Interstellar Hydrogen Atoms Research Articles

Interaction of the solar wind with the local interstellar medium

First results from a two‐dimensional (axisymmetric) time‐dependent gas dynamic model for the interaction of the solar wind with the local interstellar medium are presented. The model includes the mutual influence of the interstellar and interplanetary plasma (protons and electrons) and the neutral interstellar hydrogen atoms. Neutrals created by charge exchange with the solar wind are ignored; this allows us to approximate the dynamical evolution of the system with the isotropic fluid equations. A supersonic interstellar wind is assumed. The global structure of the heliosphere and interaction region is described for both the plasma and neutral gas. The self‐consistent model is compared to a noninteracting gas dynamic model, and dramatic differences in the size of the heliosphere are found. The distribution of neutral hydrogen in the heliosphere is also discussed.

Multidimensional interpolation using artificial neural networks: Application to an H I cloud in Perseus

view Abstract Citations (4) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multidimensional Interpolation Using Artificial Neural Networks: Applications to an HI Cloud in Perseus Serra-Ricart, Miquel ; Trapero, Joaquin ; Beckman, John E. ; Garrido, Lluis ; Gaitan, Vicens Abstract In this paper we propose a method for interpolating multidimensional unbinned data, which could also be sparse, using artificial neural network techniques. An artificial example is first presented in order to show the reliability and potential of the neural network interpolator. A robust behavior is found. We apply the technique to the mapping of a cloud of interstellar atomic hydrogen. The cloud was mapped in H I at 21 cm and we find the neural network method ideal for interpolating the unevenly sampled data, yielding a map from which the global physical parameters of the cloud can be readily obtained. Publication: The Astronomical Journal Pub Date: January 1995 DOI: 10.1086/117274 Bibcode: 1995AJ....109..312S Keywords: Data Processing; Interpolation; Neural Nets; H I Regions; Interstellar Gas; Mapping; Spline Functions; Astronomy; ISM: CLOUDS full text sources ADS |

A new way to measure the composition of the interstellar gas surrounding the heliosphere

The composition of neutral gas in the Local Interstellar Medium can be studied by direct, in situ measuring of interstellar neutral atoms penetrating into interplanetary space. A novel experimental approach for in situ atom detection, which has never been used earlier in space, is proposed. The technique is based on the conversion of neutral atoms to negative ions at a specially prepared sensitive surface. Negative ions are subsequently analyzed and detected in an essentially noise-free, multi-coincidence mode. It is shown that interstellar hydrogen, deuterium, and oxygen atoms can be measured by the proposed technique. The experiment can be performed from a high-apogee Earth-orbiting satellite or from a deep space probe.

Ionized Disk/Halo Gas: Insight from Optical Emission Lines and Pulsar Dispersion Measures

Warm (≈ 104 K), diffuse H+ is a significant component of the interstellar medium within the Galactic disk and lower halo. This gas accounts for about one quarter of the interstellar atomic hydrogen, consumes a large fraction of the interstellar power budget, and appears to be the dominant state of interstellar matter 1 kpc above the midplane. The origin of this ionized gas is not yet established; however, of the known sources of ionization only 0 stars and perhaps supernovae produce enough power to balance the “cooling” rate of the gas. If 0 stars are the source of the ionization, then the interstellar HI, including the extended “Lockman layer”, must have a morphology that allows about 14% of the Lyman continuum photons emitted by the stars to travel hundreds of parsecs within the Galactic disk and up into the lower halo.

The column density and scale height of free electrons in the galactic disk

view Abstract Citations (201) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Column Density and Scale Height of Free Electrons in the Galactic Disk Reynolds, R. J. Abstract The recent discoveries of millisecond pulsars in globular clusters located far from the Galactic plane provide the first direct measurements of the total extent and column density of diffuse ionized gas within the Galactic disk. The dispersion measures of the pulsars in the clusters, 47 Tuc at z equals about -3.3 kpc and M15 at z equals about -4.3 kpc imply a column density of (0.8-1.4) x to the 20th/sq cm between the clusters and the midplane. When combined with the dispersion measures of pulsars located closer to the Galactic plane, this result indicates a free electron scale height H(e) = 1500 pc. Diffuse H(+) thus accounts for about one-quarter of the total column density of interstellar atomic hydrogen near the solar circle and appears to be the dominant state of the interstellar gas at an absolute value of z greater than 1000 pc. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/185412 Bibcode: 1989ApJ...339L..29R Keywords: Electron Density (Concentration); Free Electrons; Milky Way Galaxy; Pulsars; Scale Height; Density Distribution; Globular Clusters; Interstellar Gas; Astrophysics; GALAXIES: THE GALAXY; INTERSTELLAR: MATTER; PULSARS full text sources ADS | data products SIMBAD (39)

