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Sandel, B. R.

Publications and source records attributed to Sandel, B. R..

At least 73 records · Page 4
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NASA NTRS1981

Extreme ultraviolet observations from Voyager 1 encounter with Saturn

Hydrogen Lyman-alpha, helium and molecular hydrogen band ultraviolet emissions from Saturn and the Titan atmosphere are considered. The Saturn H2 band excitation mechanism, while qualitatively similar to that of Jupiter, is closely related to the solar flux. The occurrence of auroras at 80 deg latitude suggests an earth-like magnetotail activity. No ion emissions have been detected from the Saturn magnetosphere. Although nitrogen emissions excited by particles have been detected in the Titan dayglow and bright limb scans, no such emission has been detected from its dark atmosphere. Enhancement of the nitrogen emission is observed in the region of interaction between Saturn's corotating plasmasphere and the Titan atmosphere.

Broadfoot, A. L.↗
NASA NTRS1981

Voyager U.V. spectrometer observations of He 584 A dayglow at Jupiter

The intensity of Jupiter's He 584 A airglow has been measured by the Voyager UV spectrometers. The disk-averaged brightness is about 4 Rs and limb darkening is present. The intensity probably varies with longitude, the variation being out of phase with the H Lyman-alpha intensity bulge. Modelling of resonance scattering of the solar He 584 A line by Jupiter's atmosphere has shown that the hydrogen and helium emissions can be explained about equally well by at least two self-consistent scenarios involving the structure (temperature and eddy diffusion coefficient) and excitation of the atmosphere. All evidence points to a dramatic change of conditions in the Jovian atmosphere in the time between Pioneer and Voyager encounters.

Mcconnell, J. C.↗
NASA NTRS1981

The Jovian hydrogen bulge - Evidence for co-rotating magnetospheric convection

The Jovian hydrogen bulge is located 180 deg away in the System II longitude from the active sector identified as the source region for Jupiter's decametric radio emission and release of energetic electrons into interplanetary space. The sector results from the large magnetic anomaly in the Jovian northern hemisphere; it is expected that a two-cell magnetospheric convection pattern is found in the Jovian atmosphere. The magnetic anomaly of the active sector produces a convection which brings the magnetospheric plasma to the upper atmosphere at the longitudes below the hydrogen bulge; the hot plasma contains electrons with energies of about 100 keV which dissociate atmospheric molecules into atomic hydrogen creating longitudinal symmetry in hydrogen Lyman alpha emission.

Dessler, A. J.↗
NASA NTRS1980

Airglow from Jupiter's nightside and crescent - Ultraviolet spectrometer observations from Voyager 2

The Voyager Ultraviolet Spectrometer has made extensive observations of airglow from Jupiter's dark-side equatorial latitudes. The brightness of H Lyman alpha, the only emission detected, varies between 700 and 1000 rayleighs (R) as a function of longitude. The dark side of Jupiter is illuminated by sky background Lyman alpha arising from resonance scattering of the solar Lyman alpha line by the neutral hydrogen of the interstellar medium. Calculations show that resonance scattering of this sky background by hydrogen in Jupiter's thermosphere will produce about 300 R of Lyman alpha emission. The additional Lyman alpha observed is probably excited by electrons and protons precipitating at equatorial latitudes. Based on the 500-R upper limit set here on the dark-side H2 Lyman and Werner bands, and the Lyman alpha measurements, the exciting particles are thought to have a soft energy spectrum and deposit about 0.04 erg/sq cm per sec in the atmosphere. There is evidence for an asymmetrical precipitation pattern associated with the longitudinal variation in Lyman alpha emission, and a suggestion of a strong day-night difference in precipitation as well.

Mcconnell, J. C.↗
NASA NTRS1980

Discovery of a longitudinal asymmetry in the H Lyman-alpha brightness of Jupiter

A strong longitude variation in the midlatitude Lyman-alpha brightness of Jupiter has been discovered using data from the Voyager ultraviolet spectrometers. A minimum brightness of 14.4 kR was observed in the System III (1965) west longitude range 200-300 deg. The intensity rises to a broad peak of 19.6 kR near a longitude of 110 deg, a 36% increase over the minimum. This intensity 'bulge' was observed to rotate with the planet and appeared at the same longitude during both Voyager encounters, four months apart. This is a surprising discovery; no other planet studied to date has such a characteristic and the upper levels of the Jovian atmosphere were expected to be uniform in longitude. It is likely that a longitudinal asymmetry in charged particle precipitation leads to selective heating of the upper atmosphere and increased dissociation of H2, increasing the hydrogen column abundance. The resulting increase in resonant scattering could produce the observed intensity bulge.

