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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Boltzmann equation for the electron gas of a nondegenerate plasma

The collision terms in the Boltzmann equation associated with various processes are derived. For processes having a Fokker-Planck (F-P) limit, the associated F-P operator is derived by means of physical arguments to determine the form of the operator; its multiplying constant is fixed by calculating the total energy exchange rate and comparing with the rate computed by other means. In this manner, the F-P operator is derived for electron-ion scattering, electron-electron scattering in the high-velocity limit, electron-atom elastic scattering, Compton scattering, and the high-velocity limit for inelastic scattering. Other processes considered are bremsstrahlung, radiative-recombination, photoionization, collisional ionization of atoms, and suprathermal-particle ionization of atoms.

Gould, R. J.↗
NASA NTRS1974

Effect of inelastic electron-atom collisions on the Balmer decrement

Calculation of the Balmer decrement in radiatively ionized hydrogen gas as a function of temperature and density, taking into account the effect of electron-atom collisions. It is found that once the electron density exceeds 10 to the 10th power per cu cm significant deviations from the normal radiative recombination decrement begin to occur. Implications of these results for the physical conditions in the line-emitting region of the Seyfert galaxy NGC 4151 are discussed briefly.

Adams, W. M.↗
NASA NTRS1975

Laser processes in plasmadynamic flows

Radiation emitted from the exhaust flow of a multi-megawatt, quasi-steady magnetoplasmadynamic accelerator has been examined photoelectrically for argon mass flows from 6 to 32 g/sec and arc currents up to 25 kA. Vacuum ultraviolet radiation, indicative of resonance transitions, has been detected photoelectrically at various axial locations downstream of the arc chamber, consistent with a relaxation length calculated using a collisional-radiative recombination model. Photoelectric measurements of particular transitions in the neutral and first ionic state of argon show that populations of excited electronic states of AII are in excess of that which would be expected on the basis of Saha-Boltzmann equilibrium at the local electron temperature and number density. Optical depth measurements indicate that for a current of 4 kA and argon mass flow of 12 g/sec, a population inversion exists between the upper and lower states of the 0.4880 micron argon ion transition.

Campbell, E. M.↗
NASA NTRS1975

The 1640.4 A H-alpha line of He II observed from Skylab

Profiles obtained from NRL Skylab solar spectra of the 1640.4 A feature identified as the H-alpha line of He II are discussed and compared with theoretical profiles derived assuming (1) collisional excitation and (2) radiative recombination as the dominant excitation mechanism. Relative intensities of this line as a function of altitude above and below the solar white-light limb are also given for a representative quiet sun region and for a polar coronal hole.

Feldman, U.↗
NASA NTRS1976

Nitric oxide gamma band emission in an aurora

Emission of the NO gamma (1,0) band at 2150 A has been observed by a rocket-borne spectrophotometer in an IBC II(+) aurora. The nu-prime progression of the gamma-system does not appear in the spectrum. The observed emission rate of the 2150 A feature increases relative to N2(+) first negative band emission with increasing altitude. We suggest radiative recombination of NO(+) ions with electrons as a possible excitation mechanism compatible with the data.

Feldman, P. D.↗
NASA NTRS1976

The solar XUV spectrum of He II

OSO-7 observations of the first five Lyman lines and the Lyman continuum of He II are given for the quiet sun, a coronal hole, prominences, filaments, and the flare of August 7, 1972. These data are calibrated and given in specific intensity units together with color and brightness temperatures for the He II continuum. It is found that He II is overionized in all features except the flare and that the continuum is formed at temperatures near 14,000 K. The He II-He III ionization equilibrium appears to be dominated by photoionizations and radiative recombinations. Schematic calculations for realistic chromosphere and transition-region models can account for the observed intensities of Ly-beta through Ly-epsilon, the Lyman continuum, and its color temperature. To account for the intensity of Ly-alpha, either an implausible 100-km plateau at temperatures near 80,000 K is needed or, more likely, the diffusion-enhanced collisional excitation should be incorporated into the models.

Linsky, J. L.↗
NASA NTRS1976

X-ray spectra of the Puppis A and the Vela supernova remnants

Low-energy X-ray spectra are presented for the entire Vela supernova remnant (SNR), the two most intense subregions of the Vela SNR, and the Puppis A SNR. Acceptable chi-squared fits are obtained from the model of a high-temperature low-density plasma with Allen cosmic abundances when thermal bremsstrahlung, radiative recombination, line emission, and interstellar absorption are included. A suggested increase of 45-fold in the Si abundance for the entire Vela SNR is found unacceptable by chi-squared tests of the present spectra. The two enhanced regions of Vela are found to have different temperatures. A deficiency of at least a factor of 3 is seen in the O VII and O VIII emission from the entire Vela SNR. Because ultraviolet observations indicate a normal abundance of oxygen, this is interpreted as evidence for the existence of nonequilibrium conditions in the emitting plasma.