Evidence of a nearby solar wind shock as obtained from distant Pioneer 10 ultraviolet glow data

Pioneer 10 UV photometric data are used to study the nature of the heliospheric boundary interactions between solar wind and inflowing partially ionized interstellar wind and the interstellar magnetic field. The radial intensity dependence of solar Ly-alpha photons backscattered by inflowing interstellar hydrogen atoms is observed. The data in the downstream direction at distances greater than about 37 AU show a distortion of the incoming hydrogen atom density profile. It is suggested that this is characteristic of a nearby solar wind shock probably located about 50 AU from the sun in January 1987.

THE OVERALL STRUCTURE OF GAS IN THE GALAXY.

ABSTRACT This paper considers the physical structure and the distribution of gas in the Galaxy, with particular attention given to the galactic distribution of the interstellar atomic and molecular hydrogen. These two components, which contain essentially all of the interstellar gas by mass, are distributed in a thin disk in which both the total gas surface density and the ratio of molecular to atomic hydrogen decrease more or less monotonically with distance from the center of the Galaxy. The molecular hydrogen is concentrated in large (up to a million solar masses) clouds in which star formation is taking place. Due to mass loss from evolving stars, about 20 percent of gas is returned to the interstellar medium. The scale height of the interstellar molecular hydrogen is about half that of the atomic gas.

NA I as a tracer of H I in the diffuse interstellar medium

view Abstract Citations (164) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS NA I as a tracer of H I in the diffuse interstellar medium. Ferlet, R. ; Vidal-Madjar, A. ; Gry, C. Abstract From a compilation of interstellar atomic sodium and neutral hydrogen absorption measurements, together with new high signal-to-noise ratio Na I observations and detailed analysis through a profile fitting method, the column-density correlation N(Na I) versus N(H) has been derived. Extending now over more than three orders of magnitude, especially toward low-column density clouds, a slope of unity is found instead of the previous quadratic dependence. Still valid for mean space densities, this new result leads to a deduction that, in the diffuse interstellar medium, the product of the recombination coefficient of Na II and the electron space density, alpha n(e), is nearly constant. The n(e) evaluations are consistent with the three-phase model of the interstellar medium, while at high densities, the shielding of UV photons by dust becomes the dominant factor. Publication: The Astrophysical Journal Pub Date: November 1985 DOI: 10.1086/163666 Bibcode: 1985ApJ...298..838F Keywords: Hydrogen; Interstellar Matter; Molecular Clouds; Sodium; Least Squares Method; Signal To Noise Ratios; Astrophysics full text sources ADS | data products SIMBAD (76)

Solar wind decrease at high heliographic latitudes detected from Prognoz interplanetary lyman alpha mapping

New evidence for a latitudinal decrease of the solar wind mass flux is presented from observations of the interplanetary Lyman alpha emission collected in 1976 and 1977 with satellites Prognoz 5 and 6. The flow of interstellar hydrogen atoms in the solar system is ionized by EUV solar radiation and charge exchange with solar wind protons which accounts for about 80% of the total ionization rate. The resulting gradual decrease of the neutral H density from the upwind region down to the downwind region observed from Ly α intensity measurements allowed the determination of the absolute value of the total ionization rate β for one H atom at 1 AU against ionization. Collected in 1976 and 1977 at five places in the solar system, the measurements are first compared to a model which assumes isotropy of the EUV and solar wind. Strong departures are obvious toward high‐latitude regions, especially when the observer is in the downwind region where the solar wind ionization has had more time to act (cumulative effect). A model was constructed which included a decrease of the ionization rate with heliographic latitude. The adjustment of data allowed for the measurement of the absolute value of the total ionization rate and implies a 50% latitude decrease of the ionization rate due to charge exchange with the solar wind, from βsw = (3.9±0.5) × 10−8; s−1 at the equator to βsw = (2.0±0.5) × 10−8 s−1 at the pole. The corresponding absolute value of the solar wind proton flux is (2.4‐3.6) × 108 cm−2 s−1 at the equator and twice less at the pole if a constant velocity is assumed for the solar wind. Even if the solar wind velocity increases from 400 to 800 km s−1, which would decrease the charge exchange cross section by 25%, there is still a decrease by about 30% of the solar wind mass flux from equator to pole. The Lyman alpha data from Mariner 10 (Kumar and Broadfoot, 1978) had already shown a similar trend in 1974, showing a persistence of the solar wind anisotropy for 3–4 years during a solar minimum. The large‐scale properties of the solar wind mass flux can therefore be monitored at all latitudes by remote sensing of Ly α interplanetary emission, since the solar wind is carving the flow of interstellar H and its anisotropies are ''printed'' on the interplanetary H distribution. With uncalibrated Ly α measurements, an absolute value of the solar wind flux can be determined at all latitudes averaged over a typical 1‐year period.

