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At least 343 records · Page 19

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NASA NTRS1986

Further evidence for lightning on Venus

Impulsive electrical signals at all frequencies recorded by the plasma wave instrument on Pioneer Venus are found to cluster around periapsis. The signals are strongest at 100 Hz and have a secondary peak at 5.4 kHz. These characteristics are consistent with generation by lightning in the Venus atmosphere and the direct propagation of whistler mode signals below the electron gyro frequency and the partial transmission of electromagnetic waves in the presence of electron density inhomogeneities above the electron gyro frequency. The scattering by electron density inhomogeneities may also weaken any correlation with surface topographical features. Thus, the source of these signals may either be intercloud lightning at high altitudes or lightning in volcanic plumes near the ground, despite the large scatter in inferred source locations.

Singh, R. N.↗

NASA NTRS1987

Numerical modeling of proton and electron bursts in a realistic magnetotail

Particle trajectories of the magnetotail ion and electron bursts were simulated using the Beard-Hirschi-Propp (1982) magnetic field model and data generated by IMP 8. Particle trajectories were computed both in the absence of the electric field, and with electric field introduced locally perpendicular to the magnetic field. The results indicate that the motions of energetic particles near the nominal neutral sheet can be divided into two types. Proton and electron parallel trajectories have most of their trajectory motion parallel to the magnetic field lines and travel long distances along field lines. Perpendicular particle trajectories travel much shorter distances with much of the trajectory motion perpendicular to the magnetic field and with proton trajectories interacting with the neutral sheet region. Only the premidnight trajectories connect to reasonable source locations, which explains the preferential occurrence of earthward energetic proton burst anisotropies in the premidnight sector.

Kutchko, Frank J.↗

NASA NTRS1987

The plasma wave environment of an auroral arc. III - VLF hiss

Auroral hiss in the vicinity of a quiet arc is studied by comparing the data obtained from a sounding rocket with predictions of the convective beam amplification (CBA) model of auroral hiss generation. It is concluded that the VLF auroral hiss observed by the payload appears to have been natural in origin. The CBA model produces predicted hiss spectra that are in good agreement with the data. The hiss intensity above the ionosphere was greater equatorward of the arc than within the arc; this pattern appears to be the result of emission geometry and refraction. Hiss intensity was not found to be better correlated with 0.1-0.7 keV fluxes than with more energetic electrons. No evidence is found for a three-wave parametric decay process. The data are consistent with a weak hiss source located at an altitude of 110-120 km.

Bering, E. A.↗

NASA NTRS1987

Exact image method for Gaussian beam problems involving a planar interface

Exact image method, recently introduced for the solution of electromagnetic field problems involving sources above a planar interface or two homogeneous media, is shown to be valid also for sources located in complex space, which makes its application possible for Gaussian beam analysis. It is demonstrated that the Goos-Hanchen shift and the angular shift of a TE polarized beam are correctly given as asymptotic results by the exact reflection image theory. Also, the apparent image location giving the correct Gaussian beam transmitted through the interface is obtained as another asymptotic check. The present theory makes it possible to calculate the exact coupling from the Gaussian beam to the reflected and refracted beams, as well as to the surface wave.

Lindell, I. V.↗

NASA NTRS1987

Localization, time histories, and energy spectra of a new type of recurrent high-energy transient source

The detection of a recurrent high-energy transient source which is neither a classical X-ray nor a gamma-ray burster, but whose properties are intermediate between the two, is reported. The energy spectra of 12 recurrent events are found to be soft, characterized by kT's of 34-56 keV. The time histories are short with rise and fall times as fast as about 10 ms. The source location is a 0.12 sq deg region about 10 deg from the Galactic center.

Atteia, J.-L.↗

NASA NTRS1987

The low-luminosity X-ray sources in Omega Centauri

UBV observations of three X-ray sources in Omega Cen (sources A, D, and E of Hertz and Grindlay, 1983), obtained with a CCD detector on the 4-m telescope at CTIO in February and August 1985, are reported. The results are presented in images, graphs, and tables and characterized in detail. No optical cataclysmic variable candidates are found down to V = 24.5 mag, corresponding to M(V) = 10.6 in the cluster. It is inferred that these low-luminosity sources, located outside the globular cluster, may be superposed unrelated objects rather than close mass-exchange binaries.

