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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Physics of compact nonthermal sources. II Determination of physical parameters

A set of restrictions upon the magnetic field and visible angular size of a canonical nonthermal source (one whose emission is incoherent electron synchrotron) has been obtained in another paper. The degree of electron anisotropy and magnetic field disordering were left as free parameters. In the present paper these restrictions are applied to the variable compact radio sources 3C 84, 3C 120, 3C 273, 3C 279, 3C 454.3, CTA 102, 3C 446, PKS 2134 + 004, VRO 42.22.01, and OJ 287. The theoretically derived angular sizes for these sources are in reasonable agreement with published VLBI sizes, provided that the characteristic pitch angles are not extremely small. The magnetic field strengths determined from the degree of circular polarization are generally compatible with field strengths derived from the self-absorption and synchrotron self-Compton conditions, again provided that the pitch angles are not too small. It is argued that the self-absorption optical depth cannot be much less than unity near the low-frequency turnovers; otherwise, serious discrepancies with observation result.

Jones, T. W.↗
NASA NTRS1983

Evaluation of non-intrusive flow measurement techniques for a re-entry flight experiment

This study evaluates various non-intrusive techniques for the measurement of the flow field on the windward side of the Space Shuttle orbiter or a similar reentry vehicle. Included are linear (Rayleigh, Raman, Mie, Laser Doppler Velocimetry, Resonant Doppler Velocimetry) and nonlinear (Coherent Anti-Stokes Raman, Laser-Induced Fluorescence) light scattering, electron-beam fluorescence, thermal emission, and mass spectroscopy. Flow-field properties were taken from a nonequilibrium flow model by Shinn, Moss, and Simmonds at the NASA Langley Research Center. Conclusions are, when possible, based on quantitative scaling of known laboratory results to the conditions projected. Detailed discussion with researchers in the field contributed further to these conclusions and provided valuable insights regarding the experimental feasibility of each of the techniques.

Miles, R. B.↗
NASA NTRS1992

A hybrid thermal/nonthermal model for the energetic emissions from solar flares

We present an alternative interpretation of the high-resolution solar flare spectra obtained in 1981 by Lin et al. In our interpretation electron heating and acceleration are simultaneous and physically linked, rather than heating being a secondary effect of particle acceleration. We show that the observed properties of solar flare X-ray emission can in general be explained through the Joule heating and electric field acceleration of runaway electrons in current channels. We have fitted a hybrid thermal/nonthermal electron distribution, consisting of hot isothermal electrons with a nonthermal tail of runaway electrons, to representative spectra obtained by Lin et al. The hybrid model relaxes the electron number and energy flux requirements for the hard X-ray emission over those of a purely nonthermal model. The low-energy 'cutoff' to the nonthermal part of the X-ray spectrum is determined by the critical velocity in the electron distribution above which runaway acceleration occurs. We relate the fit parameters to the physical properties (such as the electric field strength in the current channels) of the acceleration region.

Holman, Gordon D.↗
NASA NTRS2003

Results From Cs Activated GaN Photocathode Development for MCP Detector Systems at GSFC

We describe the development of high quantum efficiency W photocathodes for use in large area two dimensional microchannel plate based detector arrays to enable new W space astronomy missions. Future W missions will require improvements in detector sensitivity, which has the most leverage for cost-effective improvements in overall telescope/instrument sensitivity. We use new materials such as p-doped GaN, AIGaN, ZnMgO, Sic and diamond. We have currently obtained QE values > 40 % at 185 nm with Cesiated GaN, and hope to demonstrate higher values in the future. By using controlled internal fields and nano-structuring of the surfaces, we plan to provide field emission assistance for photoelectrons while maintaining their energy distinction from dark noise electrons. We will transfer these methods from GaN to ZnMgO, a new family of wide band-gap materials more compatible with microchannel plates. We also are exploring technical parameters such as doping profiles, internal and external field strengths, angle of incidence, field emission assistance, surface preparation, etc.

