Engineering Papers⌕ Search

Engineering topics

Feldman, P. D.

Publications and source records attributed to Feldman, P. D..

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

The ultraviolet spectrum of an aurora 530-1520 A

Ultraviolet spectra between 530 and 1520 A of an active auroral arc were obtained at 6.5-A instrumental resolution. Several new emission features are identified, including several bands of the N2 Birge-Hopfield (1, v-double-prime) progression and numerous N I multiplets, the latter being preponderant at low altitudes. The improved instrumental resolution over previous experiments in the wavelength region below 1200 A allows for a partial resolution of the complex structure between 900 and 1100 A reported in earlier work. The ratio of O I 1356 to N2 Lyman-Birge-Hopfield is smaller than in the dayglow at a similar altitude, and this is interpreted as evidence for a depletion of atomic oxygen in the auroral ionosphere relative to mid-latitude composition.

Feldman, P. D.↗
NASA NTRS1982

Production of CS and S in Comet Bradfield /1979 X/

High and low resolution ultraviolet spectra of carbon monosulfide (CS) in Comet Bradfield (1979 X) were obtained with the IUE satellite. The high resolution rotational profile of the (0, 0) band at 257.5 nm can be fitted with a theoretical profile derived assuming a Boltzmann temperature of 70 K. Spatial plots of the low resolution data for both S and CS show that these emissions are concentrated toward the cometary nucleus. The results that have been obtained are consistent with a Haser model for CS and S where the parent molecule is CS2. A very rapid variation of CS brightness with heliocentric distance is found.

Jackson, W. M.↗
NASA NTRS1982

IUE observations of the Jovian HI Lyman alpha emission (1979 - 1982)

Observations of the Jovian H ion Lyman emission made with the IUE observatory beginning in December 1978 just before the time of the Voyager encounters and extending through January 1982 are presented. A constant disk center brightness of about 8 kR is observed for the central meridian longitude range wavelength III approximately equal 200 deg to 360 deg and a variable brightness 9-15 kR is found for the range lambda III approximately 50 deg to 150 deg. These brightness values persisted throughout the three years of observation. The hydrogen bulge near wavelength III approximately equal 100 deg appears to be a permanent feature of the Jovian atmosphere, and no long term change of the planetary Lyman emission is seen. Since the early IUE observations were made near the two Voyager encounters, this indicates that no substantial changes in the atomic hydrogen concentration or the average atmospheric conditions took place between then and now.

Skinner, T. E.↗
NASA NTRS1982

Ultraviolet spectroscopy of the Jovian and Saturnian auroras

The results of a series of IUE observations of the north polar aurora obtained during a substantial fraction of one complete rotation of Jupiter are presented. From these data a spectrum of the aurora with high signal to noise ratio, and a resolution of about 8 A was obtained, making possible the identification of many H2 Lyman and Werner bands. The spectrum is of sufficient quality to provide reliable quantative data for a comparison with the model atmosphere calculations. The lack of an observable absorption signature makes it possible to set an upper limit on the column density of CH4 and C2H6 above the auroral emissions and hence an upper limit on the primary particle energies. A comparison of this spectrum with a laboratory spectrum of discharge excited H2 shows a remarkable similarity. The results of several IUE observations of the full disk of Saturn are also examined. The exposures were of approximately 2 hours each, and the H2 Lyman and Werner bands were observed near the north pole in two of them.

Durrance, S. T.↗
NASA NTRS1982

The ultraviolet spectrum of periodic comet Encke

A comparison of the ultraviolet spectrum of periodic comet Encke, recorded by the IUE between 1980 October 24 and November 5, with similar spectra of short and long period comets shows the gaseous composition of P/Encke to be virtually identical to that of the other comets. If P/Encke is indeed the remains of a once giant comet, this similarity implies a homogeneous structure for the cometary ice nucleus. The OH brightness distribution shows a spatial variation similar to the visible fan shaped image of the comet. The total derived water production rate is a factor of five higher than that derived from HI Lyman alpha observations during the 1970 apparition and shows a variation with heliocentric distance (r) as r to the -33 power over the range from 0.81 to 1.02 AU.