Margon, Bruce↗

NASA NTRS1987

SMM hard X-ray observations of the soft gamma-ray repeater 1806-20

Six bursts from the soft gamma-ray repeater (SGR) 1806-20 have been recorded with the SMM Hard X-ray Burst Spectrometer during a highly active phase in 1983. Rise and decay times of less than 5 ns have been detected. Time profiles of these events indicate low-level emission prior to and after the main peaks. The results suggest that SGRs are distinguished from classical gamma-ray bursts by repetition, softer nonvarying spectra, short durations, simple temporal profiles, and a tendency for source locations to correlate with Population I objects. SGR characteristics differ from those of type I X-ray bursts, but they appear to have similarities with the type II bursts from the Rapid Burster.

Kouveliotou, C.↗

NASA NTRS1987

Exact-image method for Gaussian-beam problems involving a planar interface

The exact-image method, recently introduced for the solution of electromagnetic field problems involving sources above a planar interface between two homogeneous media, is shown to be valid also for sources located in complex space, which makes its application possible for Gaussian-beam analysis. It is demonstrated that the Goos-Hanchen shift and the angular shift of a TE-polarized beam are correctly given as asymptotic results by the exact-reflection-image theory. Also, the apparent-image location giving the correct Gaussian beam transmitted through the interface is obtained as another asymptotic check. The theory described here makes it possible to calculate the exact coupling from the Gaussian beam to the reflected and refracted beams as well as to the surface wave.

Lindell, Ismo V.↗

NASA NTRS1988

Uranus radio emissions

An overview of the Voyager 2 Planetary Radio Astronomy instrument observations of the planet Uranus is presented. From these observations, a number of inferences have been made including the rotation period of the interior of the planet and the source locations of several of the radio components.

Kaiser, Michael L.↗

NASA NTRS1989

Synthesis mapping of a solar type I storm simultaneously at 90 and 20 centimeters with the VLA

A type I radio storm was observed simultaneously at 20 and 90 cm in November 1986. The characteristic signatures of the storm are detected at 90 cm only. The results indicate that the noise storm is unambiguously associated with an active region at the sun center, and that the distribution of energetic electrons responsible for the storm emission reaches low coronal heights. Polarization measurements and storm source location results suggest that the 90-cm emission is fundamental plasma radiation and that the 20-cm measurements are characteristic of free-free emission.

Habbal, Shadia Rifai↗

NASA NTRS1989

Analytical optical scattering in clouds

An analytical optical model for scattering of light due to lightning by clouds of different geometry is being developed. The self-consistent approach and the equivalent medium concept of Twersky was used to treat the case corresponding to outside illumination. Thus, the resulting multiple scattering problem is transformed with the knowledge of the bulk parameters, into scattering by a single obstacle in isolation. Based on the size parameter of a typical water droplet as compared to the incident wave length, the problem for the single scatterer equivalent to the distribution of cloud particles can be solved either by Mie or Rayleigh scattering theory. The super computing code of Wiscombe can be used immediately to produce results that can be compared to the Monte Carlo computer simulation for outside incidence. A fairly reasonable inverse approach using the solution of the outside illumination case was proposed to model analytically the situation for point sources located inside the thick optical cloud. Its mathematical details are still being investigated. When finished, it will provide scientists an enhanced capability to study more realistic clouds. For testing purposes, the direct approach to the inside illumination of clouds by lightning is under consideration. Presently, an analytical solution for the cubic cloud will soon be obtained. For cylindrical or spherical clouds, preliminary results are needed for scattering by bounded obstacles above or below a penetrable surface interface.

Phanord, Dieudonne D.↗

NASA NTRS1989

The global geochemistry of bomb-produced tritium - General circulation model compared to available observations and traditional interpretations

Observational evidence suggests that of the tritium produced during nuclear bomb tests that has already reached the ocean, more than twice as much arrived through vapor impact as through precipitation. In the present study, the Goddard Institute for Space Studies 8 x 10 deg atmospheric general circulation model is used to simulate tritium transport from the upper atmosphere to the ocean. The simulation indicates that tritium delivery to the ocean via vapor impact is about equal to that via precipitation. The model result is relatively insensitive to several imposed changes in tritium source location, in model parameterizations, and in model resolution. Possible reasons for the discrepancy are explored.