Stochastic star formation and spiral structure

Most approaches to explaining the long-range order of the spiral arms in galaxies assume that it is induced by the long-range gravitational interaction. However, it is well-known in many fields of physics that long-range order may be induced by short-range interactions. A typical example is magnetism, where the exchange interaction between magnetic spins has a range of only 10 ångströms, yet a bar magnet can be made as large as one likes. Stochastic self-propagating star formation (SSPSF) starts from the point of view of a short-range interaction and examines the spiral structure arising from it (Seiden and Gerola 1982). We assume that the energetic processes of massive stars, stellar winds, ionization-front shocks and supernova shocks, in an OB association or open cluster can induce the creation of a new molecular cloud from cold interstellar atomic hydrogen. In turn this new molecular cloud will begin to form stars that will allow the process to repeat, creating a chain reaction. The differential rotation existing in a spiral galaxy will stretch the aggregation of recently created stars into spiral features.

Association of PSR0740–28 with an H I shell in Puppis

During a survey of the interstellar atomic hydrogen (H I) distribution in the Puppis region of the Milky Way, we detected numerous, sometimes expanding, H I shells and filaments, indicative of past energetic explosive events in this area of the Galaxy. One of these shells, GS241–01 + 15, appears to be physically associated with the young pulsar PSR0740–28. In particular, the pulsar is coincident in position with an H I hotspot on the rim of the shell, and there are two nearby non-thermal radio sources, one of which lies on the rim of the H I shell coincident with the position of the open cluster Hfl l, the other lying near the centre of the shell. Both non-thermal sources have been previously identified as possible supernova remnants (SNRs)1,2. Furthermore, velocity measurements of the peak emission from the hydrogen shell and of the 21-cm absorption spectrum of the pulsar offer strong confirmation that the pulsar and the H I shell lie at the same distance from the Sun. This result is of interest because very few pulsars have been linked to known supernova remnants, and only the Crab and Vela pulsars with any degree of certainty3. Whether the pulsar PSR0740–28 is the remnant of the supernova progenitor of the H I ring, or perhaps the remnant of an early supernova in a young stellar group which was formed by the expanding H I shell, is as yet unclear. Present data seem to support the latter interpretation, suggesting that PSR0740–28 may have resulted from a ‘second-generation’ supernova event.

Model Calculations of Solar Wind Expansion Including an Enhanced Fraction of Ionizing Electrons

Collective interactions of the solar wind and newly ionized interstellar gas cause turbulent electron heating to ionizing energies analogous to laboratory experiments on the critical ionization velocity effect. Implications for solar wind and interstellar gas dynamics are calculated by simultaneously solving continuity equations for solar wind protons, interstellar hydrogen atoms, and energetic electrons. Electron impact ionization is shown to be practically as important as photoionization, giving rise to a stronger deceleration and heating of the distant solar wind, a weaker terminating shock, a smaller stand-off distance of the helio pause, and implying higher densities of the outer solar wind and the interstellar neutral gas.

Atmospheric water vapour of extraterrestrial origin: a discussion of its possible role in Sun-weather relationships

This paper discusses in detail the speculative suggestion that the influx of extraterrestrial hydrogen to the Earth's atmosphere constitutes a significant variable source of atmospheric water vapour (sometimes called ‘solar rain’ or ‘extraterrestrial rain’) which can be invoked to explain Sun-weather relationships. The continual influx of solar-flare protons with energies in the range 1–100 MeV, which penetrate down to the mesosphere or upper stratosphere, makes a negligible contribution to the normal water vapour concentration at these altitudes. During the most intense solar-proton event yet recorded (August 1972) this solar-flare contribution to atmospheric humidity amounted to no more than 2 parts in 10 9. Moreover, the long-term average influx of solar-flare protons, or galactic-cosmic-ray protons, has been totally inadequate to produce an appreciable change in the concentration of atmospheric water vapour. The continuous influx at the magnetospheric clefts of solar-wind protons with a mean energy of about 1 keV, which cannot penetrate below 150 km and hence cannot form water vapour directly, only constitutes a highly-localized reduction, or reversal, of the global efflux of hydrogen atoms from the Earth's atmosphere: a similar conclusion holds for auroral protons (∼10keV). At the present time the influx of interstellar hydrogen atoms is trivial compared with the global efflux of terrestrial hydrogen atoms. However, the influx of interstellar hydrogen could be greater than the efflux of terrestrial hydrogen if the Solar System encountered a dense interstellar cloud, but the atmospheric implications of such a large influx of interstellar hydrogen require further consideration. Apart from this one possible exception, which could only be advanced as a tentative explanation for very long-term (∼10 6y) changes of climate, the hypothesis that Sun-weather relationships can be attributed to atmospheric water vapour of extraterrestrial origin is untenable.

