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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Alfven waves, alpha particles, and pickup ions in the solar wind

Past studies of the properties of Alfven waves in the solar wind have indicated that (1) the amplitude of the velocity fluctuations is almost always smaller than expected on the basis of the amplitude of the field fluctuations, even when the anisotropy of the plasma is taken into account, and (2) the alpha particles do not participate in the wave motions because they 'surf' on the waves carried by the proton fluid. Ulysses data are used to demonstrate that (1) the discrepancy between the velocity and field fluctuations is greater at high heliographic latitudes than in the ecliptic plane, and (2) the alphas do participate in the waves, being either in phase or out of phase with the proton motions depending on whether the differential flow speed between the alphas and protons is greater than or less than the 'observed' wave speed, B(sub o)(delta v squared / delta B squared)exp 1/2, as determined from the ratio of the amplitudes of the velocity and magnetic fluctuations. It is proposed that the modification of Alfven wave propagation speed is due to pressure anisotropies resulting from asymmetric distributions of interstellar pickup ions. If the proposed explanation is correct, it indicates that scattering of pickup ions onto a (bi)spherical shell may not be as complete as generally supposed.

Goldstein, B. E.↗
NASA NTRS1993

(abstract) Interplanetary Lyman-alpha Observations with the Galileo Ultraviolet Spectrometer: Solar Wind Latitude Variations and Multiple Scattering at Solar Maximum

The Galileo Ultraviolet Spectrometer obtained a Lyman-alpha celestrial sphere map on 13,14 December 1990 near the first Earth encounter, with spacecraft near the interstellar wind downwind axis. The data show solar flux longitudinal and latitudinal asymmetries which are modeled with He 10830 Ssolar images. The difference between the observed brightness and a single scattering model is attributed to multiple scattering effects, which we also calculate. The data constrain the solar wind flux latitude variation at solar maximum. Other 1990-1992 Galileo Lyman-alpha data will be discussed.

solar wind ultraviolet spectrometer solar flux cel↗
NASA NTRS1993

Dust near luminous ultraviolet stars

This report describes research activities related to the Infrared Astronomical Satellite (IRAS) sky survey. About 745 luminous stars were examined for the presence of interstellar dust heated by a nearby star. The 'cirrus' discovered by IRAS is thermal radiation from interstellar dust at moderate and high galactic latitudes. The IRAS locates the dust which must (at some level) scatter ultraviolet starlight, although it was expected that thermal emission would be found around virtually every star, most stars shown no detectable emission. And the emission found is not uniform. It is not that the star is embedded in 'an interstellar medium', but rather what is found are discrete clouds that are heated by starlight. An exception is the dearth of clouds near the very hottest stars, implying that the very hottest stars play an active role with respect to destroying or substantially modifying the dust clouds over time. The other possibility is simply that the hottest stars are located in regions lacking in dust, which is counter-intuitive. A bibliography of related journal articles is attached.

Henry, Richard C.↗
NASA NTRS1989

Scattering by fluffy grains

There are indications (Greenberg et al., 1988), that fluffy (i.e., porous) particles are responsible for the observed 3.4 and 10 micron emissions of comet Halley. The absorption characteristics of small particles both solid and fluffy are needed in order to explain the Halley emissions. How isolated small solid particles react to an external radiation field is well known - the Rayleigh approximation. How these same small particles emit when assembled as fluffy aggregates in another question. To what degree are the emission spectra of isolated and aggregated particles comparable. In order to quantify the assertion that fluffy particles produce the observed Halley infrared emission features, the authors are performing calculations to determine the effect of porosity on the absorption characteristics of aggregates of interstellar grain-type particles. The calculations are based on an integral representation of the scattered electromagnetic field. Results are given with application to comet Halley.

Hage, J. I.↗
NASA NTRS1976

Scattering by spheres with nonisotropic refractive indices

Experimental scattering results using a microwave analog technique are described. Artificially constructed axially symmetric spheres with a graphite-like fine-layered composite structure and anisotropic refractive indices were employed to aid high-precision measurements of forward scattering under stable conditions (temperature, physical dimensions, electronic conditions) and to facilitate studies of the extinction and polarization of light by graphite particles in interstellar space. A theoretical approximation utilizing Mie theory for spheres with suitable orientation-dependent refractive indices is applied to account for changes in the complex forward-scattering amplitudes and phase in response to target orientation. The methods are extended to the full range of scattering angles, from the forward direction continuously to about 165 deg and at 180 deg. Agreement between experiment and prediction is close when the symmetry axis is parallel to the polarization.

