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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

The effect of the solar cycle on the maintenance of the nightside ionosphere of Venus

The behavior of the Venus nightside ionosphere at solar maximum and solar minimum is discussed based on Pioneer Venus radio occultation measurements. Although some solar maximum measurements are similar to those observed at minimum, which have an average peak density of about 7000/cu cm, others show much higher peak densities, reaching values of about 40,000/cu cm. These elevated peak densities also occur at higher altitudes. The integrated electron column densities for these measurements indicate the presence of substantial ionization above the main peak. The magnitudes of both the peak density and the integrated content above the peak are anticorrelated with the solar wind dynamic pressure, indicating that these enhancements during solar maximum are due to transterminator transport of O(+) ions from the dayside. The resulting ionization peak can be many times the concentration produced by energetic electron fluxes impacting the neutral atmosphere on the nightside.

Kliore, Arvydas J.↗
NASA NTRS1988

Ionisation cross sections of rare-gas atoms by electron impact

A pulsed electron beam and ion extraction method is used to measure normalized values of partial ionization cross sections for rare gases from threshold to 1000 eV. Cross sections obtained for singly ionized species are used to calibrate the mass transmission efficiency of the ion extraction/analyzer/detection system by the relative flow technique, and this mass transmission curve is then used to determine the absolute cross sections of the multiply ionized species. Total ion cross sections are found by summation of the individual partial cross sections with proper weighting for charge.

Krishnakumar, E.↗
NASA NTRS1987

Titan's atomic nitrogen torus - Inferred properties and consequences for the Saturnian aurora

A population of warm, approximately keV N(-) ions furnishing the greater part of the mass and energy input to the Titan magnetosphere, at a power level of 2 x 10 to the 11th W, is generated by ionization of Titan's nitrogen torus by both solar EUV photons and magnetospheric electron impact. Magnetospheric electrons also accelerate pickup ions to keV energies by means of lower hybrid emissions, on a time scale comparable to the plasma residence time. It is presently suggested that the aurora is excited by ion and electron precipitation from the outer magnetosphere; the energetics in question can be accounted for entirely on the basis of the titanogenic plasma supply proper to a corotation-dominated magnetosphere.

Barbosa, D. D.↗
NASA NTRS1988

Ion formation by electron impact

Dissociative attachment and polar dissociation cross sections were measured for the following molecules: HC1, NO, N2O, C6H6, SiH4, Si2H6, and LiH. Direct ionization and dissociative ionization cross sections were determined for the following molecules: H2, D2, N2, O2, He, Ne, Ar, Kr, Xe, H2O, Co, CO2, CH4, SiH4, Sih4, Si2H6, N2*, and NH3. An experimental apparatus for a pulsed extraction technique was fabricated and successfully tested.

Srivastava, Santosh K.↗
NASA NTRS1987

Tunneling-assisted impact ionization for a superlattice

Tunneling-assisted impact ionization across the conduction-band-edge discontinuity of quantum-well heterostructures is investigated and applied to a new superlattice structure. Multiquantum-well structures where the quantum-well regions are heavily doped and the undoped barrier regions are essentially insulating were considered. Incident hot electrons due to the applied electric field perpendicular to the heterointerface interact with the two-dimensional electrons confined to the quantum wells through Coulomb force. The resultant electrons can either have enough energy to get out of the wells or to tunnel through the triangular barriers. A new analytical approximation for the impact ionization rate is given which compares favorably with numerical results. The tunneling-assisted impact ionization rates and the ionization coefficients are calculated. It is shown that the tunneling effect reduces the ionization threshold and enhances the ionization rate significantly.

Chuang, S. L.↗
NASA NTRS1976

Electron impact excitation coefficients for laboratory and astrophysical plasmas

Electron impact excitation rate coefficients have been obtained for a number of transitions in highly ionized ions of interest to astrophysical and laboratory plasmas. The calculations were done using the method of distorted waves. Results are presented for various transitions in highly ionized Ne, Na, Al, Si, A, Ca, Ni and Fe.

Davis, J.↗
NASA NTRS1977

Periodic variations in Io's sodium and potassium clouds

Spectra of Io's sodium cloud taken 7.5 to 45 sec north and south of Io at a variety of magnetic latitudes of Io obtained during 17 nights of the 1975 apparition of Jupiter confirm the weakening of the cloud in the neighborhood of Jupiter's magnetic equator. The time scale for these changes is 1.5 hours or less. For impact ionization to cause these changes, a minimum flux of 100 billion/sq cm/sec 15 eV electrons or 40 billion/sq cm/sec 8 keV protons is required. Sodium velocities near 10 km/sec are required to explain the replenishment of ionized regions of the cloud. We also obtained spectra of Io's potassium cloud on eight nights during the 1975 apparition. This cloud appears to behave like the sodium cloud as Io's magnetic latitude varies. The potassium, however, may become much weaker as Io passes through the magnetic equator. The brightest intensity measured for the 7665 A resonance line was about 1 kR at a point 7.5 sec east of Io.

