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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

The Strontium Filament within the Homunculus of Eta Carinae

During a series of HST/STIS observations of Eta Carinae and associated ejecta, we noticed a peculiar emission filament located a few arcseconds north of the central source. While bright in nebular standards, it is submerged in a sea of scattered starlight until moderately high dispersion, long-slit spectroscopy with the STIS (R- 8000) brings the emission lines out. The initial spectrum, centered on 6768A with the STIS G750M grating, led to identification of twenty lines from singly-Ionized species including [Sr II], [Fe II], [Ti II], [Ni II], [Mn II], and [Co II] (Zethson, etal., 2001, AJ 122,322). No Balmer emission is detected from this filament and the Fe II 2507,9 lines, known to be pumped by Lyman alpha radiation in other regions near the central source, are not detected. Followup observations have led to detection of hundreds more emission lines from iron group elements in neutral and singly-ionized states. Thus far all are excited by less than 10 eV. This peculiar nebular emission is thought to be due to very intense stellar radiation, stripped of uv flux shortward of Lyman alpha, bathing a neutral structure. We are systematically identifying the many lines (over 90% identified) and measuring line intensities that will then be modeled to determine excitation mechanisms, temperature and density. Two [Sr II] and two Sr II lines have now been measured. Bautista, etal. (in preparation) have modeled the strontium flux ratios and find that large radiation fluxes and/or high strontium abundances may account for the detected emission. These observations were supported by STIS GTO funding and GO funding through the STScI

Gull, Theodore R.↗
NASA NTRS1996

The Electron Excitation Function of H Lyman-(alpha) from Threshold to 1.8 keV

The excitation function of prompt Lyman-(alpha) radiation, produced by electron impact excitation of atomic hydrogen, has been measured for the first time over an extended energy range from threshold to 1.8 keV. Measurments were obtained in a crossed-beams experiment using both magnetically confined and electrostatically focused electrons in collision with atomic hydrogen produced by an intense discharge source.

hydrogen atomic hydrogen Lyman-alpha radiation↗
NASA NTRS1996

(abstract) Measurement of the Total Cross Section for Excitation of the 2p State of Atomic Hydrogen by Electron Impact

The excitation function of prompt Lyman-alpha radiation, produced by electron impact excitation of atomic hydrogen in the energy range from threshold to 1.8 keV, has been measured in a crossed-beam experiment. The present data are significantly different from earlier experimental results and are in good agreement with recent theoretical convergent close coupling calculations over a two order of magnitude range in impact energy. Multistate coupling affecting the excitation function to 1 keV is apparent in both the present experimental and recent theoretical results.

Lyman-alpha radiation atomic hydrogen excitation f↗
NASA NTRS1993

Heliospheric hydrogen beyond 15 AU - Evidence for a termination shock

The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman alpha radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman alpha intensities in the upstream direction fall as r exp -0.75+/-0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r exp -1.07+/-0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. The Voyager H Lyman alpha intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r exp -1, whereas between 30 and 35 AU they fall as r exp -0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond about 25 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman alpha observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.

Hall, D. T.↗
NASA NTRS1972

Backscatter of solar resonance radiation. II.

Calculated curves are presented for the intensity of the scattered solar Lyman alpha radiation as it would be observed at various distances from the sun by a deep space probe during the late 1970s and early 1980s. Near the sun, most of the neutral gas, and hence the emission maximum, is in the pole direction. At 2 AU from the sun, the maximum is quite broad. At greater heliocentric distances, most of the emission lies between the probe and the sun; thus, the angle which the emission maximum makes with the pole increases steadily as the probe moves out.

Johnson, H. E.↗
NASA NTRS1989

Line-of-sight effects on spectroscopic measurements in the inner solar wind region

The effect of the integration along the line of sight on the spectral line profiles of the resonantly scattered Lyman alpha radiation emitted by low-density coronal holes at heights above 1.5 Rs from sun center is investigated. It is shown how the spectral lines from this region are influenced by the Lyman alpha emission from surrounding regions with higher densities. The coronal hole and the surrounding areas are described by a two-fluid solar wind model. It is shown that the line-of-sight effects can be important for the interpretation of the Lyman alpha spectral line measurements in the outer corona and inner solar wind.

Esser, Ruth↗
NASA NTRS1983

H I Lyman-alpha in the sun - The effects of partial redistribution in the line wings

A form for the Lyman-alpha redistribution function that is modified to include the effects of collisional coupling between different energy states is presented. It is noted that this extends the work of Yelnik et al. (1981) and Roussel-Dupre(1982). By separating the source function into its primary components at all heights, the formation of Lyman-alpha radiation is investigated in detail. Frequencies in the line wing can be mapped back in such a way that the heights of formation for a change in lambda of plus or minus 0.5-6 A span the entire solar chromosphere. The usefulness of the Lyman-alpha wing as an atmospheric probe, however, is limited by long thermalization lengths, which result from almost coherent scattering outside of the Doppler core. When the line profiles calculated from the chromospheric models of Vernazza et al. (1981) are compared with the quiet-sun observations of Roussel-Dupre, agreement is found in integrated core intensities and in the line wing shape and normalized center-to-limb behavior. The calculated wing intensities, however, are a factor of four too small.

