Distribution Of Hydrogen Atoms Research Articles

Io's atmosphere and ionosphere: New limits on surface pressure from plasma models

We have studied charge particle impact as a mechanism for the production of Io's ionosphere. Pioneer 10 thermal plasma measurements and magnetospheric plasma models which explain the observed spatial distribution of neutral hydrogen and sodium atoms in the vicinity of Jupiter’s satellite Io imply electron fluxes of ∼1010 cm−2 sec−1. The fluxes and the temperature (∼100 eV) of this plasma suggest that electron impact ionizaion is the dominant process in forming the ionosphere of Io. It is found that the surface number density of the neutral species required to match the observed electron density profiles is ∼109 cm−3 or less. This value is two orders of magnitude lower than previous estimates.

Winter bulge and diurnal variations in hydrogen inferred from AE‐C composition measurements

The atomic hydrogen distribution at 250 km during December 1974 solstice has been inferred, considering charge exchange equilibrium, from Atmosphere Explorer‐C measurements of n(H+), n(O+), and n(O). An empirical model, derived from the observations by least square analysis in terms of spherical harmonics, has the following characteristics: (a) n(H) increases by as much as a factor of two between the summer and winter hemispheres, (b) the n(H) diurnal variation is largest at the equator where nmax/nmin = 3.2, the maximum occurring at 03h LT, and (c) the diurnal variation is larger in the winter hemisphere ( nmax/nmin = 2.6 at +40°) than in the summer (nmax/nmin = 2.3 at −40°). Similar analysis of the gas temperature derived from n(N2) measurements reveals that all n(H) and Tg spherical harmonic coefficients are anticorrelated. Both the diurnal and latitudinal (annual) n(H) and Tg amplitudes are in substantial agreement with the “zero flux condition,” in which exospheric flow dominates the hydrogen distribution. The observed diurnal phase lag of n(H) with respect to Tg is about one hour, agreeing with theory. During the period of measurement the observed mean global values of Tg and n(H) were 800K and 3.6 × 105 cm−3, respectively, the latter exceeding the Kockarts‐Nicolet model concentration by a factor of two.

Distribution of hydrogen and its Lyman-alpha intensity in the atmosphere of Titan

Several models of the Titan atmosphere are derived, and the corresponding vertical distribution of atomic hydrogen and its Lyman-alpha (1216 Å) emission are determined.

Influence of the Earth's rotation and of possible perturbations, on the exobase and exospheric hydrogen densities

In this study, we try to refine the relation existing between the exobase temperature and density distributions of atomic hydrogen around the Earth (assuming that the zero net ballistic flux condition is satisfied all over the critical level). We find essentially that neither local heating in high latitude regions, nor the addition of proton fluxes around the Earth, induce large perturbations in the equatorial density distribution (less than 10 per cent). On the other hand, certain local heating can give large perturbations in the global density distribution (more than 50 per cent).The effect of the Earth's rotation is also studied. We find that at the exobase the density distribution of atomic hydrogen lags about one hour behind the temperature distribution. At higher altitudes this time lag increases, reaching 5–6 hr at 20 Earth radii.We show also that, due to a density inversion which takes place at 2 Earth radii, if the minimum of density at the exobase is on the dayside, above 2 Earth radii, a maximum of density is then on the dayside when going higher, due to the rotational effect, that density maximum shifts towards the evening, reaching early parts of the night at 20 Earth radii.

Energy distributions of charged and neutral hydrogen atoms backscattered from metal surfaces bombarded with 5 to 18 keV protons

Polycrystalline targets of Be, V, Nb, Mo and Ta have been bombarded with protons with primary energies in the range from 5 to 18 keV. The energy distributions of the charged and the neural particles backscattered at 135° with respect to the primary beam direction have been measured between 200 eV and 18 keV. The energy distribution of the neutrals has a pronounced maximum between 0.5 and 1 keV whose position does not depend on the primary energy or the target material, or the angle of emergence of the scattered particles. The energy distribution of the charged particles shows a less pronounced maximum between 1 and 1.5 keV. Only slight differences in the shape of the energy distributions have been observed for different target materials. The fractional number of charged backscattered particles increases from ≈; 3% at 300 eV to ≈; 40% at 18 keV.