Norton, Tim↗
NASA NTRS1991

Electric field measurements during supercharging events on the MAIMIK rocket experiment

Data are presented on the evolution of electric field structures near the MAIMIK mother-daughter tethered rocket during active electron emissions from the rocket, which consisted of two separately instrumented payload sections which were detached in-flight. The results of the E field data analysis show that the temporal behavior of the fields was consistent with ion responses to the rapid charging of the active payload. A qualitative model of the interaction between the active payload and the ambient plasma background is developed to explain the electric field data, including the apparent causal relationship between the radial and tangential impulse components.

Denig, W. F.↗
NASA NTRS1975

Direction-finding measurements of type 3 radio bursts out of the ecliptic plane

Direction-finding measurements with the plasma wave experiments on the HAWKEYE 1 and IMP 8 satellites are used to find the source locations of type 3 solar radio bursts in heliocentric latitude and longitude in a frequency range from 31.1 kHz to 500 kHz. Using an empirical model for the emission frequency as a function of radial distance from the sun the three-dimensional trajectory of the type 3 radio source can be determined from direction-finding measurements at different frequencies. Since the electrons which produce these radio emissions follow the magnetic field lines from the sun these measurements provide information on the three-dimensional structure of the magnetic field in the solar wind. The source locations projected into the ecliptic plane follow an Archimedian spiral. Perpendicular to the ecliptic plane the source locations usually follow a constant heliocentric latitude. With direction-finding measurements of this type it is also possible to determine the source size from the modulation factor of the received signals.

Baumback, M. M.↗
NASA NTRS1988

Low-energy Galactic centre gamma-rays from low-mass X-ray binaries

Nonthermal processes in low-mass X-ray binaries concentrated in the Galactic bulge are proposed as the direct source of the three continuum components of the emission from the Galactic center region (GCR) and also, possibly, as the indirect source of the 511-keV electron-positron annihilation line. It is suggested that the softer power-law component of the GCR continuum arises from synchrotron emission of relativistic electrons in the strongly nonuniform magnetic field of the neutron star and, more tentatively, that the MeV bump is the result of interaction of harder gamma rays with power-law photons. The hardest power law may be due to Compton scattering of relativistic electrons or photons.

Kluzniak, W.↗
NASA NTRS2001

Divine-garrett model and Jovian synchrotron emission

Simulations of synchrotron emission from relativistic electrons trapped in Jupiter's magnetic field are used to evaluate the energetic electron distribution of the Divine-Garrett Jupiter radiation belt model at radial distances less than 4 Jovian radii.

Radial diffusion-models electrons jupiter↗
NASA NTRS2001

Modeling Jupiter's synchrotron radiation

We have constructed a computer model to simulate synchrotron emission from relativistic electrons trapped in Jupiter's magnetic field.

Synchrotron emmission magnetic field↗
NASA NTRS1973

Energetic particles as a source of flare energy

The production of solar flares by the reservoir of energy generated by sunspot magnetic fields is discussed. Electrons play an important part in the flare process since high energy electrons are responsible for X-ray and radio emission. The energy required in the form of protons trapped in sunspot magnetic fields to produce large solar flares is analyzed. A trigger mechanism is described where the particles can be rapidly dumped into the chromosphere when fast electrons are produced by Coulomb collisions and through pion production by the highest energy protons.

Elliot, H.↗
NASA NTRS1996

Electron Emission Observations from As-Grown and Vacuum-Coated Chemical Vapor Deposited Diamond

Field emission has been observed from chemical vapor deposited diamond grown on Mo and Si substrates. Emission was observed at fields as low as 20 kV/cm. The samples were tested in the as-grown form, and after coating with thin films of Au, CsI, and Ni. The emission current was typically maximum at the onset of the applied field, but was unstable, and decreased rapidly with time from the as-grown films. Thin Au layers, approximately 15 nm thick, vacuum deposited onto the diamond samples significantly improved the stability of the emission current at values approximately equal to those from uncoated samples at the onset of the applied field. Thin layers of CsI, approximately 5 nm thick, were also observed to improve the stability of the emission current but at values less than those from the uncoated samples at the onset of the applied field. While Au and CsI improved the stability of the emission, Ni was observed to have no effect.

