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Shemansky, D. E.

Publications and source records attributed to Shemansky, D. E..

At least 73 records · Page 4
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NASA NTRS1981

The Voyager 1 EUV spectrum of the Io plasma torus

The Voyager 1 EUV spectrum of the hot Io plasma torus obtained before encounter in 1979 at a range of 4,800,000 km is analyzed in terms of a collisional model with improved collision strengths. Model calculations are based entirely on steady-state equilibrium between electron excitation-deactivation and radiative decay in the subspecies structure, and plasma temperature and species number densities are derived by fitting model spectra to the observations. Lines of S II, S III, S IV, O II, and O III are found to be dominant in the plasma torus, at torus half width densities of 44, 160, 220, 50, and 340/cu cm, respectively, and lines of S V and K III may also be present. An effective electron temperature of 80,000 K is estimated for the central dense region near 6 Jupiter radii. Model calculations are in substantial agreement with other spectroscopic observations of the hot torus; however, the observations exhibit strong morphological differences with recent ground-based observations of S II emission, and calculated ion densities differ from in situ Voyager 1 plasma science measurements.

Shemansky, D. E.↗
NASA NTRS1981

Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter

The observations of a number of objects by the Voyager EUV instruments are summarized. The summary is considered to demonstrate the wide ranging application of the EUV spectroscopy. It also marks an important step forward in spectrography and emphasizes the continuing importance of the search and discovery nature of spectroscopic techniques.

Broadfoot, A. L.↗
NASA NTRS1981

Visible aurora in Jupiter's atmosphere

The darkside limb pictures obtained by the imaging experiment on Voyager 1 have been reexamined. It is concluded that the observed luminosity is very likely due at least in part to Io torus aurora. If the effective wavelength of the emission lies in the 4000- to 5000-A region, the slant intensity is estimated to be about 20 kR. The observed double structure may be due to a number of causes such as horizontal structure in auroral emission, aurora plus twilight or photochemical airglow plus aurora.

Cook, A. F., II↗
NASA NTRS1981

The interaction of helium with alpha-quartz

A detailed method of calculating the three-dimensional interaction of gas atoms with solid surfaces has been developed in order to provide a quantitative means of determining atmospheric evolution and global distribution on exospheric planetary bodies. The computational method involves accurate structural models of the surface and subsurface, and numerical solution of 60-dimensional differential equations of motion in a Monte Carlo calculation of the collision process. Preliminary results for the He-alpha-quartz system are described.

Kunc, J. A.↗
NASA NTRS1981

Composition and thermal profiles of the Jovian upper atmosphere determined by the Voyager ultraviolet stellar occultation experiment

During the occultation of the star Regulus (B7 type) by Jupiter as seen from the Voyager 2 spacecraft on July 9, 1979, two absorbing regions were detected. Between 911 and 1200 A, H2 was absorbing over a 600 km altitude range. Above 1300 A, the rapid increase of the absorption by the hydrocarbons was observed over an altitude interval of approximately 100 km with a height resolution of 3 km. The analysis of these absorption features has provided the height profiles of molecular hydrogen, methane, ethane, and acetylene, as well as the thermal profile in the upper atmosphere of Jupiter. Combining the Voyager ultraviolet spectrometer results with other data, such as those obtained by the Voyager infrared and radioscience instruments, has yielded a comprehensive model of the composition and structure of the atmosphere of Jupiter.

Festou, M. C.↗
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 NTRS1980

Mass-loading and diffusion-loss rates of the Io plasma torus

Limits to the mass-loading and diffusion-loss rates of ions in the Io plasma torus have been calculated on the assumption that observed optical emissions are controlled by electron-ion collisions. Calculations of the yield of emission from the vicinity of Io limit the mass-loading rate to the order of 10 to the 27th per s for S II or O II, on the grounds that electron-excited emissions associated with the location of Io have not been observed in the optical spectrum. This mass-loading limit is dependent on the assumptions that Io is the source of torus particles and that most of the neutral atoms are converted to ions within 1 R(J) of Io. According to the calculations presented below, the observed partitioning of sulfur ion species in the hot torus at the time of Voyager 1 encounter indicates that the diffusion-loss time of the ions is of the order of 1/D = 100 days. The two results limiting the mass-loading and diffusion-loss rates are compatible and suggest that the energy required to maintain the observed radiated power cannot be supplied by acceleration of ions formed at Io in Jupiter's rotating magnetic field.

Shemansky, D. E.↗
NASA NTRS1980

Radiative cooling efficiencies and predicted spectra of species of the Io plasma torus

Calculations of the physical condition of the Io plasma torus have been made based on the recent Voyager EUV observations. The calculations represent an assumed thin plasma collisional ionization equilibrium among the states within each species. The observations of the torus are all consistent with this condition. The major energy loss mechanism is radiative cooling in discrete transitions. Calculations of radiative cooling efficiencies of the identified species leads to an estimated energy loss rate of at least 1.5 x 10 to the 12th watts. The mean electron temperature and density of the plasma are estimated to be 100,000 K and 2100/cu cm. The estimated number densities of S III, S IV, and O III are roughly 95, 80, and 190-740/cu cm. Upper limits have been placed on a number of other species based on the first published Voyager EUV spectrum of the torus. The assumption that energy is supplied to the torus through injection of neutral particles from Io leads to the conclusion that ion loss rates are controlled by diffusion, and relative species abundances consequently are not controlled by collisional ionization equilibrium.

Shemansky, D. E.↗
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 NTRS1978

Monte Carlo modeling of exospheric bodies - Mercury

In order to study the interaction with the surface, a Monte Carlo program is developed to determine the distribution with altitude as well as the global distribution of density at the surface in a single operation. The analysis presented shows that the appropriate source distribution should be Maxwell-Boltzmann flux if the particles in the distribution are to be treated as components of flux. Monte Carlo calculations with a Maxwell-Boltzmann flux source are compared with Mariner 10 UV spectrometer data. Results indicate that the presently operating models are not capable of fitting the observed Mercury exosphere. It is suggested that an atmosphere calculated with a barometric source distribution is suitable for more realistic future exospheric models.

Smith, G. R.↗
NASA NTRS1977

Interaction of the surfaces of the moon and Mercury with their exospheric atmospheres

A model of atmosphere-surface interactions on the moon and on Mercury is presented. This model rejects the traditional assumption that the atmospheric particle source is a 'saturated' absorbed surface layer of gas and instead assumes that interaction is with solid atoms bound in a lattice structure so that most collisions with the light atmospheric particles are free-free rather than free-bound. The energy accommodation per collision for He and H is dominated by first-order interaction terms and is generally less than 0.1 per collision. It follows that estimated thermal escape is drastically reduced for the light atoms on both bodies. Thus the model requires much reduced solar wind collection efficiencies for protons and alpha particles. Some peculiarities of the Mercury He and H atmospheres might possibly be explained by the nature of the gas-surface coupling.

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

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.↗