Wang, R. T.↗
DOE OSTI2022

Using host galaxy spectroscopy to explore systematics in the standardization of Type Ia supernovae

We use stacked spectra of the host galaxies of photometrically identified Type Ia supernovae (SNe Ia) from the Dark Energy Survey (DES) to search for correlations between Hubble diagram residuals and the spectral properties of the host galaxies. Utilizing full spectrum fitting techniques on stacked spectra binned by Hubble residual, we find no evidence for trends between Hubble residuals and properties of the host galaxies that rely on spectral absorption features (<1.3σ), such as stellar population age, metallicity, and mass-to-light ratio. However, we find significant trends between the Hubble residuals and the strengths of [O ii] (4.4σ) and the Balmer emission lines (3σ). These trends are weaker than the well-known trend between Hubble residuals and host galaxy stellar mass (7.2σ) that is derived from broad-band photometry. After light-curve corrections, we see fainter SNe Ia residing in galaxies with larger line strengths. We also find a trend (3σ) between Hubble residual and the Balmer decrement (a measure of reddening by dust) using H β and H γ. The trend, quantified by correlation coefficients, is slightly more significant in the redder SNe Ia, suggesting that the bluer SNe Ia are relatively unaffected by dust in the interstellar medium of the host and that dust contributes to current Hubble diagram scatter impacting the measurement of cosmological parameters.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1973

Observations of the inner F and K coronas below 2220-A wavelength.

The inner coronal continuum has been observed and measured below 2220 A on the slitless spectra obtained by Speer et al. (1970) at the March 7, 1970 eclipse. These observations set some constraints on the brightness of the inner F corona and hence on the scattering efficiency of the inner interplanetary dust cloud particles in the far ultraviolet. They neither confirm nor reject the possibility that the inner dust cloud has the same sharp upturn in scattering efficiency below 2220 A observed in the zodiacal light and in the interstellar medium.

Orrall, F. Q.↗
NASA NTRS2006

Vibrational and rotational quenching of CO by collisions with H, He, and H2

Collisional quenching of molecular species is an important process in a variety of astrophysical environments including interstellar clouds, photodissociation regions, and cool stellar/planetary atmospheres. In this work, quantum mechanical scattering calculations are presented for the rotational and vibrational relaxation of rotationally-excited CO due to collisions with H, He and H2 for collision energies between 10(exp -6) and approx.15000/cm. The calculations were performed using the close-coupling approach and the l-labeled form of the coupled-states approximation. Cross sections and rate coefficients for the quenching of the v=0-2, j=0-6 levels of CO are presented and comparisons with previous calculations and measurements, where available, are provided.

Yang, Benhui↗
NASA NTRS2008

Radiation Transport of Heliospheric Lyman-alpha from Combined Cassini and Voyager Data Sets

Heliospheric neutral hydrogen scatters solar Lyman-alpha radiation from the Sun with '27-day' intensity modulations observed near Earth due to the Sun's rotation combined with Earth's orbital motion. These modulations are increasingly damped in amplitude at larger distances from the Sun due to multiple scattering in the heliosphere, providing a diagnostic of the interplanetary neutral hydrogen density independent of instrument calibration. This paper presents Cassini data from 2003-2004 obtained downwind near Saturn at approximately 10 AU that at times show undamped '27-day' waves in good agreement with the single-scattering models of Pryor et al., 1992. Simultaneous Voyager 1 data from 2003- 2004 obtained upwind at a distance of 88.8-92.6 AU from the Sun show waves damped by a factor of -0.21. The observed degree of damping is interpreted in terms of Monte Carlo multiple-scattering calculations (e.g., Keller et al., 1981) applied to two heliospheric hydrogen two-shock density distributions (discussed in Gangopadhyay et al., 2006) calculated in the frame of the Baranov-Malama model of the solar wind interaction with the two-component (neutral hydrogen and plasma) interstellar wind (Baranov and Malama 1993, Izmodenov et al., 2001, Baranov and Izmodenov, 2006). We conclude that multiple scattering is definitely occurring in the outer heliosphere. Both models compare favorably to the data, using heliospheric neutral H densities at the termination shock of 0.085 cm(exp -3) and 0.095 cm(exp -3). This work generally agrees with earlier discussions of Voyager data in Quemerais et al., 1996 showing the importance of multiple scattering but is based on Voyager data obtained at larger distances from the Sun (with larger damping) simultaneously with Cassini data obtained closer to the Sun.