Feldman, P. D.↗
NASA NTRS1982

Ultraviolet photometry of A-type stars at high galactic latitudes

Ultraviolet photometry on the International Ultraviolet Explorer (IUE) was used to study four stars located in one target of an earlier diffuse background sounding-rocket experiment. The resulting stellar correction is much smaller than that previously estimated, giving a higher diffuse background at this target. Visible photometry appears to be a better indicator of ultraviolet flux than spectral type. The discrepancy between previous predictions and the present observations is explained in terms of: (1) misclassification of two stars; (2) use of a spectral type/effective temperature calibration hotter than more recent determinations; and (3) inadequacy of the Kurucz models, in the far ultraviolet, for A-type stars.

Landsman, W. B.↗
NASA NTRS1982

Ultraviolet spectroscopy of comae

Vacuum ultraviolet observations from sounding rockets and satellite observatories of the gaseous comae of several recent comets are reviewed. The earliest of these led to discovery of the hydrogen envelope extending for millions of km from the nucleus. Subsequent observations of H I Lyman alpha, the OH (0,0) band and the oxygen resonance triplet have provided strong evidence for the water-ice model of the cometary nucleus. Several new species were discovered in the coma, including C, C(+), CO, S and CS. High-resolution spectroscopy and the spatial variation of the observed emissions provide means to elucidate the production and excitation mechanisms of these species. The similarity of the spectra of the half-dozen comets observed to date argues for a common, homogeneous composition (with the exception of dust and CO) of the cometary ice and a minimal effect on the neutral species due to molecular collisions in the inner coma.

Feldman, P. D.↗
NASA NTRS1981

Water production models for comet Bradfield /1979 X/

Whipple (1950, 1951) has proposed that water might be the dominant volatile constituent of the cometary nucleus. The considered investigation is concerned with the clues which observations of the cometary coma provide regarding the parents of the dominant observed coma species. In the early part of 1980, comet Bradfield (1979 X) was observed with the aid of the International Ultraviolet Explorer (IUE). These measurements are the first which provide a basis for a comprehensive study of all three water dissociation products, H, O, and OH, simultaneously. The IUE observations of comet Bradfield are compared with the predictions derived from an H2O model. The obtained results indicate that the observed brightnesses of H, O, and OH are certainly consistent with a common water source for all three species.

Weaver, H. A.↗
NASA NTRS1981

Spectroscopy of the extreme ultraviolet dayglow during active solar conditions

The spectral characteristics of the mid-latitude daytime airglow observed between 530 and 1500 A under conditions of high solar activity are compared with those obtained at the same location during markedly lower solar activity. The spectral observations were made by two scanning spectrometers and an N2 3371 A photometer carried aboard Astrobee-F rockets launched from White Sands Missile Range, NM, on January 9, 1978 and June 27, 1980. The more recent data allow the partial resolution of the emission spectrum between 800 and 1200 A into a large number of weak N I, O I and N2 transitions. Data taken at 220 km altitude suggest an increase in atomic nitrogen density of more than a factor of 3 between the two observations, along with a doubling of the solar EUV flux at wavelengths less than 688 A. No evidence of a corresponding increase in the 10 to 50 eV photoelectron flux is found, however, an ionospheric sounding data indicate the peak electron density to have decreased during this period. The mechanism for this electron flux decrease in the face of increased EUV flux and only a three-fold increase in N concentration remains unknown.

Gentieu, E. P.↗
NASA NTRS1981

The forbidden oxygen lines in comets

High spatial resolution observations of comet Bradfield (1979X) with the IUE spectrograph have made it possible to tentatively identify a feature at 2972 A as the 1S-3P forbidden transition of oxygen atoms. It is shown here that the most likely source of this emission is the photodissociation of water molecules, mainly by solar Lyman-alpha photons, but that a significant contribution of CO2 cannot be excluded.