Roussel-Dupre, D.↗
NASA NTRS1987

Cometesimals in the inner solar system

Spectra obtained by the ultraviolet spectrometer on the Voyager 2 spacecraft between 1 and 2.5 AU are shown to provide evidence for a very large number of 'cometesimals' with radii between a few meters and a few tens of meters in the neighborhood of the earth. The evidence consists of a component in the interplanetary Lyman-alpha radiation that decreases rapidly with heliocentric distance. It is proposed that the source consists of cometesimals which are ice-coated, porous, low-density refractory boulders that may be the building blocks of ordinary comet nuclei. It is shown that the cometesimals required to produce the observed Lyman-alpha emission can also account for all the lunar craters with diameters between 200 m and 1500 m produced during the phase 3200 million years at sites such as Mare Tranquillitatis.

Donahue, T. M.↗
NASA NTRS1983

Tentative confirmation of an aurora on Uranus

There have been three recent reports of the detection of Lyman-alpha radiation (due to atomic hydrogen, H, at wavelength 1,216 A) from Uranus by means of the International Ultraviolet Explorer (IUE) satellite. The interpretations of these results differ. Two reports conclude that there is a strong aurora on Uranus, but the third concludes that the source is resonance scattering of solar Lyman-alpha. This paper reports the detection of emission features due to molecular hydrogen, H2, near 1,600 A. This detection is near the limit of the IUE sensitivity. If it is real, the detection of H2 emission strongly supports the conclusion that Uranus has an aurora comparable in strength with those of the inner two giant planets, Jupiter and Saturn. The first published IUE spectra of the Saturn aurora are also presented.

Caldwell, J.↗
NASA NTRS1995

Composition/Structure/Dynamics of comet and planetary satellite atmospheres

This research program addresses two cases of tenuous planetary atmospheres: comets and Io. The comet atmospheric research seeks to analyze a set of spatial profiles of CN in comet Halley taken in a 7.4-day period in April 1986; to apply a new dust coma model to various observations; and to analyze observations of the inner hydrogen coma, which can be optically thick to the resonance scattering of Lyman-alpha radiation, with the newly developed approach that combines a spherical radiative transfer model with our Monte Carlo H coma model. The Io research seeks to understand the atmospheric escape from Io with a hybrid-kinetic model for neutral gases and plasma given methods and algorithms developed for the study of neutral gas cometary atmospheres and the earth's polar wind and plasmasphere. Progress is reported on cometary Hydrogen Lyman-alpha studies; time-series analysis of cometary spatial profiles; model analysis of the dust comae of comets; and a global kinetic atmospheric model of Io.

Combi, Michael R.↗
NASA NTRS1981

The carbon monoxide fourth positive bands in the Venus dayglow. I - Synthetic spectra

Recent observations of the Venus dayglow spectrum from 1200 to 1800 A are discussed, and identifications of the CO4+ bands are given using synthetic spectra. The main excitation mechanism is found to be resonant and fluorescent scattering of solar radiation, so the theory for this process is covered in some detail. A synthetic fluorescence spectrum is generated with the aid of high-resolution solar flux data, and emission rate factors are calculated. The influence of solar emission line excitation on this spectrum is demonstrated. In particular, it is shown that the uncertainties in interpreting the Venus dayglow are overcome by the identification of CO4+ bands in the v prime = 14 progression resulting from the fluorescent scattering by CO of solar Lyman alpha radiation.

Durrance, S. T.↗
DOE OSTI2025

Hints of primordial magnetic fields at recombination and implications for the Hubble tension

Primordial Magnetic Fields (PMFs), long studied as relics of the early Universe, accelerate recombination and have been proposed as a way to relieve the Hubble tension. However, previous studies relied on simplified toy models. Here, in this study, we use recent evaluations of recombination with PMFs, incorporating full magnetohydrodynamic (MHD) simulations and detailed Lyman-alpha radiative transfer, to test PMF-enhanced recombination (bΛCDM) against observational data from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), and Type Ia supernovae (SN). Focusing on non-helical PMFs with a Batchelor spectrum, we find a preference for presentday total field strengths of approximately 5-10 pico-Gauss. Depending on the dataset combination, this preference ranges from mild (∼ 1.8σ with Planck + DESI) to moderate (∼ 3σ with Planck + DESI + SH0ES-calibrated SN) significance. The bΛCDM has Planck + DESI χ 2 values equal to or better than ΛCDM while predicting a higher Hubble constant. Future high-resolution CMB temperature and polarization measurements will be crucial for confirming or further constraining PMFs at recombination. Field strengths of 5-10 pico-Gauss align closely with those required for cluster magnetic fields to originate entirely from primordial sources, without the need for additional dynamo amplification.

Jedamzik, Karsten [Centre National de la Recherche↗
NASA NTRS1969

A hydrogen bulge over the springtime pole.

Hydrogen bulge above springtime pole indicated by Lyman alpha radiation in earth geocorona, discussing hydrogen horizontal transport and hydrogen compounds dissociation as causes

Donahue, T. M.↗