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Moos, H. W.

Publications and source records attributed to Moos, H. W..

At least 37 records · Page 2
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NASA NTRS1989

IUE observations of the Jovian dayglow emission

IUE spectra of Jupiter are examined in light of recent models put forward to explain the anomalously bright ultraviolet emissions seen from the upper atmospheres of the outer planets. Chi-squared fits of the IUE spectra with model spectra produced by two proposed excitation mechanisms, electron impact and fluorescence of solar radiation, result in consistently higher chi-squared values for the solar fluorescence model. No conclusive evidence is found in the IUE data for the dominance of solar fluorescence over electron excitation in producing the Jovian dayglow emission.

Mcgrath, M. A.↗
NASA NTRS1988

Io: IUE observations of its atmosphere and the plasma torus

Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent.

Ballester, G. E.↗
NASA NTRS1988

Jovian equatorial H2 emission from 1979-1987

Ninety two IUE observations of the Jovian equatorial region taken between 2 Dec. 1978 and 1 Feb. 1988 were averaged together by date of observation, resulting in 22 averaged spectra which were fit with a model to determine the amount of H2 Lyman band emission in the region 1552 to 1624A. The data suggest that the H2 emission may vary with time. Especially suggestive is the marked downward trend of the emission between 1983 and 1987, during which time the strength of the emission in the 1552 to 1624A region decreases by a factor of 10. Uncertainty in the existing data and a gap in the data in 1980 and 1981 preclude a positive identification of a correlation between the brightness of the H2 emission and the major solar cycle.

Mcgrath, M. A.↗
NASA NTRS1987

Detection of neutral oxygen and sulfur emissions near Io using IUE

IUE spectra have shown several O I and S I emissions near Io. The optical thickness of the S I 1814 A multiplet indicates that the S column density is greater than about 2 x 10 to the 12th/sq cm. The presence of an S I 1479 A feature suggests that electron collisions with SO2 could be a major source of the emissions. It is likely that particle excitation in the denser collision-dominated part of the atmosphere is also responsible for a substantial part of the observed emissions.

Ballester, G. E.↗
NASA NTRS1987

Laboratory study of the spectrum of highly ionized calcium in the 100-250 A range applied to solar flare diagnostics

Calcium was introduced into the TEXT tokamak, and its spectral emission was recorded in the 50-360 A range by an absolutely calibrated grazing incidence spectrometer. These observations of highly ionized species of calcium at known conditions of plasma electron temperature and density allow testing of line brightness ratio predictions based on theoretical values of temperature-dependent electron excitation rates. The confirmation of the expected ratios in Be I-like to O I-like calcium allows more confident use of these ratios as a density diagnostic of remote astrophysical sources such as solar flares.

Lippmann, S.↗
NASA NTRS1987

IUE observations of hydrogen and deuterium in the local interstellar medium

High-resolution Ly-alpha spectra of the late-type stars Epsilon Eri, Procyon, Altair, Capella, and HR 1099 taken with the short-wavelength camera on IUE are presented. The density, velocity dispersion, and bulk velocity of the interstellar H I toward each of the stars is derived from the spectra. Lower limits on the deuterium-to-hydrogen ratio toward these stars are obtained.

Murthy, J.↗
NASA NTRS1987

A comparison of the C III, O V, F VI, and Ne VII Delta n = 0 (2-2) line emissions from a laboratory plasma with theoretical predictions and astrophysical observations

Spectra of the Delta n = 0 (2-2) transitions of Be I-like ions, C III, O V, F VI, and Ne VII emitted from the TEXT tokamak, were measured with photometrically calibrated instrumentation and compared to the predictions of several models which differ in their treatment of electron impact excitation, using either the distorted wave or R-matrix approach. It was found that the ions from C III to Ne VII were located near the edge of the plasma, at densities between 10 to the 12th and 13th/cu cm. The experimental line ratios were compared with several sets of computations. Agreement is obtained between the experimental data and computations by using the R-matrix technique. This leads to the conclusion that the effect resonances must be included in collision strength calculations, particularly at low nuclear charge. The results show that the line ratios studied may be used with confidence as electron density diagnostics for laboratory or astrophysical plasmas.