Trafton, L.↗
NASA NTRS2011

The EUV Emission in Comet-Solar Corona Interactions

The Atmospheric Imaging Assembly (AlA) on the Solar Dynamics Observatory (SDO) viewed a comet as it passed through the solar corona on 2011 July 5. This was the first sighting of a comet by a EUV telescope. For 20 minutes, enhanced emission in several of the AlA wavelength bands marked the path of the comet. We explain this EUV emission by considering the evolution of the cometary atmosphere as it interacts with the ambient solar atmosphere. Water ice in the comet rapidly sublimates as it approaches the Sun. This water vapor is then photodissociated, primarily by Ly-alpha, by the solar radiation field to create atomic Hand O. Other molecules present in the comet also evaporate and dissociate to give atomic Fe and other metals. Subsequent ionization of these atoms can be achieved by a number of means, including photoionization, electron impact, and charge exchange with coronal protons and other highly-charged species. Finally, particles from the cometary atmosphere are thermalized to the background temperature of the corona. Each step could cause emission in the AlA bandpasses. We will report here on their relative contribution to the emission seen in the AlA telescopes.

Bryans, Paul↗
NASA NTRS1996

The 825-1110 angstroms EUV spectrum of Venus

On 15 August 1994 we launched the EUVS sounding rocket payload to observe the 825-1110 angstrom region of Venus's far ultraviolet airglow spectrum. The EUVS telescope/spectrograph obtained good data at five times higher spectral resolution than was previously available in the far ultraviolet. We present these data and compare our results to those obtained by the Galileo UVS and Venera 11/12 UV spectrophotometers. We identify several new spectral emission features, including both singly ionized nitrogen and molecular nitrogen in Venus's spectrum. We also see evidence for electron-impact-induced emission from CO. Finally, the EUVS data indicate that the "Ar" emissions detected in Venus's far ultraviolet spectrum by Venera 11/12 spectrophotometers are in fact not due to argon, thus eliminating the discrepancy between in situ and remote sensing measurements.

Flight Experiment↗
NASA NTRS1979

Theory of the intercluster medium

A set of computer models for the evolution of intercluster matter (ICM) in expanding cosmologies which includes processes of electron impact ionization/excitation, radiative and dielectronic recombination, thermal bremsstrahlung, Compton scattering, and photoionization is presented. The thermal, ionization, and excitation histories of major constituents were found as functions of time, and predictions of emission spectra, the possible distortions of the cosmic 3 K microwave background, and optical depths of absorption troughs in QSO continua at various redshifts were obtained. It is concluded that the present Friedmann cosmology can be dominated by hot intercluster matter which would contain H and He in their cosmic ratio and traces of heavier elements. For a narrow range of parameters it is possible to generate cosmologies with a closure density of ICM which do not exceed the diffuse X-ray measurements. In open cosmologies distortion of the 3 K microwave background occurs, while closed cases show negligible change from the unperturbed spectrum.

Sherman, R. D.↗
NASA NTRS1993

Molecular line emission models of Herbig-Haro objects. II - HCO(+) emission

We present time-dependent models of the chemistry and temperature of interstellar molecular gas clumps that are exposed to the radiation from propagating stellar-jet shocks. The X-ray, EUV, and FUV radiation from the shock initiates ion chemistry and also heats the gas in the clumps. Using representative parameters, we show that, on the shock transit time between the clumps, the abundances of the ionized molecular species that are produced in the clumps can exceed the values determined from steady state models by several orders of magnitude. Collisional excitation by the heated gas can lead to measurable line emission from several ionized species; as in previous investigations of X-ray-irradiated molecular gas, we find that electron impacts contribute significantly to this process. We apply these results to the interpretation of the HCO(+) line emission that has already been detected in several Herbig-Haro objects. We demonstrate that this picture provides a natural explanation of the fact that the line intensity typically peaks ahead of the associated shock, as well as of the reported low line-center velocities and narrow line widths. We tabulate several diagnostic line intensities of HCO(+) and other molecular species that may be used to infer the physical conditions in the emitting gas.

Wolfire, Mark G.↗
NASA NTRS1994

Semiclassical Study of the Wave Vector Dependence of the Interband Impact Ionization Rate in Bulk Silicon

We present calculations of the interband impact ionization rate calculated using a wave vector dependent (k-dependent) semiclassical formulation of the transition rate. The transition rate is determined using Fermi's golden rule from a two-body screened Coulomb interaction assuming energy and momentum conservation. The transition rate is calculated for the first two conduction bands of silicon by numerically integrating over the full Brillouin zone. The overlap integrals in the expression for the transition rate are determined numerically using a 15 band k-p calculation. It is found that the transition rate depends strongly on the initiating electron wave vector (k vector) and that the transition rate is greatest for electrons originating within the second conduction band than the first conduction band. An ensemble Monte Carlo simulation, which includes the numerically determined ionization transition rate as well as the full details of the first two conduction bands, is used to calculate the total impact ionization rate in bulk silicon. Good agreement with the experimentally determined electron ionization rate data is obtained.