Solar wind expansion beyond the heliosphere

A model of solar wind expansion beyond the heliosphere is given on the basis of the assumption of a continuous, stationary out-flow. The distributions of the hot solar wind protons, and the constituents resulting from charge-exchange collisions of these protons with the cool interstellar atomic hydrogen, like hot neutral atomic hydrogen and cold protons, have been investigated for the region beyond the heliosphere. The effect of the relative motion between cool interstellar atomic hydrogen and the solar wind protons in the interstellar space has been included. It is shown on the basis of the continuity of the mass, energy and magnetic flux that the assumption of a steady-state solar wind flowing continuously from the heliosphere into the interstellar space without strong shocks which cause instabilities and turbulences, breaks down when the angle θ between radial flow of the solar wind protons in the heliosphere and the direction of the interstellar magnetic field exceeds about 50–60 degrees. For angles smaller than 50°, the solar wind plasma is able to flow through an infinite cylindrical channel, its axis coincides with the direction of the interstellar magnetic field. The radius of the cylinder is that of the heliosphere. The shocks at the heliopause are found to be non-symmetric around the Sun contrary to most recent ideas concerning the termination of the solar wind.

The termination of the solar wind

It has previously been suggested that the solar wind might terminate at distances of 5 AU to 20 AU from the Sun, and that the solar wind might be drastically slowed down by charge exchange and photoionization of interstellar hydrogen atoms which approach the Sun. However, recent satellite measurements of resonantly scattered Lyman alpha radiation, together with pulsar dispersion and Faraday rotation measures, imply very small values for the interstellar hydrogen density (0.05 cm −3) and magnetic field strength (3 μG). As a result, the solar wind is not expected to be slowed down by more than about 30% inside the termination distance, which is expected to be about 100 AU. Analytic solutions for the solar wind flux and velocity as a function of distance have been obtained, yielding estimates of the slowing down and termination distance as a function of the various input parameters (solar wind flux, velocity, solar Lyman-continuum flux, interstellar hydrogen density and velocity, and magnetic field strength).

Rocket Observation of Interstellar Molecular Hydrogen

view Abstract Citations (174) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rocket Observation of Interstellar Molecular Hydrogen Carruthers, George R. Abstract The Lyman resonance-absorption bands of interstellar molecular hydrogen have been observed in the far-ultraviolet spectrum of the star Persei. The column density of H2 is estimated to be about 1.3 X 1O cm . The colurnn density of interstellar atomic hydrogen, determined from the La absorption line in the same spectrum, is about 4.2 >c 1 cm . Hence, in this line of sight, where visual total extinction by dust is about 1 mag, nearly half of the total hydrogen may be in molecular form. This is in agreement with theoretical predictions. Publication: The Astrophysical Journal Pub Date: August 1970 DOI: 10.1086/180575 Bibcode: 1970ApJ...161L..81C full text sources ADS | data products SIMBAD (5)

Atomic and molecular hydrogen in interstellar space

This paper reviews the present state of knowledge of the abundances and physical state of interstellar atomic and molecular hydrogen. Much new data in this area have been obtained in recent rocket observations. There have also been new developments as a result of ground-based infrared and 21-cm observations, and theoretical research.

Mariner 5 measurements of ultraviolet emission from the Galaxy

The Mariner 5 ultraviolet measurements obtained while the spacecraft was in interplanetary flight are interpreted as Lyman-α radiation. This radiation may arise from (1) the scattering of solar Lyman-α radiation by interplanetary atomic hydrogen, (2) the scattering of solar Lyman-α by atomic hydrogen that is present at the edge of the solar system, or (3) diffusely scattered Lyman-α radiation from the Galaxy. The Mariner 5 measurements show a symmetry with respect to the galactic equator which suggests that the major source of the observed emission is the diffuse galactic radiation. In the Münch model for the diffuse Lyman-α radiation from an HI region, the sources of the Lyman-α photons are stellar chromospheres, the photons are scattered a large number of times by interstellar atomic hydrogen, and are absorbed by interstellar dust. Some of the Mariner 5 measurements may be attributed to Lyman-α emission from an HII region since the field of observation scanned the edge of the Gum nebula.