Finkenthal, M.↗
NASA NTRS1987

Planetary atmospheres and aurorae

Observations of planetary atmospheres and auroras obtained by the IUE satellite observatory during the first 10 years of its operation are reviewed. Topics examined include the value of UV studies of atmospheric phenomena, the kinds of observations available prior to the launch of IUE in 1978, the composition and structure of the upper atmospheres below the homopause, the effects of the magnetosphere on the atmosphere above the homopause, excitation processes, and fundamental questions and scientific goals. Data on Jupiter, Saturn, Neptune and Uranus, and the Io plasma torus are presented in tables and graphs and briefly characterized. It is pointed out that the IUE has greatly advanced knowledge of the plantary atmospheres, despite the fact that its design was not optimized for planetary observations.

Moos, H. W.↗
NASA NTRS1986

IUE study of the very local interstellar medium

The IUE and Copernicus results for the very local interstellar medium are compared. Despite its lower resolution, IUE produces results of comparable quality, giving important confirmation of Copernicus results on the density, temperature, turbulence, and deuterium-to-hydrogen ratio in the region within 10 pc of the Sun. The stars observed are in a very low-density quarter of the galaxy: multicomponent structure seen in other directions may not be present in the direction of most of the observed stars. The exceedingly low densities observed in certain directions encourages the idea that EUV studies of certain normal stars may be possible.

Henry, R. C.↗
NASA NTRS1986

IUE observations of interstellar hydrogen and deuterium toward Alpha Centauri B

A high dispersion profile is presented of the Lyman-alpha emission toward Alpha Cen B as recorded in two images taken with the IUE spacecraft. The spectra were examined with a three-parameter Gaussian or five-parameter solar-type profile to derive the intrinsic background stellar emission. Voight absorption profiles were calculated for the intervening H I and D I gas. A uniform, thermally broadened medium was assumed, with the calculations being based on the free stellar parameters of density, velocity dispersion and the bulk velocity of H I, and the density of D I. The use of a small aperture is shown to have been effective in eliminating geocoronal and interplanetary diffuse Ly-alpha contamination. The H I absorption profile toward Alpha Cen B is found to be equivalent to that toward Alpha Cen A, indicating that the H I profiles derived are essentially independent of stellar emission. Less success, however, was attained in obtaining any definitive D I profile, although an asymmetry in the blue and red wings of the Lyman-alpha emissions did show the presence of absorption by interstellar deuterium and allow setting a lower limit of 0.00001 for the D I/H I ratio.

Landsman, W. B.↗
NASA NTRS1986

IUE observations of neutral hydrogen and deuterium in the local interstellar medium

Small-aperture, high-dispersion IUE spectra have been obtained of seven late-type stars that, in general, confirm previous Copernicus results concerning the distribution of hydrogen and deuterium in the local interstellar medium. In addition, the IUE Ly Alpha spectra of Altair, and of the Alpha Cen components, suggest that multiple velocity components exist in these two directions.

Landsman, W. B.↗
NASA NTRS1986

A two-dimensional intensified photodiode array for imaging spectroscopy

The Johns Hopkins University is currently developing an instrument to fly aboard NASA's Space Shuttle as a Spartan payload in the late 1980s. This Spartan free flyer will obtain spatially resolved spectra of faint extended emission line objects in the wavelength range 750-1150 A at about 2-A resolution. The use of two-dimensional photon counting detectors will give simultaneous coverage of the 400 A spectral range and the 9 arc-minute spatial resolution along the spectrometer slit. The progress towards the flight detector is reported here with preliminary results from a laboratory breadboard detector, and a comparison with the one-dimensional detector developed for the Hopkins Ultraviolet Telescope. A hardware digital centroiding algorithm has been successfully implemented. The system is ultimately capable of 15-micron resolution in two dimensions at the image plane and can handle continuous counting rates of up to 8000 counts/s.