Lamouri, A.↗
NASA NTRS1986

The Venus ultraviolet aurora - Observations at 130.4 nm

Continuous and highly variable emissions at 130.4 nm are observed on the night-side of Venus by the Pioneer Venus Orbiter ultraviolet spectrometer. The emissions have a typical intensity of 10-20 R, occasionally exceed 100 R, and have a dawn-dusk symmetry in brightness during periods of unusual intensity. Possible energy source for this 'aurora' is the precipitation of suprathermal electrons. Field and particle observations suggest an additional source for the brightest emissions which is associated with solar flares and/or interplanetary shocks.

Phillips, J. L.↗
NASA NTRS1972

Calculated limits for particle fluxes in Jupiter's Van Allen belts

Electron and proton fluxes in Jupiter's radiation belts are calculated, along with the envelopes of dose rates. The following assumptions are made: the particles in the Jupiter belts are influenced only by the magnetic field of the planet; the particles act correspondingly to the particles in the Earth's belts and the Earth's belts can be used as a model; the magnetic field of Jupiter is essentially a dipole; the radiation of a decimetric nature received from Jupiter is synchrotron radiation due to the electrons, and to a first approximation it is emitted isotropically; and the strength of the emission in the decimetric wavelength range gives an upper bound considering how strong the field can be and how many electrons there are. The point dose rates for tissue and 0.1 gram/cm aluminum shielding at about 3 Jupiter radii are 10000 rads/hr for electrons and 1000 rads/hr for protons.

Haffner, J.↗
NASA NTRS1993

Electron cyclotron maser emission at oblique angles

Possible causes of observations of electron cyclotron maser emissions (ECMEs) at oblique angle (about 60 deg) to the magnetic field are investigated. The paper discusses general concepts of ECME in terms of resonant ellipses and considers electron distributions required to produce ECME at oblique angles, as well as the ways in which the most favorable of these distributions may be produced. A mechanism is proposed that might produce an appropriate 'spiraling beam' distribution, with a peak in velocity space at speed v - v(0) and pitch angle alpha = alpha(0) not equal to 0.

Melrose, D. B.↗
NASA NTRS1973

Work function measurements by the field emission retarding potential method.

Description of the theoretical foundation of the field electron retarding potential method, and review of its experimental application to the measurement of single crystal face work functions. The results obtained from several substrates are discussed. An interesting and useful fallout from the experimental approach described is the ability to accurately measure the elastic and inelastic reflection coefficient for impinging electrons to near zero-volt energy.

Strayer, R. W.↗
NASA NTRS1990

A relationship between the integrated CO intensity and the radio continuum emission in spiral galaxies

In an effort to determine the role played by cosmic ray electrons and interstellar radiation fields on the collapse of molecular clouds, a survey was begun to investigate the relationship between the radio continuum brightness emission and the integrated CO intensity in spiral galaxies. The investigation was done on two scales; a global galaxy to galaxy comparison of integrated disk values, and a ring-averaged study over the disks of individual galaxies. For the large-scale survey, radio continuum flux densities integrated over the full disk at 1.49 GHz were taken from Condon (1987) and the total CO fluxes were taken from Verter (1985). The galaxies with values included in the two catalogs are displayed. It can be seen that a good correlation exists between the integrated CO emission and radio continuum emission.

Adler, D. S.↗
NASA NTRS1979

Possible radio emission mechanism for pulsars

A mathematical model is presented and discussed as a possible mechanism to describe radio emission from pulsars. The model determines that the magnetic field in the neutron proton electron (npe) layer of a neutron star results from a quasistationary eddy current of superconducting and normal protons relative to normal electrons, which generates radio emission by the Josephson effect. The radiation propagates in the magnetically active medium, from the optically thick npe layer to the magnetosphere through breaks in the crust. As a result, hot radio spots form on the surface of the star, and a radiation pattern forms near the magnetic poles, the cross section of which gives the observed pulse structure. Due to the specific properties of the mechanism, variations of the quasistationary current are converted to amplitude frequency variations of the radiation spectrum. Variations of the fine structure of the spectrum pulse amplitude and spectral index, as well as their correlation are discussed.

Kovalev, Y. A.↗