Far-Ultraviolet Spectra of Six Stars in Orion.

view Abstract Citations (20) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-Ultraviolet Spectra of Six Stars in Orion. Morton, Donald C. Abstract An Aerobee rocket carried two ultraviolet objective spectrographs to 186 km on the morning of 13 October 1965 and pointed them towards Orionis within 1 ~. The cameras and stabilization system were similar to those used to obtain spectra of stars in Scorpius in June 1965. This time one of the f/2 Schmidt cameras had a lithium fluoride corrector, while the other used calcium fluoride as be- fore. An unexpected shock during re-entry severed the mirror and film cassettes from the calcium fluoride camera so that films from only the lithium fluoride one were recovered. Development of these films showed ultraviolet spectra of the six OB stars in Orion listed below with their wavelength ranges. Spec- Wavelength Star V B-V trum range Velocity 1 Ori2.20 -0.21 09.511 1200 to 1415A -1400 km sec~ Ori 1.70 -0.19 BOla 1220 1595 -1900 ~On 2.05 09.51b 1200 1740 -1900 ~ Ori3.35 -0.19 BO.5V 1220 1400 (weak) Ori 2.77 -0.25 09111 1235 2100 -3000: Ori 2.04 -0.18 BO.51a 1630 2795 An exposure of 180 sec gave the best spectra, wiil~ a resolution of about 3 A, somewhat poorer than the 1 A obtained on the previous flight. Nevertheless, many absorption lines and a few emission lines are definitely visible in the spectra. Zero-order images of other stars on the same film permit an absolute calibration of wavelengths within 1 A. Emission lines of Si IV at 1402.7 A and C IV at 1549.5 A are identified in ~ and E. Strong absorption lines of both components of the Si IV doublet and the unresolved C IV doublet are found in ~, E, and t shifted to shorter wavelengths by several angstroms. The Si IV doublet is also present in ~, shifted in a similar way. The velocities corresponding to these shifts are indicated in the table. The value for is uncertain because of poor lines and a lack of unshifted features for checking the wavelength scale. These displaced absorption features must be formed by gas escaping from the stars at high velocities indicating a significant mass loss from these early type giants and supergiants. Rough limits on the rate of mass loss can be obtained by assuming that 4~e broadening of the lines 15 entirely by either Maxwellian Doppler velocities or radiation damping. For the widths of the lines in ~ Orionis the Doppler model gives ~10-~ M0 yr-1 while damping gives ~10-~ M0 yr-'. Absorption lines about 8 A wide are also visible at 1216 A in ~ and 8, and are attributed to Lyman-~ absorption by interstellar hydrogen atoms. Assuming broadening by radiation damping, the width corresponds to ~lO~~ atoms Cm-- 1n il~e line of sight. Publication: The Astronomical Journal Pub Date: 1966 DOI: 10.1086/110137 Bibcode: 1966AJ.....71S.172M full text sources ADS | data products SIMBAD (6)

Interstellar Molecule Formation.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar Molecule Formation. Stecher, Theodore P. ; Williams, David A. Abstract The formation of interstellar molecules is shown to be the result of chemical reactions on the surfaces of the interstellar grains. Previous investigation has shown that the formation of molecules by physical adsorption of the molecular constituents on the grain with a subsequent three- body reaction is inoperative on ice. It is shown here that it is also of no consequence for graphite and Platt particles. The formation process described involves exchange reactions between incident atoms and atoms chemically bound into the grain. The theory of chemical exchange is shown to apply to all types of grain and is then illustrated with graphite flakes. Production rates are given for H2, CH, OH, NH, CN, CO, and N2 as functions of temperature and pressure. Destruction of the molecules is due to photodissociation by starlight and other chemical exchange reactions. The varying conditions in the interstellar medium are considered. No molecules are formed when the gas is quiescent. With the higher temperatures of doud~loud collisions H2 is formed, and it is shown that a minimum of 43 of all interstellar hydrogen atoms are in H2 molecules. CH +CH+, OH, and CN are produced by chemical exchange in the observed amounts when graphite grains are driven at supersonic speeds through the gas by radiation pressure. The process predicts the formation of H2, CO, NH, N2 from graphite flakes and possibly Platt particles. Publication: The Astronomical Journal Pub Date: 1966 DOI: 10.1086/110169 Bibcode: 1966AJ.....71T.399S full text sources ADS |