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At least 217 records · Page 12

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NASA NTRS1977

Thermodynamics of Thomas-Fermi screened Coulomb systems

We obtain in closed analytic form, estimates for the thermodynamic properties of classical fluids with pair potentials of Yukawa type, with special reference to dense fully ionized plasmas with Thomas-Fermi or Debye-Hueckel screening. We further generalize the hard-sphere perturbative approach used for similarly screened two-component mixtures, and demonstrate phase separation in this simple model of a liquid mixture of metallic helium and hydrogen.

Firey, B.↗

NASA NTRS1978

An investigation of the reduction of carbon dioxide in a silent electric discharge

The reduction of CO2 to O2 and CO in a silent electric discharge was studied. It was found that current alone (in the ionized plasma induced by the silent electric discharge) was reponsible for the CO2 reduction process. Voltage and frequency were important only in so far as they induced current in the plasma. Pressure and temperature were of minimum influence in the process. The large power consumption in the process was recognized as resulting from the low power factor of the reactor vessel which electrically behaved like a capacitor. The power factor was subsequently improved by adding an inductive element to make the reactor vessel capacitance part of a resonant circuit. It was found that the CO2 reduction process was most efficient in terms of power vs reduction rate when a voltage was employed that was only slightly higher than that needed to induce the plasma.

Luce, R. S.↗

NASA NTRS1978

Discovery of iron line emission in the Hercules X-1 low-state spectrum with HEAO 1

Results are reported for HEAO 1 observations of Her X-1 which confirm the existence of the iron line-emission feature discovered with OSO 8 in the high-state spectrum and also reveal an iron emission feature in the low-state spectrum. The iron line-emission feature in the low-state spectrum was detected at a significance level greater than 5 sigmas and is characterized by a line energy of 6.4 keV, an equivalent width larger than that of the high-state line, a binary phase dependence of the line intensity, and a line width that is intrinsically narrow in contrast to the broad high-state line. The observed line energy and narrow line width are shown to be consistent with an interpretation in terms of fluorescence emission due to X-ray heating. It is suggested that HZ Her contributes no more than about 10% of the predicted line flux, that this emission is reflected from a highly ionized plasma, and that a hot coronal gas is an important source of the low-state X-rays.

Pravdo, S. H.↗

NASA NTRS1979

The IR energy distribution of SS433

The infrared energy distribution of the emission-line object SS 433 in the range 1 to 10 microns is determined. Photometric measurements were made with a 2.2-m telescope using an InSb detector and a 6-arcsec diaphragm for 1.2- to 4.8-micron observations and a gallium-doped germanium bolometer with a 6-arcsec diaphragm for 8- to 13-micron observations. Results indicate the presence of substantial night-to-night flux density and color variations, and a decrease in flux density at wavelengths greater than 5 microns. The energy cut-off is used to estimate a physical size of 1 AU for the emitting region and an electron density of 10 to the 11th/cu cm on the basis of a model of reddened free-free emission from an ionized plasma, with self-absorption of radiation over 5 microns. The greater variability observed at longer wavelengths is also interpreted in terms of this model.

Wynn-Williams, C. G.↗

NASA NTRS1979

Laboratory simulation of ionospheric double-layers

A mechanism for double-layer formation in partially- or fully-ionized plasmas is described, founded on beam-plasma interaction: RF growth along the beam excites a rectified, ponderomotive electric field, which in turn causes charge separation. Laboratory studies of the mechanism are described.

Crawford, F. W.↗

NASA NTRS1980

An empirical polytrope law for solar wind thermal electrons between 0.45 and 4.76 AU - Voyager 2 and Mariner 10

In this paper empirical evidence is presented that between 0.4 and 5 AU the thermal portion (but not all) of the solar wind electron population obeys a polytrope relation. It is also shown that this functional relationship is a member of a broader class of possible laws required of a steady state, fully ionized plasma whose proper frame electric field is dominated by the polarization electric field. The empirically determined, thermodynamically interesting value of the polytrope index (1.175) is virtually that predicted (1.16) by the theoretical considerations of Scudder and Olbert (1979). Strong, direct, empirical evidence for the nearly isothermal behavior of solar wind electrons as has been indirectly argued in the literature for some time is provided.