Moore, W. E.↗
NASA NTRS1976

Tropical F region winds from O I 1356-A and forbidden OI 6300-A emissions. I - Theory

The paper theoretically investigates the relationship of neutral winds and E x B plasma drifts to the time-dependent distribution of 1356-A and 6300-A airglow intensities in the region of the Appleton anomaly. Radiative recombination, ion-ion neutralization, and dissociative recombination are considered as main mechanisms in producing the atomic oxygen tropical nightglow. A quantitative relation is determined between the neutral wind velocities and the resultant asymmetry in the emissions about the dip equator. The height difference at northern and southern conjugate points is shown to be linearly related to the vector sum of the wind velocity components along the magnetic meridian at the latitude of the conjugate points. The height difference at conjugate points may be used to study the pattern of neutral wind velocities in the tropical ionosphere.

Bittencourt, J. A.↗
NASA NTRS1976

Formation of the 0.511 MeV line in solar flares

The slowing down and annihilation of positrons and the formation of positronium in a solar flare plasma are investigated to determine how the width of the 0.511-MeV line and its strength relative to the three-photon continuum from positronium decay depend on the temperature and density of the medium in which the positron comes to rest. The calculations are limited to the cases of annihilation in a completely ionized plasma, in a partially ionized plasma with an electron/neutron density ratio of 1.0 or 0.1, and in an atomic gas with a very small ion density. Thermally averaged rate coefficients are obtained for the free annihilation of positrons and for positronium formation through radiative recombination in the fully ionized plasma. Positronium formation rates and the resultant energy distributions of the positronium atoms in the partially ionized medium are determined by numerically solving the Fokker-Planck equation in a medium where the ambient free electrons have a Maxwell-Boltzmann distribution of finite temperature but the density of the medium is sufficiently low that positronium atoms decay without further collisions following their formation. A Monte Carlo calculation is performed for the positron energy loss, positronium formation through charge exchange, and positronium breakup in the weakly ionized medium. The energy distributions of decaying positronium atoms and the relative number of triplet to singlet positronium decays are evaluated for ion concentrations and densities characteristic of the solar photosphere.

Crannell, C. J.↗
NASA NTRS1977

Nuclear-pumped lasing of /He-3/-Xe and /He-3/-Kr

The letter reports direct nuclear pumping of (He-3)-Xe and (He-3)-Kr using the volumetric He-3(n,p)H-3 nuclear reaction. Lasing in (He-3)-Xe was achieved at the 2.027-micron transition of Xe I; lasing in (He-3)-Kr is assumed to have occurred at the 2.52-micron transition of Kr I. Experimental results show that laser output in (He-3)-Xe increases and tends to saturate with increasing pressure, the lasing neutron-flux threshold decreases with decreasing neutron pulse, and maximum output occurs between 0.1% and 0.5% Xe with the total pressure held constant at 400 torr and with an average neutron flux of 6 by 10 to the 15th power per sq cm/sec. It is noted that the (He-3)-Xe laser has the lowest lasing-threshold neutron flux of any He-3 nuclear-pumped laser to date, but the (He-3)-Kr laser has a very high threshold flux. The primary lasing mechanism is believed to be Penning ionization of Xe and Kr followed by collisional radiative recombination and cascading into the upper laser level.

De Young, R. J.↗
NASA NTRS1979

A study of the excitation and radiative decay of the 3s-prime 3D-0 and 3d 3D-0 levels of atomic oxygen

The absolute cross-sections for the excitation of the 989 A, 1027 A, 7990 A, 8446 A, 1.1287 micron and 1.3164 micron multiplets of atomic oxygen by electron impact dissociation of O2 are reported. The radiative branching ratios for these transitions are calculated from these results and compared with the NBS compilation of Wiese et al. (1966) and the recent theoretical calculations of Pradhan and Saraph (1977). The cascade models of O(+) radiative recombination and of electron-impact excitation of the O I(3S) state in the terrestrial airglow are discussed in the light of the laboratory measurements, and the effects of the resonant absorption of components of the lambda 989 A and lambda 1027 A multiplets by the Birge-Hopfield band system of N2 are investigated. This process is shown to depend sensitively on the N2 vibrational temperature and to cause characteristic changes in the OI EUV emission spectrum in auroras and in the sunlit F-region at high exospheric temperatures.

Zipf, E. C.↗
NASA NTRS1979

Ultraviolet night airglow of Venus

The night airglow spectrum of Venus in the ultraviolet is dominated by the V-prime = 0 progressions of the gamma and delta bands of nitric oxide. The bands are produced by two-body radiative recombination of nitrogen and oxygen atoms. Since the source of these atoms is in the dayside thermosphere, the night airglow is a tracer of the day-to-night thermospheric circulation. The airglow is brightest at equatorial latitudes and at longitudes on the morning side of the antisolar meridian.