Pryor, W.↗
NASA NTRS1994

An IUE survey of interstellar H I Ly alpha absorption. 2: Interpretations

We present an analysis of interstellar neutral hydrogen column densities obtained from archival Ly alpha absorption line data toward the complete sample of B2 and hotter stars observed at high spectral resolution with the IUE satellite. The full sample includes 554 stars and more than doubles the number of lines of sight previously observed. Many of the B2 and B1.5 stars exhibit contamination from stellar Ly alpha absorption, and were excluded from the subsequent analysis. The final working sample includes 393 stars. We present statistical averages for a number of quantities derived for the sample. The stars range in distance from 0.12 to 11 kpc with an average distance of 2.1 kpc. Values for average sight-line density range from 0.017 to 8.62 atoms cm(exp -3), with an overall average of 0.23 atoms cm(exp -3). Neutral hydrogen and dust are well correlated, with N(H I)/E(B - V) = 4.93 x 10(exp 21) cm(exp -2) mag(exp -1), and N(H I)/E(Bump) = 2.17 x 10(exp 21) cm(exp -2) mag(exp -1). There is an increase in the H I to dust ratios for the densest sight lines. Evidently, the H I to dust ratios in dense clouds are higher than in the lower density medium between the clouds and in interarm directions. The effect is probably an indication of dust modification in the dense clouds. The H I to dust ratios are approx. 17% smaller for the sight lines to O stars compared to the ratios found toward B stars. We have identified sight lines with anomalously large and small values of N(H I)/E(B - V) and H(H I)/E(Bump). Some of these sight lines are well-known examples of peculiar extinction (i.e., HD 147933 and HD 37061 (NU Ori)). Others are less well known and may provide important new examples of regions with highly modified dust. The number of stars with anomalously low H I to dust ratios is disproportionately larger than the number of stars with anomalously high H I to dust ratios. The distribution of the gas away form the Galactic plane for the sample of objects is complex. For the subset of 375 stars whose lines of sight have a statistically low probablility of intersecting a large cloud, the distribution may be roughly approximated by an exponential density distribution, with a midplane density of 0.366 atoms cm(exp -3), a scale height of 195 pc, and a random logarithmic scattering parameter sigma (sub p) of 0.159 dex. For the same sample of stars, interstellar dust is found to be confined closer to the plane than the H I, with (E(B - V)/r)(sub 0) = 0.257 mag kpc(exp -1), scale height h = 152 pc, and sigma(sub p) = 0.263 dex. The dust distribution is more inhomogeneous than the gas distribution. The observed distribution of H I away from the Galactic plane can also be fitted with a two-component exponential model including the effects of patchiness. Unfortunately, the foreground contamination produced by the cloudy and more confined component of H I makes it difficult to estimate the scale height of the extended component without invoking an independent estimate of its midplane density. Assuming n(sub 2)(0) = 0.16 cm(exp -3) from the Copernicus satellite H I/H(sub 2) survey, we obtain n(sub 1)(0) = 0.247 cm(exp -3), h(sub 1) = 73 PC, and h(sub 2) + 357 pc.

Diplas, Athanassios↗
DOE OSTI2025

A Numerical Model for the Dynamics of Pickup Ions Outside the Heliopause and IBEX “Ribbon” Observation

The leading mechanism for the origin of the Interstellar Boundary Explorer (IBEX) “ribbon” of enhanced energetic neutral atoms (ENAs) from the outer heliosphere is the so-called secondary ENA process. In this mechanism, neutralized ions escape the heliosphere, and after several times of charge exchange collision,s some propagate back toward Earth in directions nearly perpendicular to the local interstellar magnetic field. However, the physical processes governing the distribution of the pickup ions (PUIs) outside the heliopause are still unclear. In this study, we build a new global model of the IBEX ribbon, where a key component is to calculate the dynamics of the PUIs outside the heliopause by solving the gyrophase-averaged focused transport equations on top of the background magnetohydrodynamic-kinetic model for the heliosphere-interstellar medium interaction. We discuss how the properties of the simulated ribbon change with different scattering parameters and show simulation results of the ENA sky maps, ribbon centers, ribbon profiles at several azimuthal slices, the ribbon ENA source region, and the velocity distribution of PUIs in the ENA source region for different scattering frequencies. Our results show that the model can provide reasonable ribbon sky maps comparable to the observed ribbon when the PUI mean free path is on the order of several thousand astronomical units.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