Changes in concentration of oxygen and hydrogen constituents in the mesosphere during an eclipse

Numerical solutions of the diffusion and continuity equations are used to examine the changes in concentration of constituents in an oxygen—hydrogen atmosphere during a solar eclipse. The height distributions of atomic oxygen, ozone and atomic hydrogen at particular times during and following the eclipse are presented, and the relative variations of these and other constituents at representative heights are also given.

Spatial and temporal variations of the Lyman-alpha airglow and related atomic hydrogen distributions

Spatial variations of the atomic hydrogen Lyman-alpha airglow were observed at 550 km from the OSO-4 spacecraft. Near the subsolar position, the zenith, horizon and nadir emission rates were 27,50 and 35 kR, respectively, while near the antisolar position, the respective emission rates were 2·0, 2·7 and 1·2 kR on 27 October 1967. Analyses of these results shows that the mean vertical optical depth above satellite altitude is in the range 0·87–3·5 with 1·3 as the preferred value. Atomic hyrdogen undergoes a diurnal density variation of the order of a factor of 1·7 with a maximum in the early morning and a minimum in the afternoon. A hydrogen altitude profile corresponding to 1100 K exospheric temperature is adequate to explain the data. Temporal variations of the airglow observed from OGO-4 show that the solar line-center Lyman-alpha flux should typically increase by 50 per cent for an increase in Zurich sunspot count from 30 to 210.

Angular Distribution of Metastable Hydrogen Formed by Dissociation ofH2+

A study has been made of ${\mathrm{H}}_{2}^{+}$ dissociation by impact on targets of He, Ar, ${\mathrm{H}}_{2}$, and ${\mathrm{N}}_{2}$. Cross sections were measured for the angular distributions of metastable hydrogen, protons, and neutral hydrogen atoms resulting from dissociation. Projectile energies are in the range 4-12 keV; the angular range of the measurements extended from 0.5\ifmmode^\circ\else\textdegree\fi{} to 5.0\ifmmode^\circ\else\textdegree\fi{}. The angular distributions of metastables are consistent with their formation through excitation of the ${\mathrm{H}}_{2}^{+}$ projectile into the $2s{\ensuremath{\sigma}}_{g}$ and $3p{\ensuremath{\sigma}}_{u}$ repulsive states of ${\mathrm{H}}_{2}^{+}$. In all cases the dissociation cross section appears to be strongly peaked towards small angles between the internuclear axis of the molecule and the direction of the ${\mathrm{H}}_{2}^{+}$ projectile; in the case of $\mathrm{H}(2s)$ formed by dissociation on He, the cross section appears to vary as approximately the square of the cosine of that angle. Angular distributions vary quite markedly from one target to another.

Mariner 6 and 7 Ultraviolet Spectrometer Experiment: Analysis of hydrogen Lyman-alpha data

Four Lyman alpha airglow measurements of the limb and disk of Mars, made by ultraviolet spectrometers on Mariner 6 and 7 in 1969 and Mariner 9 in 1971, are analyzed to determine the amount and distribution of atomic hydrogen above 80 km. The variation of atomic hydrogen with altitude is calculated by using time-independent chemical diffusion models from 80 to 250 km, and an exospheric model is used above 250 km. By employing radiative transfer theory that includes effects of pure absorption and accounts for temperature variations in the atmosphere, a spherical model of the airglow Lyman alpha emission is used to produce theoretical intensities for comparison with the data. It is found that (1) the exospheric temperature and distribution in 1971 are consistent with those determined in 1969, (2) the vertical optical depth above 80 km was 2.2 in 1969 and 5 in 1971, and (3) the derived atomic hydrogen distribution from 80 to 250 km requires a source of atomic hydrogen above 80 km. Comparison of observed profiles with chemical diffusion models implies a large downward flow of atomic hydrogen at 80 km coupled with a large upward flow of molecular hydrogen.-

Maps of the Distribution of Atomic Hydrogen in a Region in Centaurus

Maps of the Distribution of Atomic Hydrogen in a Region in Centaurus

Galactic Lyman Alpha and the Interstellar Shock Wave

A THEORY originally proposed by Patterson et al.1 and recently extended by Blum and Fahr2 suggests that the collision of interstellar hydrogen with the solar system produces an anisotropic distribution of fast neutral hydrogen atoms. This theory has been used by Chambers et al.3 to interpret their Vela 7 Lyman alpha results. They have shown that a peak in the Lyman alpha brightness occurs in a direction very close to the velocity vector of the solar system as given by Allen4, implying that the radiation originates in the shock front which surrounds the solar system. Despite the plausibility of the shock wave theory, it seems that the interpretation of Chambers et al. of their data may be erroneous.