Sandel, B. R.↗
NASA NTRS1979

Extreme ultraviolet observations from Voyager 2 encounter with Jupiter

The extensive Voyager 2 EUV spectral observations of the Jovian planetary system have produced a number of significant results. The positions of the auroral zones have been defined with the aid of support imaging to the extent that the aurora is now known to correspond to the location of the planetary magnetic field lines intersecting the Io plasma torus. The observations do not preclude the possibility of a simultaneous magnetotail aurora, but if present it must not be a dominant component. Emission in molecular hydrogen has been detected from the equatorial regions of Jupiter, which indicates planetwide electron precipitation. Hydrogen Lyman alpha from the dark side of the planet has been measured at an intensity of about 1 kilorayleigh. Occultation of Alpha Leonis by Jupiter has been observed, and the data are being analyzed in detail.

Sandel, B. R.↗
NASA NTRS1979

Voyager spectral observations of the Cygnus Loop nebula, 600-1700 A

Observations of the Cygnus Loop objects NGC 6995 and NGC 6992, which are defined by bright filamentary structure in visible radiation, are reported which were made with the Voyager 2 UV spectrometer in the wavelength range from 600 to 1700 A. The results are compared with far-UV observations performed during the Apollo 16 mission. It is found that measurable emission in the region from 912 to 1050 A is sharply confined to the location of the optical filaments, that emissions in the range from 1200 to 1680 A exhibit a sharp decline 0.5 deg of arc west of the filaments toward the center of the Loop, and that the most likely emissions in the 912-1050-A region are C II at 1037 A, C III at 977 A, and O I at 936 and 999.5 A. The results suggest that the radiation in the 912-1050-A range is associated with cooler postshock regions and that the shock velocities range from 70 to 100 km/s

Shemansky, D. E.↗
NASA NTRS1979

Extreme ultraviolet observations from Voyager 1 encounter with Jupiter

Observations of the optical extreme ultraviolet spectrum of the Jupiter planetary system during the Voyager 1 encounter have revealed previously undetected physical processes of significant proportions. Bright emission lines of S(+2), S(+3), O(+2) indicating an electron temperature of 100,000 K have been identified in preliminary analyses of the Io plasma torus spectrum. Strong auroral atomic and molecular hydrogen emissions have been observed in the polar regions of Jupiter near magnetic field lines that map the torus into the atmosphere of Jupiter. The observed resonance scattering of solar hydrogen Lyman alpha by the atmosphere of Jupiter and the solar occultation experiment suggest a hot thermosphere (greater than or equal to 1000 K) with a large atomic hydrogen abundance. A stellar occultation by Ganymede indicates that its atmosphere is at most an exosphere.

Broadfoot, A. L.↗
NASA NTRS1979

Observations of the diffuse interstellar EUV radiation field

Results are reported for observations and a preliminary theoretical analysis of the diffuse interstellar EUV radiation field exhibiting positive measurable fluxes in the wavelength range from 912 to 1150 A. The observations were made with the UV spectrometers on the two Voyager spacecraft, and a spectral half-width of 30 A was obtained. The procedures used in the analysis are described in detail, particularly those relating to the elimination of thermal and cosmic-ray noise, internal instrumental scattering from bright lines, and possible contributions from direct starlight. Measurements in several directions reveal diffuse intensities at 975 A ranging from a maximum of about 10 to the -7th erg/sq cm per (sec sr A) near the galactic plane to an upper limit of 10 to the -8th erg/sq cm per (sec sr A). A comparison of the present measurements with various theoretical predictions and previous measurements indicates reasonable agreement with some theoretical predictions of the interstellar EUV intensity.

Sandel, B. R.↗
NASA NTRS1979

Voyager observations of the interstellar medium in the 500- to 1700-A spectral region

Observations of the interstellar medium have been obtained in the 500-1700-A spectral region by the ultraviolet spectrometers installed on the Voyager spacecraft. A preliminary analysis of the least complex of the spectra obtained to date indicates measurable emission in spectral lines at 584, 1026, and 1216 A. These emissions are identified with resonance scattering of solar radiation in transitions of helium and atomic hydrogen. The observations in the direction alpha 324 deg, delta-23 deg contain no measurable direct emission or scattering of stellar radiation by the interstellar medium to an upper limit of 0.0005 R/A at 975 A. Spectra obtained in other directions indicate measurable sources of apparent continua having relatively broad spatial extent.

