
NASA NTRS1990

Bragg crystal polarimeters

A Bragg crystal oriented at 45 deg to an incoming beam of X-rays acts as a polarization analyzer. This crystal geometry preferentially reflects those X-rays that satisfy the Bragg condition and whose electric vectors are perpendicular to the plane defined by the incident and reflected photons. X-rays with electric vectors parallel to this plane of incidence are photoelectrically absorbed. The energy bandwidth of nearly perfect crystals is extremely small, which makes them very inefficient X-ray polarimeters. This limitation is particularly acute for observations of the relatively weak X-ray continuum of stellar sources. The bandwidth can be greatly increased by employing mosaic or ideally imperfect crystals. Mosaic crystals possess a high integrated reflectivity, which results in a large increase in the reflection of continuum radiation. A review of the theory and performance characteristics of crystal polarimeters designed for observations of cosmic X-ray sources is presented.

Silver, E.↗
NASA NTRS1990

Low-frequency radio emissions at Neptune

The Voyager 2 plasma wave receiver detected weak radio emissions from Neptune's magnetosphere in the frequency range of 3 - 60 kHz. The emissions occurred in bursts lasting for typically 1.5 hours, often occurring twice per planetary rotation. Most of these radio bursts were detected within several degrees of the magnetic equatorial plane. During the passage through the magnetosphere, electrostatic upper hybrid resonance bands were observed close to the magnetic equator in conjunction with intensifications of the radio emissions at frequencies close to and above the upper hybrid bands. Further, near closest approach, the radio emissions were observed to cross the right-hand cutoff frequency with no apparent attenuation. It is concluded that the Neptunian radio emissions below about 60 kHz are produced by mode conversion from the upper hybrid waves and propagate in the ordinary mode into beams within about 12 deg of the magnetic equator. There is also evidence of an extraordinary mode emission at about 60 kHz which is apparently generated by an entirely different source from the escaping continuum radiation.

Kurth, W. S.↗
NASA NTRS1991

High resolution measurements of density structures in the Jovian plasma sheet

A recent effort to digitize the plasma density by using the low frequency cutoff of trapped continuum radiation in the vicinity of the Jovian plasma sheet has revealed the existence of sharply defined density structures in the plasma sheet. These structures typically have a plasma density which is relatively constant but of order 50 percent greater or less than in the surrounding plasma. At the boundaries of these structures, the transitions from low to high density occur on time scales of about ten seconds, which correspond to spatial dimensions on the order of a few ion Larmor radii. The structures themselves last for intervals from less than a minute to more than five minutes, corresponding to size scales from a fraction of a Jovian radius to more than a Jovian radius, depending of the velocity of the structure relative to the spacecraft. In view of the importance of near corotation plasma flows, these structures are likely to be limited in both the longitudinal and radial dimensions and, therefore, could represent flux tubes with greatly varying plasma content. These observations are presented as among the first to directly address the theoretically proposed interchange instability.

Ansher, J. A.↗
NASA NTRS1990

Images in the rocket ultraviolet - Young clusters in H II regions of M83

UV images of M83 at 1540 and 2360 A reveal 18 compact sources that are associated with H II regions. E(B - V) values were estimated individually from the observed UV and optical colors and the Galactic UV extinction curve, using theoretical flux distributions. The dereddened colors are consistent with ages up to 3 x 10 to the 6th yr. A maximum possible age of 6.5 x 10 to the 6th yr is obtained assuming foreground reddening only. The distribution of observed colors is consistent with the Galactic reddening curve but not with enhanced far-UV extinction, as in the LMC 30 Dor curve. The H-alpha fluxes suggest either that dust within the H II regions absorbs up to 70 percent of the Lyman continuum radiation or that a similar fraction of the H-alpha flux is below the surface brightness detection limit. Cluster mass estimates depend on the range of stellar masses present but are probably in the range 10,000-100,000 solar masses.

Bohlin, Ralph C.↗
NASA NTRS1990

Flare heating and ionization of the low solar chromosphere. II - Observations of five solar flares

Two neutral Mg spectral lines formed in the temperature-minimum region and the low chromosphere, at 4571 and 5173 A, are used to quantify the changes in the atmospheric structure as a function of time during five solar flares. Eight proposed flare heating and ionization mechanisms and predictions of the effects of each on the temperature minimum region are discussed. Two Mg spectral observations made at the National Solar Observatory (Sacramento Peak), along with observations of hard and soft X-rays from the SMM and GOES satellites, are compared to the predictions of the eight proposed mechanisms. The initial effects in all five flares are consistent with backwarming by enhanced Balmer- and Paschen-continuum radiation originating in the upper chromosphere. Extended heating observed in two of the flares is most likely due to UV irradiation. In all cases heating by the dissipation of nonreversed electric currents, collisions with an electron or proton beam, irradiation by soft X-rays, and dissipation of Alfven waves are eliminated.