Stewart, A. I.↗
NASA NTRS1979

IUE and visual spectrophotometry of 3C 120 and Markarian 79

Low-resolution ultraviolet spectra have been obtained with IUE and combined with similar visual observations. The continua of both objects show an excess of flux in the neighborhood of 3000 A. In both objects there appears to be a depression near 2200 A, which is probably the ultraviolet interstellar absorption feature centered at 2175 A. The strength of this feature suggests E(B-V) values of 0.22 and 0.38 for Mrk 79 and 3C 120, respectively. The ratio of intensities of L-alpha to H-beta is abnormally small compared with the radiative recombination ratio; the discrepancy is removed in 3C 120 and much reduced in Mrk 79 if the reddening values just mentioned are adopted. The C IV to L-alpha line intensity ratio is abnormally high in these objects compared with that for quasars. Although both objects have strong Fe II lines in the visual, there is no Fe II absorption or emission in the ultraviolet.

Oke, J. B.↗
NASA NTRS1979

Simplified model of a volumetric direct nuclear pumped He-3-Ar laser

A physical model for the volumetric nuclear pumped He-3-Ar laser is described and the source terms arising from the He-3(n, p)H-3 reaction as well as the rate equations are given. Steady state solutions are derived for the densities of the species vs the fraction of argon. The dominant pumping mechanism is found to be collisional radiative recombination of the argon atomic ions and subsequent cascading into the upper laser level. The reactions following recombination are described and an expression is derived for the difference in densities of the argon 3d and 4p levels. Comparison with experiments shows good agreement.

Harries, W. L.↗
NASA NTRS1980

The equatorial aurora in the extreme ultraviolet

The extreme ultraviolet telescope on the Apollo-Soyuz mission observed the equatorial aurora from an altitude of 220 km on four separate occasions in July 1975, in quiet geomagnetic conditions (Ap = 6). In all cases signals well above ambient background in the 50-150, 114-150, 170-600, and 500-780 A bands were recorded as the spacecraft moved across the equator and the instrument viewed the atmosphere below it. The observed emissions are confined to a band roughly 10 to 20 deg in width with the peak emission occurring in the range -15 to +2 deg magnetic latitude. No enhancement on the 1350-1550 A channel was noted. The observed signals are interpreted as recombination radiation of energetic helium and, possibly, oxygen ions originating in the terrestrial ring current.

Paresce, F.↗
NASA NTRS1981

Meteoric ion production near Jupiter

Meteoric ion layer formation within the Jovian atmosphere is examined with attention to metallic ion production in the lower ionosphere. The Fe(+) impact ionization rate within the Jovian atmosphere peaks above the mesopause with a magnitude of approximately 0.5 cu cm/sec and is much less than the ambient ionosphere photoionization rates near the late afternoon Pioneer 10 ionosphere occultation. Charge exchange of the ablated neutral Fe atoms with ambient ions can result in an Fe(+) production rate of about 10 cu cm/sec. Ignoring transport, steady state Fe(+) density maxima of about 10,000 or 1,000,000 cu cm can be maintained when Fe(+) loss is through radiative association or radiative recombination respectively. Even if an estimated lower limit to the incident meteoroid flux is used based on a meteoroid spatial density which does not vary with distance from the sun, the corresponding Fe(+) peak densities are 1,000 and 500,000 cu cm, respectively. Meteoric ion densities may thus be important in the Jovian lower ionosphere.

Grebowsky, J. M.↗
NASA NTRS1981

The ionization state in a gas with a non-Maxwellian electron distribution

The inferred degree of ionization of a gas is often used in astrophysics as a diagnostic of the gas temperature. In the solar transition region and corona, in the outer atmospheres of cool stars, and in some portions of the interstellar medium), photoionization can be neglected, and the ionization state is fixed by the balance between ion-electron collisional ionization and dielectronic and/or radiative recombination. Under these conditions, higher degrees of ionization result from higher energy ion-electron collisions which are common in a high temperature gas. Actually, ionization occurs through collisions with electrons that have kinetic energies greater than the ionization potential of the given ion, and so the ionization rate depends on to the number of such high-energy electrons in the tail of the electron velocity distribution. High-velocity electrons move across large distances between effective coulomb collisions, and, in a strong temperature or density gradient, the tail can be overpopulated relative to Maxwell-Boltzmann distribution of equivalent energy density. Thus, the ionization rate can also be greatly increased. These effects for a parameterized form of the electron distribution function with an enhanced high-velocity tail, namely the kappa distribution are illustrated.

Owocki, S. P.↗
NASA NTRS1985

Population mechanisms for the He(+) n = 3 levels determined from measurements of the solar 1640 A emission

The 1640 A line profile is analyzed using solar data obtained by the NRL slit spectrograph on Skylab. Data from coronal hole regions, quiet sun regions, active regions, prominence regions, and flare regions are presented. The relative densities of 3s, 3p, and 3d levels are determined from the data, and the dominant population mechanisms for these levels are inferred using a rate equation model for the He(+) ion. The relative importance of collisional excitation, radiative recombination, and photoexcitation is determined for each of the solar regions.

Seely, J. F.↗