Anisotropic Atomic Hydrogen Distribution in Interplanetary Space

Blum and Farr1 have suggested modifications of current theories2,3 concerning the interaction of the solar wind with interstellar hydrogen. One feature of their model involves the motion of the solar system. This motion, coupled with the velocity of the interstellar hydrogen, results in a velocity of 10 to 40 km s−1 for interstellar hydrogen relative to the solar system. According to Blum and Fahr the relative velocity gives rise to an anisotropic distribution of neutral hydrogen atoms in interplanetary space. The neutral hydrogen density is enhanced in the direction of approach of the interstellar hydrogen and diminished in the opposite hemisphere. The motion of the interstellar hydrogen is assumed to cause a highly aspherical transition region between the hydrogen and the solar wind.

Hydrogen Exchange in Proteins as a Measure of Solvent Exclusion due to Ligands: NUCLEASE AND MYOGLOBIN

The kinetic distribution of hydrogen atoms back-exchanging from tritium-labeled proteins has been estimated from the results of interrupted flow and slow, continuous-flow gel filtration. Staphylococcal nuclease exchanges almost all of it bound tritium atoms in a few hours at pH 8.1, but in the presence of calcium ions and deoxythymidine 3′,5′-diphosphate about 34 hydrogen atoms remain associated with the protein for periods longer than 8 hours. The effect of temperature on the binding of tritium, in the presence and in the absence of these ligands, has been studied. Sperm whale myoglobin shows an analogous trapping of tritium atoms upon the addition of hematin to apomyoglobin. The hematin causes the retention of about 25 hydrogen atoms for periods longer than 20 hours at pH 7.0; protoporphyrin IX traps fewer hydrogen atoms. These results indicate that the addition of the ligands to these two proteins causes the exclusion of solvent water from otherwise exchangeable protein hydrogen atoms. The mechanism of this effect probably involves some combination of direct shielding of the protein from the solvent by the ligand, and changes in conformation and motility.

Daytime ion composition in the height range of 150–1500 km for solar minimum condition

An ion composition model for the height range 150–1500 km, likely to hold for day-time at moderate latitudes for minimum solar activity ( F 10.7 = 70 units † † 1 unit = 10 −22 W/m 2/cps of 10.7 cm solar radiation. ) has been developed. It is based on the neutral atmospheric model given by Bhatnagar and Mitra (1966), solar ionizing flux data given by Hinteregger et al. (1965) and the electron and ion temperatures given by evans (1965) close to minimum solar activity period. The topside ionosphere has been considered as a ternary mixture of atomic oxygen, helium and atomic hydrogen ions. The calculated distribution of O + agrees with that observed in the bottomside F2-region, if the rate coefficients k 1 and k 2 for the charge exchange between O + and N 2, and O + and O 2 have values of about (0.9–1.0) × 10 −12 cm 3/s and (0.9–1.0) × 10 −11 cm 3/s respectively, at F-region temperatures. These values agree within a factor of 2 with measured laboratory values. In the case of helium ions a reasonable agreement between the calculated and observed values is obtained, if the rate coefficients k 3 and k 4 for the charge exchange between He + and N 2, and He + and O 2 have values of about 10 −11 cm 3/sec. In the ease of atomic hydrogen ions, agreement between the calculated and observed values is obtained, if the neutral atomic hydrogen distribution from the atmospheric model is increased by a factor of about 5–10. It is concluded that the electron and ion temperatures in the topside ionosphere above about 500 km, obtained from incoherent scatter technique are larger than those required to explain the observed ionic composition at dip values around 60° for low solar activity conditions.