Sittler, E. C., Jr.↗

NASA NTRS1983

Non-equilibrium ionization in Puppis A

High resolution X-ray spectroscopy of the brightest knot of emission in the Puppis A supernova remnant shows that it is made up of ionizing plasma, far from equilibrium. Flux measurements in several X-ray lines make it possible to determine the nonequilibrium conditions: electron temperature, ion populations, and time since the knot was heated by the supernova shock. Imaging and spectroscopic data from the Einstein Observatory together suggest that this knot is a cloud of density about 10 per cu cm which has recently been shocked to a temperature 7,000,000 K. Radio and optical data on the region appear consistent with this picture.

Winkler, P. F.↗

NASA NTRS1986

Laboratory observations of ion cyclotron waves associated with a double layer in an inhomogeneous magnetic field

Observations of coherent electrostatic ion cyclotron (EIC) waves associated with a strong, magnetized double layer are presented. The double layers are produced in a weakly ionized argon plasma by applying a positive potential to an electrode plate located in the diverging magnetic field region of a cylindrical plasma column. Ionization within the electrode sheath is essential to the formation of these double layers. The resulting V-shaped potential structures have extended parallel, oblique and perpendicular (to B) electric field components. The frequency of the ion cyclotron instability is dependent upon the magnetic field strength at the position of the parallel potential structure. The properties of EIC waves in the presence of the double layer are discussed in relation to the possible excitation mechanisms (field-aligned currents, ion and electron beams, and perpendicular E fields).

Alport, M. J.↗

NASA NTRS1987

Rapid temporal evolution of radiation from non-thermal electrons in solar flares

Solutions of the time dependent Fokker-Planck equation was found for accelerated electrons undergoing Coulomb collisions in a magnetized, fully ionized plasma. An exact solution was found for arbitrary pitch angle and energy distribution in a uniform background plasma. Then, for an inhomogeneous plasma, a solution was found for particles with small pitch angles. These solutions were used to calculate the temporal evolution of bremsstrahlung x-rays from short bursts of nonthermal electron beams, and these spectra were compared with observed high time resolution spectra of short timescale solar hard x-ray bursts. It is shown that the observed softening in time of the spectra rules out a homogeneous background and therefore the possibility of electrons being confined to the corona either because of converging magnetic field or high densities. The inhomogeneous solution was also applied to a model with constant coronal density and exponentially rising chromospheric density. The spectra are shown to be consistent with that produced by a collimated beam of electrons accelerated in the corona with certain given conditions. These conditions could be violated if large pitch angle electrons are present.

Lu, Edward T.↗

NASA NTRS1986

Modelling extended chromospheres

Attention is given to the concept that the warm, partially ionized plasma (presently called chromosphere) associated with such stars as Alpha Boo and Rho Per extends outwards at least several photospheric radii. Calculations are presented for the Mg II K line in light of two input model atmospheres. Specific predictions are deduced from the results obtained by each of the two models.

Linsky, J. L.↗

NASA NTRS1987

X-ray and ultraviolet radiation from accreting white dwarfs. IV - Two-temperature treatment with electron thermal conduction

Results are reported from two-temperature calculations of the structures and X-ray spectra of radiation shocks generated by accretion onto nonmagnetic white dwarfs. The approach was necessitated by the domination of bremsstrahlung in the emission region by Compton cooling. Features of the shock model, which includes steady, spherical infall of fully ionized plasma and dominance of the stand-off shock by collisional processes, are summarized. A maximum hard X-ray temperature of about 50 keV and a maximum hard X-ray luminosity of 2 x 10 to the 36th ergs/sec were obtained. The results prove that the bulk of accretion energy cannot be transported to the star by electron thermal conduction, provided that bremsstrahlung cooling is dominant over cyclotron cooling.

Imamura, J. N.↗

NASA NTRS1988

Rapid temporal evolution of radiation from nonthermal electrons in solar flares

Solutions of the time dependent Fokker-Planck equation was found for accelerated electrons undergoing Coulomb collisions in a magnetized, fully ionized plasma. An exact solution was found for arbitrary pitch angle and energy distribution in a uniform background plasma. Then, for an inhomogeneous plasma, a solution was found for particles with small pitch angles. These solutions were used to calculate the temporal evolution of bremsstrahlung X-rays from short bursts of nonthermal electron beams, and these spectra were compared with observed high time resolution spectra of short timescale solar hard X-ray bursts. It is shown that the observed softening in time of the spectra rules out a homogeneous background and therefore the possibility of electrons being confined to the corona either because of converging magnetic field or high densities. The inhomogeneous solution was also applied to a model with constant coronal density and exponentially rising chromospheric density. The spectra are shown to be consistent with that produced by a collimated beam of electrons accelerated in the corona with certain given conditions. These conditions could be violated if large pitch angle electrons are present.