Circumstellar dust emission models

We present the wavelength-dependent absorption coefficient K(sub lambda) the scattering coefficient o(sub lambda), the albedo w(sub lambda), and the average cosine of the scattering phase function g(sub lambda) between 0.0912 micrometers and 1000 micrometers for four interstellar medium grain models. These grain models are used in a radiation transfer code to calculate the properties of dust shells surrounding a newly formed O star. For each shell model a distribution of 25 grain sizes and two compositions were used in our calculations. The spectral type of the central star (O6 ZAMS), the geometry (shell), and circumstellar density distribution (constant) are the same in all models, so that different model predictions result entirely from differences in grain properties. For each grain type the models predict the emergent spectral energy distribution with wavelength, the optical depth with wavelength, and the mean dust temperature with distance from the central star. In addition, we find the emitted envelope flux (total flux minus the direct stellar contribution) included within an angular radius theta for several wavelengths between 2.2 micrometers and 100 micrometers. It is found that large differences in the emitted spectrum can occur when grains with different optical constants and size distributions are used.

Wolfire, Mark G.↗
NASA NTRS1994

Observations of interplanetary Lyman-alpha with the Galileo Ultraviolet Spectrometer: Multiple scattering effects at solar maximum

The Galileo Ultravilet Spectrometer Experiment (UVS) obtained a partial celestial sphere map of interplanetary Lyman-alpha (IP L alpha) on 13-14 December 1990 during the first Earth encounter. The Galileo spacecraft was near the downwind axis of the local interstellar medium flow. These UVS measurements sampled the downwind, anti-sunward hemisphere. The data were modeled using a hot model of the interplanetary hydrogen density distribution with the goal of studying multiple scattering effects in the inner solar system. The derived ratio in the downwind direction of the observed brightness and a single scattering model brightness, both normalized to unity in the upwind direction, is 1.82 +/- 0.2. This brightness ratio requires a multiple scattering correction which is 36% larger than can be accounted for by theoretical calculations. The hot model may require: (1) a temperature perturbation of the interstellar wind velocity distribution or (2) an additional downstream source of interplanetary hydrogen. However, a more likely exlanation which affects the hot model is the latitude dependence of the radiation pressure. This dependence, based on the known solar L alpha flux latitude variation at solar maximum, causes a downwind brightness enhancement by preferential focusing of H-atoms with trajectory planes containing the solar poles. This result implies that radiation pressure near the solar poles is nearly independent of solar cycle and is insufficient to lead to a net repulsion of hydrogen atoms by the sun, as can occur near the ecliptic plane during the solar maximum. In addition, the UVS performed 13 observations of IP L alpha while in cruise between Venus and the Earth in 3 directions fixed in ecliptic coordinates.

Ajello, J. M.↗
NASA NTRS1985

The role of cosmic rays in magnetic hydrodynamics of interstellar medium

Cosmic ray (CR) propagation in the Galaxy and generally in the cosmic plasma is usually considered in the diffusion approximation. The diffusion is regarded to result from CR scattering due to their interaction with a magnetic and an electric field. In most cases the fields are assumed to be given. Meanwhile, in the Galaxy the CR energy density w sub cr is similar to I eV/cm, i.e., it is comparable with the energy densities of the magnetic field and turbulent motions in the interstellar gas. Therefore, for the Galaxy it becomes necessary to take into account the influence of CR on the gas dynamics and on the magnetic fields in this gas. The simplest way to this is to use the hydrodynamic approximation, but this is possible only on scales greatly exceeding the CR free path lambda before scattering and only for times larger than lambda/v approx. equals lambda/c. One should thus obtain corresponding MHD equations and establish the limits of their applicability.