Festou, M. C.↗
NASA NTRS1981

Possible detection of far-ultraviolet line emission from a hot galactic corona

The presence of an emission-line component to the radiation field is suggested by rocket observations, at low resolution, of the spectrum of the diffuse, far-ultraviolet background near the north galactic pole. Removal of the line emission leaves a residual uniform cosmic ultraviolet background radiation of only 150 + or - 50 photons/sq cm s sr A, or about half that previously reported. The lines, which are at the wavelengths of the collisionally-excited atom emissions that have been predicted to arise from a hot galactic corona, suggest that there is no need to explain observed high-ionization states at high galactic latitudes as being due to photoionization caused by neutrino decay.

Feldman, P. D.↗
NASA NTRS1981

Ultraviolet spectroscopy of comets using sounding rockets, IUE and Spacelab

Ultraviolet spectroscopy is a very powerful tool for the study of cometary atmospheres since the four elemental cosmic species, H, C, N and O, as well as several simple molecules made from these species, have their strong resonance transitions in the ultraviolet region of the spectrum. However, due to the opacity of the atmosphere to ultraviolet, these observations must be made from space. Results are presented for observations of sevral comets along with plans for further cometary spectroscopy.

Feldman, P. D.↗
NASA NTRS1981

Ultraviolet spectroscopy of comets using sounding rockets, IUE and Spacelab

Information was obtained from the IUE observations as a result of the high spatial resolution capability of the IUE spectrographs and the ability to observe the comet over an extended range of heliocentric distance. The IUE spectrographs also have a high dispersion capability which so far has been used to obtain high resolution spectra of the OH and CS bands.

Feldman, P. D.↗
NASA NTRS1981

IUE observations of faint comets

Ultraviolet spectra of seven comets taken with the same instrument are given. The comets P/Encke (1980), P/Tuttle (1980 h), P/Stephan-Oterma (1980 g), and Meier (1980 q) were observed in November and December 1980 with the IUE satellite, and comets P/Borrelly (1980 i) and Panther (1980 u) were observed with the IUE on March 6, 1981. The spectra of these comets are compared with one another, as well as with comet Bradfield (1978 X), which was extensively studied earlier in 1980 with the IUE. To simplify the interpretation of the data and to minimize the dependence upon a specific model, the spectra are compared at approximately the same value of heliocentric distance whenever possible. Effects arising from heliocentric velocity, geocentric distance, and optical depth are also discussed. All of the cometary spectra are found to be remarkably similar, suggesting that these comets may have a common composition and origin.

Weaver, H. A.↗
NASA NTRS1981

IUE monitoring of the spatial distribution of the H Ly-alpha emission from Jupiter

North-south spatial maps of the Jovian H Lyman-alpha emission observed with the IUE satellite support the identification of a marked longitudinal asymmetry in the equatorial brightness, with the peak around lambda III(1965) = 50-100 deg longitude, and show a weaker planet-wide equatorial brightening above the level predicted by a plane-parallel layer model. IUE observations made 18 months after the initial discovery of the localized emission bulge establish that it is fixed with respect to Jupiter's magnetic longitude (lambda III system) and not with respect to atmospheric longitude (lambda II system). In addition, a decrease in brightness of 20% in the region of the emission bulge has been observed over a one yr interval, while the rest of the planet remained roughly constant in brightness. This decrease is more than would be expected as a result of changes in the incident solar Lyman-alpha flux, indicating a real decrease in reflectivity in this region of Jupiter's upper atmosphere.

Clarke, J. T.↗
NASA NTRS1981

The ultraviolet dayglow. IV - The spectrum and excitation of singly ionized oxygen

The emission line spectrum of singly ionized atomic oxygen (O II) dominates the day airglow spectrum in the extreme ultraviolet below 834 A. The strongest resonance line, at 834 A, is optically thick and an analysis of height profiles obtained from rocket observations between 140 and 265 km in specific viewing directions indicates that the principal excitation source is direct photoionization of neutral atomic oxygen. Strong emission at 538-539 A is most likely due to the quartet rather than the doublet transitions, which both occur at these wavelengths and which are not spectrally resolved in the data. The intensities of the weaker lines are consistent with recent laboratory measurements of transition branching ratios. Several strong O II lines near He I 584 produce severe contamination of low-altitude (less than 400 km) measurements of geocoronal helium emission made with thin-film broadband photometers.

Feldman, P. D.↗