Tennyson, P. D.↗
NASA NTRS1985

Long-term stability of the Io high-temperature plasma torus

The short wavelength camera of the International Ultraviolet Explorer satellite was used to measure S II 1256, S III 1199, semiforbidden S III 1729, and semiforbidden S IV 1406 emission from the high-temperature region of the Io plasma torus. Observations over a period of five years (1979-1984) indicate that the Io plasma parameters have relatively small variations, particularly in the case of the mixing ratio for the dominant constituent S(++), and electron temperature. A simple three-dimensional model of the plasma torus was used to obtain the ion mixing ratios and the plasma density for each observation. The results are compared with Voyager 1 data for mixing ratio (ion density divided by electron density); ionization balance; and plasma density. The results of the comparison are discussed in detail.

Moos, H. W.↗
NASA NTRS1984

Observations of Interstellar HI Toward Nearby Late-type Stars

High-disperson Copernicus and IUE observations of chromospheric Ly alpha emission are used to study the distribution of HI in the local interstellar medium. Interstellar parameters are derived toward 3 stars within 5 pc of the Sun, and upper limits are given for the Ly alpha flux from 9 other stars within 10 pc.

Landsman, W. B.↗
NASA NTRS1984

Comparison of the Jovian north and south pole aurorae using the IUE observatory

New results on the spatial and temporal variability of the auroral emissions from Jupiter have been obtained from three IUE observations of the south pole made during the period July 1983 to March 1984. The current observations, together with previous IUE studies of the north pole aurora, provide convincing evidence for persistent longitudinal asymmetries in the Jovian auroral emissions. The strongest emissions appear to originate from regions centered near lambda-III of about 0 deg at the south pole and lambda-III of about 185 deg at the north pole. Differences in surface magnetic field strength seem inadequate to explain the extent to which particles precipitating along field lines into a given longitude sector in one hemisphere are inhibited from precipitating along the same field lines into the opposite hemisphere. Thus, the IUE auroral results present a challenge to existing models of auroral production.

Skinner, T. E.↗
NASA NTRS1984

Observations of interstellar hydrogen and deuterium toward Alpha Centauri A

A composite profile is presented of the Ly-alpha emission line of Alpha Cen A, obtained from 10 individual spectra with the high-resolution spectrograph aboard the International Ultraviolet Explorer (IUE) satellite. There is excellent overall agreement with two previous Copernicus observations. Interstellar deuterium is detected, and a lower limit is set on the deuterium to hydrogen ratio of nDI/nHI greater than 8 x 10 to the -6th. In addition, the deuterium bulk velocity appears blueshifted by 8 + or - 2 km/s with respect to interstellar hydrogen, suggesting a nonuniform medium along the line of sight.

Landsman, W. B.↗
NASA NTRS1984

IUE observations of longitudinal and temporal variations in the Jovian auroral emission

The IUE's short wavelength spectrograph has been used to monitor the auroral emissions from Jupiter's northern hemisphere, yielding eight observations between January 1981 and January 1982 of H I Lyman-alpha and the H2 Lyman and Werner bands. Attention is given to an apparent periodic emission flux fluctuation, through detailed modeling of the emission geometry. Two possible auroral zones are defined at the north pole by mapping the magnetic field lines from the Io torus and the magnetotail onto the planet's atmosphere. The observed variation in flux with central meridian longitude is not consistent with a uniform brightness as a function of magnetic longitude in either auroral zone. The data can be fitted by confining the emissions to the region of the northern torus auroral zone, in qualitative agreement with the magnetic anomaly model. A similar emission from the magnetotail auroral zone cannot be ruled out.

Skinner, T. E.↗