Metcalf, Thomas R.↗
NASA NTRS1991

Infrared emission from isolated dust clouds in the presence of very small dust grains

Models of the effects of small grain-generated temperature fluctuations on the IR spectrum and surface brightness of externally heated interstellar dust clouds are presently constructed on the basis of a continuum radiation transport computer code which encompasses the transient heating of small dust grains. The models assume a constant fractional abundance of large and small grains throughout the given cloud. A comparison of model results with IRAS observations indicates that the observed 12-25 micron band emissions are associated with about 10-A radius grains, while the 60-100 micron emission is primarily due to large grains which are heated under the equilibrium conditions.

Lis, Dariusz C.↗
NASA NTRS1991

Molecular hydrogen polarization images of OMC-1

An image of the polarization of the shocked H2 v = 1-0 S(1) line emission in the core of OMC-1 has been obtained. Along the molecular outflow of the source, the line is dichroically polarized by a medium of aligned grains located between the earth and the shock fronts. The polarization pattern traces the magnetic field direction, which is parallel to the outflow axis and to the large-scale field direction determined from far-IR continuum measurements. Close to the IR source IRc2, the likely source of the outflow, the aligned vectors twist, indicating that the magnetic field direction changes. Modeling the line ratios of scattered H2 lines in the reflection nebula, it is concluded that the size distribution of grains there is typical of the small grains in the diffuse interstellar medium. By contrast, the scattered continuum radiation from the core region suggests that the grains there are larger than this.

Burton, Michael G.↗
NASA NTRS1991

Gamma-ray spectroscopy: The diffuse galactic glow

The goal of this project is the development of a numerical code that provides statistical models of the sky distribution of gamma-ray lines due to the production of radioactive isotopes by ongoing Galactic nucleosynthesis. We are particularly interested in quasi-steady emission from novae, supernovae, and stellar winds, but continuum radiation and transient sources must also be considered. We have made significant progress during the first half period of this project and expect the timely completion of a code that can be applied to Oriented Scintillation Spectrometer Experiment (OSSE) Galactic plane survey data.

Hartmann, Dieter H.↗
NASA NTRS1991

A mysterious plasma wave emission and the determination of plasma densities in Neptune's inner magnetosphere

One of the strongest plasma wave signals observed during the Voyager 2 encounter with Neptune is a narrowband emission between 3.0 and 4.3 kHz that was detected over a period of roughly 2 hours around closest approach. The emission occurs below the electron cyclotron frequency and the low-frequency cutoff of the radio continuum radiation. Of the naturally occurring signals in the earth's auroral zone and in Jupiter's magnetosphere this emission most resembles trapped Z mode waves found near the left-hand cutoff frequency. Using this identification, a plasma density profile is obtained that is independent of the plasma temperature. These densities greatly exceed those measured by the plasma science instrument on Voyager but are lower than estimates based on other models of Neptunian plasma wave phenomenology. If this wave mode is not a natural emission, it might arise from an unusual interaction of the spacecraft with the cold, dense ambient plasma.

Moses, S. L.↗
NASA NTRS1991

The heating of the solar chromosphere

The solar atmosphere associated with magnetic fields is brighter in characteristic field emitters than the magnetic field-free portion of the atmosphere. The chromosphere can accordingly be identified with the atmosphere within such magnetic elements as flux tubes. Attention is presently given to bright points associated with the intranetwork magnetic fields, which are heated by large-amplitude compressive waves whose periods approximate the 3-min acoustic cutoff period. Relevant line and continuum radiation observations are studied; it is concluded that energy dissipated by the 3-min waves may suffice to heat the low and middle chromosphere, in the bright points, to the temperatures observed.

Kalkofen, Wolfgang↗
NASA NTRS1992

A liquid xenon imaging telescope for gamma ray astrophysics: Design and expected performance

A high resolution telescope for imaging cosmic x ray sources in the MeV region, with an angular resolution better than 0.5 deg is being developed as balloon-borne payload. The instrument consists of a 3-D liquid xenon TPC as x ray detector, coupled with a coded aperture at a distance of 1 meter. A study of the actual source distribution of the 1.809 MeV line from the decay of Al-26 and the 511 keV positron-electron annihilation line is among the scientific objectives, along with a search for new x ray sources. The telescope design parameters and expected minimum flux sensitivity to line and continuum radiation are presented. The unique capablity of the LXe-TPC as a Compton Polarimeter is also discussed.

Aprile, E.↗