Shemansky, D. E.↗
NASA NTRS1977

Ultraviolet spectrometer experiment for the Voyager mission

An objective grating spectrometer covering the wavelength range of 500 to 1700 A with a 10-A resolution is employed for the Voyager ultraviolet spectrometer experiment. In determining the composition and structure of the atmospheres of Saturn, Jupiter and several satellites, the ultraviolet spectrometer will rely on airglow mode observations to measure radiation from the atmospheres due to resonant scattering of solar flux, and the occultation mode for assessments of the atmospheric extinction of solar or stellar radiation as the spacecraft enters shadow zones. Since it is capable of prolonged stellar observations in the 500 to 1000 A wavelength range, the spectrometer is expected to make important contributions to exploratory studies of UV sources.

Broadfoot, A. L.↗
NASA NTRS1977

Self-scanned anode array with a microchannel plate electron multiplier - The SSANACON

A photoelectron counting detector utilizing a 128-element linear self-scanned anode array to collect the output of a dual microchannel plate electron multiplier is described. The operation of the detector, its spatial resolution and detection efficiency, and radiation behavior are discussed. The low-noise detector was developed for the Mariner Jupiter/Saturn mission UV spectrometer, and offers a large dynamic range and count rate of 0.003 per anode-sec. It can measure simultaneously two spatially separated signals differing by more than six orders of magnitude in count rate.

Broadfoot, A. L.↗
NASA NTRS1977

Microchannel plate life tests

The behavior of the gain of a microchannel plate (MCP) during operation of the latter at moderate output currents was investigated. The total charge per unit area, Q, extracted from the MCP is shown to be the relevant quantity for evaluating MCP performance. The shape of the gain versus Q curve provides information on the gain decay process. Long periods of stable gain, frequently exhibited by channeltrons, were not observed in the present experiment.

Sandel, B. R.↗
NASA NTRS1976

Photoelectron counting with an image intensifier tube and a self-scanned photodiode array

A self-scanned photodiode array was fiber-optically coupled to an ITT proximity-focused image intensifier tube. It was determined that a single photoelectron event produces 34,000 charge carriers in a photodiode. This charge is only a factor of 2.8 above the rms noise level of the preamplifier used in the tests, but standard charge-sensitive techniques are at least a factor of 12 better. The image tube and photodiode array combination can be packaged in a disk 5.1 cm in diameter and 2.6 cm thick.

Sandel, B. R.↗
NASA NTRS1973

Measurement of auroral Birkeland currents and energetic particle fluxes.

A rocket-borne experiment containing a vector magnetometer and a set of charged-particle detectors was launched from Poker Flat, Alaska, at 2217 LT on February 13, 1971, over a single auroral arc. Particle data in the energy range 0.5-20 kev, obtained during part of the flight, show a peak incident flux of 40 millions electrons per sq cm sec ster kev in the energy range 1-2 kev. The pitch angle distributions of these precipitating electrons were found to be fairly isotropic from 0 to 60 deg and to decrease from 60 to 90 deg. The results of the vector magnetometer indicate the existence of a system of Birkeland currents with magnitudes of 5 microamperes/sq m in the vicinity of the auroral arc. Two different models that fit the data are discussed.

Cloutier, P. A.↗
NASA NTRS1972

Rocket-based vector magnetic measurements of Sq ionospheric currents near sporadic E.

A rocket-borne vector magnetometer has been used to infer the current magnitude, direction, and distribution of midlatitude ionospheric currents near sporadic E. Complete vector information returned throughout the flight indicates that a southward equivalent surface current (vertically integrated current) of 0.15 amp/m was encountered between the altitudes of 104 and 118 km in both ascending and descending portions of the flight. Vertical distribution of the current within the layer was approximately uniform, and the layer was found to be horizontal within the accuracy of the experiment. The data are consistent with the assumption that a sporadic-E layer observed by Wallops Island ionosondes significantly altered the usual Sq current flow pattern by concentrating the current into a narrow altitude range, and by horizontally focusing the current flow into the sporadic-E region with attendant reduction of current density outside the region.

Cloutier, P. A.↗