Koster, Randal D.↗

NASA NTRS1990

New arcs associated with the smooth high-frequency radiation from Uranus

New vertex-early kilometric arcs have been found to be associated with the dominant smooth high-frequency radio emission from Uranus. These arc features had vertices at about 300 kHz and bandwidths of about 200 kHz. Based on the fact that they exhibited a different temporal occurrence, spectral character, source location, and beaming pattern compared to the more dominant smooth component, it is concluded that they are a separate and independent radio component, different from those previously identified for this planet.

Farrell, W. M.↗

NASA NTRS1990

Offset, tilted dipole models of Uranian smooth high-frequency radio emission

The smooth high-frequency (SHF) component of the radio emission detected during the Voyager 2 encounter with Uranus (January 1986) is studied. An offset tilted dipole (OTD) investigation of the SHF emission at L shells is carried out within the range of the bursty source locations. A viable high L shell model is presented. It is suggested that Miranda, which reaches a minimum L shell at L = 5, may be related to the timing of several types of radio emissions.

Schweitzer, Andrea E.↗

NASA NTRS1990

Observations of the earth's continuum radiation in the distant magnetotail with ISEE 3

Observations of the earth's nonthermal continuum radiation made with the radio receiver on ISEE 3 are analyzed, emphasizing rapid crossings of the magnetopause in the distant magnetotail. The intensity and anisotropy of the radiation field are always found to be larger in the lobes than in the magnetosheath. The radiation is interpreted as arising from a source located at or near the plasmapause. It is suggested that the observed variations across the magnetopause of the intensity and anisotropy can be understood by taking into account the facts that the magnetotail is a plasma waveguide whose walls are sometimes rough and leaky and that there are density fluctuations in the magnetosheath. The efficiency with which the continuum power at 30 kHz is collected in the near-earth regions and ducted toward the distant tail is a strong, increasing function of the magnetosheath plasma frequency (MPF). The percentage of that power which travels in the magnetosheath decreases with increasing MPF.

Steinberg, J.-L.↗

NASA NTRS1990

Cleft contribution to ring current formation

The storm time transport of ionospheric plasma from the 'cleft fountain' to the plasma sheet and ring current is investigated by means of three-dimensional trajectory codes. Using observations to define the source location and flow rate, test particles are traced during a 'taillike' to 'dipolelike' reconfiguration of the geomagnetic field. Emphasis is placed on the behavior of heavy ions of low charge state, O(+). As a result of their long periods of gyration, these ions are highly sensitive to rapid field variations and possibly display transient nonadiabatic motions. It is demonstrated that O(+) which have originated in the high-latitude ionosphere but which find themselves in the vicinity of the equator at substorm onset can experience considerable energization (from several keV up to several hundred keV) and pitch angle increase leading to trapping, as an effect of the induced electric field.

Delcourt, D. C.↗

NASA NTRS1989

A test of magnetic field draping induced Bz perturbations ahead of fast coronal mass ejecta

ICE plasma and magnetic field data are examined to look for observational evidence of IMF draping ahead of fast coronal mass ejections (CMEs). The utility of the draping model for predicting the Bz perturbations and hence geomagnetic activity associated with the sheath regions ahead of such CMEs is also examined. A simple prediction scheme based on the upstream radial field component is developed and a set of interplanetary shock events previously associated with interplanetary type II bursts, and hence solar source locations, is used. Of 17 events the radial component predictor developed here correctly predicts the direction considered of the Bz perturbations for 13 events (76 percent). While this result is certainly not conclusive, it is considered to be supportive of the draping scenario.

Mccomas, D. J.↗

NASA NTRS1991

A model for the disc Lyman alpha emission of Uranus

A new efficient radiative transfer algorithm for nonhomogeneous model atmospheres has been applied to the Uranian atmosphere. The contribution of the scatter solar Lyman-alpha to the Uranain emission is of the order of 300 R, and the Rayleigh contribution may reach 450 R for small values of the eddy diffusion coefficient (EDC). The total solar contribution may then reach about 750 R for a solar flux of 2.5 x 10 to the 11th photons/sq cm/s/A. A level of up to 400 R is confirmed in some directions for the interstellar wind contribution. The values of the atmospheric EDC necessary to mimic the observations are 50-100 sq cm/s. A small additional source located on the dayside Uranian atmosphere seems necessary correctly to fit the shape of the limb to limb intensity variation, especially near the limbs. Its contribution to the emergent intensity would range from 100 to 500 R.

Ben Jaffel, L.↗