Wang, Yang↗
NASA NTRS2006

Cross Sections for Electron Impact Excitation of Astrophysically Abundant Atoms and Ions

Electron collisional excitation rates and transition probabilities are important for computing electron temperatures and densities, ionization equilibria, and for deriving elemental abundances from emission lines formed in the collisional and photoionized astrophysical plasmas. Accurate representation of target wave functions that properly account for the important correlation and relaxation effects and inclusion of coupling effects including coupling to the continuum are essential components of a reliable collision calculation. Non-orthogonal orbitals technique in multiconfiguration Hartree-Fock approach is used to calculate oscillator strengths and transition probabilities. The effect of coupling to the continuum spectrum is included through the use of pseudostates which are chosen to account for most of the dipole polarizabilities of target states. The B-spline basis is used in the R-matrix approach to calculate electron excitation collision strengths and rates. Results for oscillator strengths and electron excitation collision strengths for transitions in N I, O I, O II, O IV, S X and Fe XIV have been produced

Tayal, S. S.↗
NASA NTRS1986

On the direct and dissociative excitation of the O(3s 3S0) state by electron impact on atomic and molecular oxygen

The ratio of the cross sections for the direct and dissociative excitation of the OI(3s 3S0-2p 3P; 1304 A wavelength) transition, sigma A/sigma D, are accurately determined, and the sigma A/sigma D ratio is directly normalized to the ratio of the O(+) and O2(+) ionization cross sections using a high-density diffuse gas source, an electrostatically focused electron gun, a vacuum-ultraviolet monochromater, and a quadrupole mass spectrometer for simultaneous optical and composition measurements. Using revised sigma A(1304 A) values calculated with new calibration standards, the shape of the cross section for the excitation of the O(3s 3S0) state agrees well with previous results, though the absolute magnitude of sigma A(1304 A) is smaller than the results of Stone and Zipf (1974) by a factor of 2.8. The revised cross sections agree well with recent quantum calculations when cascade excitation of the 3s 3S0 state is taken into account.

Zipf, E. C.↗
NASA NTRS1992

Formation of negative ions by electron impact on SiF4 and CF4

First measurements of cross sections for the formation of negative ions by electron attachment to SiF4 and CF4 are reported for an electron impact energy range of 0 to 50 eV. The energies at which the various ions appear and the positions at which the ionization efficiency curves peak have been obtained and compared with previous measurements. Thermochemical data have been used to predict and identify the various channels of dissociation. Cross sections for the production of ion pairs through the process of polar dissociation have also been measured.

Iga, I.↗
NASA NTRS1995

The Development of a Nonequilibrium Radiative Heat Transfer Computational Model for High Altitude Entry Vehicle Flowfield Methods

This final report will attempt to concisely summarize the activities and accomplishments associated with NASA Grant and to include pertinent documents in an appendix. The project initially had one primary and several secondary objectives. The original primary objective was to couple into the NASA Johnson Space Center (JSC) nonequilibrium chemistry Euler equation entry vehicle flowfield code, INEQ3D, the Texas A&M University (TAMU) local thermodynamic nonequilibrium (LTNE) radiation model. This model had previously been developed and verified under NASA Langley and NASA Johnson sponsorship as part of a viscous shock layer entry vehicle flowfield code. The secondary objectives were: (1) to investigate the necessity of including the radiative flux term in the vibrational-electron-electronic (VEE) energy equation as well as in the global energy equation, (2) to determine the importance of including the small net change in electronic energy between products and reactants which occurs during a chemical reaction, and (3) to study the effect of atom-atom impact ionization reactions on entry vehicle nonequilibrium flowfield chemistry and radiation. For each, of these objectives, it was assumed that the code would be applicable to lunar return entry conditions, i.e. altitude above 75 km, velocity greater, than 11 km/sec, where nonequilibrium chemistry and radiative heating phenomena would be significant. In addition, it was tacitly assumed that as part of the project the code would be applied to a variety of flight conditions and geometries.

Carlson, Leland A.↗
NASA NTRS1976

Efficiencies for production of atomic nitrogen and oxygen by relativistic proton impact in air

Relativistic electron and proton impact cross sections are obtained and represented by analytic forms which span the energy range from threshold to 1 GeV. For ionization processes, the Massey-Mohr continuum generalized oscillator strength surface is parameterized. Parameters are determined by simultaneous fitting to (1) empirical data, (2) the Bethe sum rule, and (3) doubly differential cross sections for ionization. Branching ratios for dissociation and predissociation from important states of N2 and O2 are determined. The efficiency for the production of atomic nitrogen and oxygen by protons with kinetic energy less than 1 GeV is determined using these branching ratio and cross section assignments.

Porter, H. S.↗