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Broadfoot, A. L.

Publications and source records attributed to Broadfoot, A. L..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Extreme ultraviolet observations from the Voyager 2 encounter with Saturn

Combined analysis of helium (584 A) airglow and the atmospheric occultations of the star delta Scorpii imply a vertical mixing parameter in Saturn's upper atmosphere of K (eddy diffusion coefficient) of approximately 8 x 10 to the 7th sq cm per second, an order of magnitude more vigorous than mixing in Jupiter's upper atmosphere. Atmospheric H2 band absorption of starlight yields a preliminary temperature of 400 K in the exosphere and a temperature near the homopause of 200 K. Certain auroral emissions can be fully explained in terms of electron impact on H2, and auroral morphology suggests a link between the aurora and the Saturn kilometric radiation. Absolute optical depths have been determined for the entire C ring and parts of the A and B rings. A new eccentric ringlet has been detected in the C ring. The extreme ultraviolet reflectance of the rings is fairly uniform at 3.5 to 5 percent. Collisions may control the distribution of H in Titan's H torus, which has a total vertical extent of about 14 Saturn radii normal to the orbit plane.

Sandel, B. R.↗
NASA NTRS1982

Io's hot plasma torus - A synoptic view from Voyager

A study of the morphology of Io's hot plasma torus has encompassed hundreds of Voyager UVS measurements of torus intensity. The long-term average state of the torus can be characterized by an axial asymmetry in the brightness of the prominent S III 685-A feature manifested as an enhancement in brightness whose peak is fixed near 1900 local time. No long-term correlation of brightness with magnetic longitude is present. On time scales of a few Jupiter rotations, the torus can differ markedly from its average axial asymmetry, and its brightness is correlated with magnetic longitude for short times.

Sandel, B. R.↗
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

Morphology of Saturn's aurora

Aurorally-excited emissions of atomic and molecular hydrogen come from a narrow circumpolar band near 80 deg north and south latitude on Saturn. The aurorae, which lie near the edge of the polar cap region, are continuously excited in both the north and south. If the strong variations observed in the auroral intensity are temporal, rather than longitudinal, they may be related to the periodic structure in the Saturn kilometric radiation.

Sandel, B. R.↗
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 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 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

Jovian upper atmospheric temperature measurement by the Voyager 1 UV spectrometer

A measurement of the neutral temperature in the exosphere of Jupiter is reported. The observations were carried out by the Voyager 1 Ultraviolet Spectrometer during the solar occultation phase beginning 4.5 hours after the Jupiter encounter. The range of the exospheric temperature is found to be 1450 (+300, -250) K. The uncertainty in the determination of the temperature appears to be largely a consequence of the lack of knowledge of the brightness distribution on the solar disk at the time of the observations. The high exospheric temperature appears to result from a combination of upward-propagating inertia gravity waves, magnetospheric soft electrons, and auroral electrons, including those from the Io-plasma torus.

Atreya, S. K.↗
NASA NTRS1979

The Lyman-alpha observations of comet Kohoutek from Mariner 10

The paper presents the Lyman-alpha data from postperihelion observations of comet Kohoutek made with Mariner 10 UV spectrometer during the 1974 January 11-24 period when the comet distance from the sun varied from 0.521 AU to 0.860 AU. These data are analyzed to determine the hydrogen production rates. It is noted that observations against the low interplanetary background allowed a measurement of the hydrogen corona over 30 million km (or 15 minutes) as seen from the Mariner 10 spacecraft. The bulk of the observations can be fitted with a non-steady-state model assuming two velocity components (20 km/sec and 8 km/sec) of equal proportions. A 70% increase in the hydrogen production rate is found within 2 days from January 17 to 19 while the comet receded from the sun. Finally, it is noted that the postperihelion production rates from the Mariner 10 observations are comparable to the preperihelion production rates reported previously.

Kumar, S.↗
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

Signatures of solar wind latitudinal structure in interplanetary Lyman-alpha emissions - Mariner 10 observations

A detailed analysis is conducted which shows that signatures in the interplanetary Lyman-alpha emissions observed in three different data sets from Mariner 10 (corresponding to different locations of the spacecraft) provide firm evidence that the intensity departures are correlated with a decrease in solar wind flux with increasing latitude. It is suggested that observations of the interplanetary emission can be used to monitor average solar wind activity at high latitudes. The asymmetry in the solar radiation field as a source of observed departures in L-alpha data is considered and attention is given to the interstellar hydrogen and helium density.

Kumar, S.↗
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.↗