Lu, Edward T.↗

NASA NTRS1988

Self-excitation of auroral arcs in a three-dimensionally coupled magnetosphere-ionosphere system

This paper presents the first full three-dimensional dynamic simulation of auroral arc formation. The magnetospheric and ionospheric dynamics are represented by one-fluid magnetohydrodynamic equations and two-fluid weakly ionized plasma equations, respectively. The feedback coupling between magnetospheric Alfven waves and ionospheric density waves results in a spontaneous generation of longitudinally elongated striations of field-aligned currents and ionospheric electron densities, which compare very well with many features of quiet auroral arcs.

Watanabe, Kunihiko↗

NASA NTRS1990

An assessment of the potential of continuous-wave ranging for measuring the distance to a highly reflective, infinite sheet

The feasibility of a continuous-wave, distance-measuring technique for measuring the distance from a spacecraft antenna to a highly ionized plasma surface is examined. The reflection coefficient angle is computed for several aperture models. It is concluded that aperture size and the presence of a nonablating dielectric cover over the antenna are critical factors.

Hearn, C. P.↗

NASA NTRS1991

Adiabatic properties of pulsating DA white dwarfs. I - The treatment of the Brunt-Vaisala frequency and the region of period formation

The fundamental issue of the region of period formaton in a degenerate star is examined, with special attention given to the treatment of the Brunt-Vaisala frequency. It is shown that, in order to obtain reliable numerical results in degenerate stellar models, the Brunt-Vaisala frequency must be appropriately transformed, because it is defined in terms of a difference between two numbers which become nearly equal in highly degenerate matter, causing serious numerical problems and systematic errors. An alternative expression is derived, which is valid for multicomponent nonideal partially degenerate and partially ionized plasmas such as those encountered in white dwarf envelopes. This expression is used to compute the period structure of the same white dwarf considered by Pesnell (1987). It is shown that the implicit numerical differencing used in the Lagrangian pulsation code of Pesnell leads to very serious difficulties when used with models of degenerate stars.

Brassard, P.↗

NASA NTRS1991

Fluid hydrogen at high density - Pressure dissociation

A model for the Helmholtz free energy of fluid hydrogen at high density and high temperature is developed. This model aims at describing both pressure and temperature dissociation and ionization and bears directly on equations of state of partially ionized plasmas, as encountered in astrophysical situations and high-pressure experiments. This paper focuses on a mixture of hydrogen atoms and molecules and is devoted to the study of the phenomenon of pressure dissociation at finite temperatures. In the present model, the strong interactions are described with realistic potentials and are computed with a modified Weeks-Chandler-Andersen fluid perturbation theory that reproduces Monte Carlo simulations to better than 3 percent. Theoretical Hugoniot curves derived from the model are in excellent agreement with experimental data.

Saumon, Didier↗

NASA NTRS1991

Effect of an arcjet plume on satellite reflector performance

The effect of an arcjet plume on the performance of satellite reflector antennas is studied. The arcjet plume is modeled as a weakly ionized plasma. The spatial permittivity distribution of the plume is approximated using the measured electron density profile and a cold plasma model. Geometrical optics is applied to determine the ray paths as well as the transmitted fields through the inhomogeneous plume. The ray optics results are compared against several exact solutions for scattering from inhomogeneous dielectrics, and good agreement is observed for sufficiently large scatterer size. The far-field antenna patterns of the reflector in the presence of the plume are calculated from the transmitted ray fields using a ray-tube integration scheme. For arcjet prototypes in the 1-kW class, the plume effect on the antenna performance is small. As the electron density increases, the main beam and sidelobe level gradually degrade. The main beam also tends to squint away from the plume region.

Ling, Hao↗

NASA NTRS1992

Kinetic description of electron beams in the solar chromosphere

We formulate the relativistic Fokker-Plank equation for a beam of accelerated electrons interacting with a partially ionized plasma. In our derivation we conserved those terms contributing to velocity diffusion and found that this effect cannot be neglected a priori. We compute the terms accounting for elastic and inelastic collisions with neutral hydrogen and helium. Collisions with neutral hydrogen are found to be dominant throughout the chromosphere, except at the uppermost layers close to the transition region. As an application, we compute the loss of energy and momentum for a power-law beam impinging on the solar chromosphere, for a particular case in which the Fokker-Planck equation can be integrated analytically. We find that most of the beam energy is deposited in a relatively thin region of the chromosphere, a result which is largely insensitive to the theoretical method employed to compute the energy deposition rate.

Gomez, Daniel O.↗