On the Spiral Structure of Disk Galaxies. III. Comparison with Observations

view Abstract Citations (566) References (32) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Spiral Structure of Disk Galaxies. III. Comparison with Observations Lin, C. C. ; Yuan, C. ; Shu, Frank H. Abstract The density-wave theory of galactic spirals is developed in a form slightly more general than that outlined by Lin and Shu in an earlier short communication. Only self-sustained waves are studied in this paper, and the problem of the origin of the spiral structure is barely discussed. The implications of the theory are examined, both in general terms and in detail. The conclusions are compared with observa- tions. Specifically, we consider (1) the distribution of atomic hydrogen in the Galaxy, (2) the systematic motion of the gas, (3) the distribution of young stars and other optical objects, and (4) the migration of moderately young stars. Good agreements are obtained in all cases if we adopt a pattern speed of about 13 km sec' kpc', and a spiral gravitational field equal to about 5 per cent of the symmetrical field. General discussions are also given on (a) the structure of the magnetic field and its role on the systematic motion of the gas, (b) the role of the density wave in the process of star formation, and (c) the distribution of liii regions as revealed by the 109a radio observations Publication: The Astrophysical Journal Pub Date: March 1969 DOI: 10.1086/149907 Bibcode: 1969ApJ...155..721L full text sources ADS |

Dust and atomic hydrogen nearρ Ophiuchi

We discuss the distribution of dust and atomic hydrogen in and around a small dark cloud nearρ Ophiuchi. Although the obscuration reaches values as high as 8 magnitudes, the values ofNH derived inside the cloud do not differ markedly from those found at nearby, less obscured points. At low values of the obscuration, dust and hydrogen isophotes show satisfactory agreement, i.e.,NH increases withNG. However, we find thatNH starts decreasing for sufficiently large values ofNG. In the entire range of optical extinctions investigated our observations are satisfied by an expression of the typeNH=ANG exp (-BNG). Interpretation of this relationship suggests that an appreciable part of the hydrogen has been converted to molecular form inside the cloud. A tentative method for calculating the time elapsed since the conversion process began is proposed.

Differential Charge-Transfer Cross Section for Collisions ofH+onO2

The angular distribution of fast hydrogen atoms from the exothermic reaction ${\mathrm{H}}^{+}$+${\mathrm{O}}_{2}$\ensuremath{\rightarrow}H+$\mathrm{O}_{2}^{}{}_{}{}^{+}$ is measured for proton energies of 30-300 eV and scattering angles of 0-30\ifmmode^\circ\else\textdegree\fi{}. By means of the known total cross section for this reaction, absolute differential cross sections are also derived. In spite of the relatively large energy defect of 1.5 eV, the distributions are found to be highly peaked in the forward direction. Theoretical implications of this result are discussed qualitatively. Measurements of the differential charge-exchange cross section of the system ${\mathrm{He}}^{+}$ + He showed good agreement with theoretical expectations.

Comparison of the distribution and motion of atomic hydrogen in our own Galaxy and in other spiral systems

Our own stellar systemhas been studied in some detail by the Leiden and Sydney groups, whose maps of the large-scale hydrogen distribution are well-known; these are discussed by Lindblad in Paper 24 of this Symposium volume. The distances in our Galaxy are sufficiently small to reveal significant structure on the order of size of spiral arms, but the lack of a direct distance scale, with an indirect scale to be inferred from velocity models, is a serious handicap in reaching definite conclusions about the symmetry, number of spiral features, and inclination of spiral arms. In fact it is very difficult to decide whether the gas arms coincide with the spiral arms as defined by stars. By the study of external galaxies one can remove the scale uncertainty, but only at the cost of a serious lack of detailed information on the structure, because angular resolution is relatively poor, even with the largest existing telescopes. The largest, the 300-foot (91-m) transit telescope of the National Radio Astronomy Observatory at Green Bank, West Virginia, has a beamwidth of 10′ (arc) at 21 cm, which is larger than the angular dimensions of all but the closest external galaxies.

Atomic and molecular hydrogen in the thermosphere

Calculations have been performed to determine the altitude distribution of atomic and molecular hydrogen in the Earth's atmosphere following a suggestion by Bates and Nicolet that molecular hydrogen may be considerably more abundant than atomic hydrogen at the 100-km level. It is shown in this paper, on the basis of the Lyman-α absorption measurements of Purcell and Tousey, that the molecular hydrogen concentration at 100 km may only be comparable to the atomic hydrogen concentration there.

Atmospheric hydrogen

Attention is drawn to the possibility that H + HO 2 → H 2 + O 2 may be an important source of hydrogen molecules in the lower thermosphere. It is suggested that the fractional volume abundance of hydrogen atoms (free and combined) in this region is determined by, and is not very much less than, the corresponding fractional volume abundance in the stratosphere. Any hydrogen molecules in the lower thermosphere would affect the altitude distribution of hydrogen atoms and augment their rate of escape.