Ginzburg, V. L.↗
NASA NTRS2002

Laboratory Studies of the Optical Properties and Condensation Processes of Cosmic Dust Grains

A laboratory facility for conducting a variety of experiments on single isolated dust particles of astrophysical interest levitated in an electrodynamics balance has been developed at NASA/Marshall Space Flight Center. The objective of the research is to employ this experimental technique for studies of the physical and optical properties of individual cosmic dust grains of 0.1-100 micron size in controlled pressure/temperatures environments simulating astrophysical conditions. The physical and optical properties of the analogs of interstellar and interplanetary dust grains of known composition and size distribution will be investigated by this facility. In particular, we will carry out three classes of experiments to study the micro-physics of cosmic dust grains. (1) Charge characteristics of micron size single dust grains to determine the photoelectric efficiencies, yields, and equilibrium potentials when exposed to UV radiation. (2) Infrared optical properties of dust particles (extinction coefficients and scattering phase functions) in the 1-30 micron region using infrared diode lasers and measuring the scattered radiation. (3) Condensation experiments to investigate the condensation of volatile gases on colder nucleated particles in dense interstellar clouds and lower planetary atmospheres. The condensation experiments will involve levitated nucleus dust grains of known composition and initial mass (or m/q ratio), cooled to a temperature and pressure (or scaled pressure) simulating the astrophysical conditions, and injection of a volatile gas at a higher temperature from a controlled port. The increase in the mass due to condensation on the particle will be monitored as a function of the dust particle temperature and the partial pressure of the injected volatile gas. The measured data will permit determination of the sticking coefficients of volatile gases and growth rates of dust particles of astrophysical interest. Some preliminary results based on measurements of photoelectric emission and radiation pressure on single isolated 0.2 to 6.6 micron size silica particles exposed to UV radiation at 120-200 nm and green laser light at 532 nm are presented.

Abbas, M. M.↗
NASA NTRS1976

Gas absorption cell photometer for rocket observations of local interstellar helium

A photometer sensitive in the 584-A line of He I, incorporating a helium-gas absorption cell, has been developed. The helium is confined at a pressure of 0.2 torr between thin-metal-foil broad-band ultraviolet filters. The cell contains sufficient helium to absorb 584-A radiation scattered from atmospheric helium. The ratio of fluxes seen with the cell full and empty provides a valuable datum for fitting models of the local interstellar medium, which is independent of solar 584-A flux, interstellar helium density, and photometer sensitivity.-

Freeman, J.↗
NASA NTRS1993

Model scattering envelopes of young stellar objects. II - Infalling envelopes

We present scattered light images for models of young stellar objects surrounded by dusty envelopes. The envelopes are assumed to have finite angular momentum and are falling in steady flow onto a disk. The model envelopes include holes, such as might be created by energetic bipolar flows. We calculate images using the Monte Carlo method to follow the light scattered in the dusty envelope and circumstellar disk, assuming that the photons originate from the central source. Adopting typical interstellar medium dust opacities and expected mass infall rates for protostars of about 10 exp -6 solar mass/yr, we find that detectable amounts of optical radiation can escape from envelopes falling into a disk as small as about 10-100 AU, depending upon the viewing angle and the size of the bipolar flow cavity. We suggest that the extended optical and near-IR light observed around several young stars is scattered by dusty infalling envelopes rather than disks.

Whitney, Barbara A.↗
NASA NTRS1988

Properties of interstellar dust in reflection nebulae

Observations of interstellar dust in reflection nebulae are the closest analog in the interstellar medium to studies of cometary dust in our solar system. The presence of a bright star near the reflection nebula dust provides the opportunity to study both the reflection and emission characteristics of interstellar dust. At 0.1 to 1 micrometer, the reflection nebula emission is due to starlight scattered by dust. The albedo and scattering phase function of the dust is determined from observations of the scattered light. At 50 to 200 micrometers, thermal emission from the dust in equilibrium with the stellar radiation field is observed. The derived dust temperature determines the relative values of the absorption coefficient of the dust at wavelengths where the stellar energy is absorbed and at far infrared wavelengths where the absorbed energy is reradiated. These emission mechanisms directly relate to those seen in the near and mid infrared spectra of comets. In a reflection nebula the dust is observed at much larger distances from the star than in our solar system, so that the equilibrium dust temperature is 50 K rather than 300 K. Thus, in reflection nebulae, thermal emission from dust is emitted at 50 to 200 micrometer.

Sellgren, Kristin↗