Search NASA⌕ Search

SEARCH · Search NASA

Results for “PLASMA SPECTRUM”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

HF-induced parametric decay in presence of field-aligned irregularities

A nonlinear theory for the saturation of the HF-induced parametric decay instability is formulated in terms of the normal modes of a field-aligned plasma wave duct. Coupling coefficients between modes are derived, and a saturation spectrum is computed numerically for a typical irregularity. The computed spectrum shows that unstable waves may propagate at larger angles to the magnetic field than would be the case in a regular medium. The relevance of this work to the experiment of Muldrew and Showen (1977), concerning the height of the HF-induced plasma line at Arecibo, is discussed. A possible explanation is also found for the 'decay line' feature in comparison with the 'broad bump' in the observed backscatter spectrum.

Rypdal, K.↗
NASA NTRS1979

Ionospheric wave spectrum measurements

The local spectrum S(k, omega) of either potential or electron-density fluctuations can be used to determine macroscopic-plasma characteristics such as the local density and temperature, transport coefficients, and drift current. This local spectrum can be determined by measuring the cross-power spectrum. The paper examines the practicality of using the cross-power spectrum analyzer on the Space Shuttle to measure ionospheric parameters. Particular attention is given to investigating the integration time required to measure the cross-power spectral density to a desired accuracy.

Harker, K. J.↗
NASA NTRS2007

Turbulence and Global Properties of the Solar Wind

The solar wind shows striking characteristics that suggest that it is a turbulent magnetofluid, but the picture is not altogether simple. From the earliest observations, a strong correlation between magnetic fluctuations and plasma velocity fluctuations as noted. The high corrections suggest that the fluctuations are Alven waves. In addition, the power spectrum of the magnetic fluctuation showed evidence of an inertial range that resembled that seen in fully-developed fluid turbulence. Alfven waves, however, are exact solutions of the equations of an incompressible magnetohydrodynamics. Thus, there was a puzzle: how can a magnetofluid consisting of Alfven waves be turbulent? The answer lay in the role of velocity shears in the solar wind that could drive turbulent evolution. Puzzles remain: for example, the power spectrum of the velocity fluctuations is less steep than the slope of the magnetic fluctuations. The plasma in the magnetic tail of Earth's magnetosphere also shows aspects of turbulence, as does the plasma in the dayside magnetosphere near the poles--the so-called dayside cusps.

Goldstein, Melvyn L.↗
NASA NTRS2010

Turbulence and Global Properties of the Solar Wind

The solar wind shows striking characteristics that suggest that it is a turbulent magnetofluid, but the picture is not altogether simple. From the earliest observations, a strong correlation between magnetic fluctuations and plasma velocity fluctuations was noted. The high corrections suggest that the fluctuations are Alfven waves. In addition, the power spectrum of the magnetic fluctuation showed evidence of an inertial range that resembled that seen in fully-developed fluid turbulence. Alfven waves, however, are exact solutions of the equations of incompressible magnetohydrodynamics. Thus, there was a puzzle: how can a magnetofluid consisting of Alfven waves be turbulent? The answer lay in the role of velocity shears in the solar wind that could drive turbulent evolution. Puzzles remain: for example, the power spectrum of the velocity fluctuations is less steep than the slope of the magnetic fluctuations. The plasma in the magnetic tail of Earth's magnetosphere also shows aspects of turbulence, as does the plasma in the dayside magnetosphere near the poles the dayside cusps. Recently, new analyses of high time resolution magnetic field data from Cluster have offered a glimpse of how turbulence is dissipated, thus heating the ambient plasma.

Goldstein, Melvyn L.↗
NASA NTRS1992

Precise tracking of the Magellan and Pioneer Venusorbiters by same-beam interferometry. Part 1: Dataaccuracy analysis

Simultaneous tracking of two spacecraft in orbit about a distant planet by two widely separated Earth-based radio antennas provides more-accurate positioning information than can be obtained by tracking each spacecraft separately. A demonstration of this tracking technique, referred to as same-beam interferometry (SBI), is currently being done using the Magellan and Pioneer 12 orbiters at Venus. Signals from both spacecraft fall within the same beamwidth of the Deep Space Station antennas. The plane-of-sky position difference between spacecraft is precisely determined by doubly differenced phase measurements. This radio metric measurement naturally complements line-of-sight Doppler. Data was first collected from Magellan and Pioneer 12 on August 11-12, 1990, shortly after Magellan was inserted into Venus orbit. Data were subsequently acquired in February and April 1991, providing a total of 34 hours of same-beam radio metric observables. Same-beam radio metric residuals have been analyzed and compared with model measurement error predictions. The predicted error is dominated by solar plasma fluctuations. The rms of the residuals is less than predicted by about 25 percent for 5-min averages. The shape of the spectrum computed from residuals is consistent with that derived from a model of solar plasma fluctuations. This data type can greatly aid navigation of a second spacecraft when the first is well-known in its orbit.

Border, J. S.↗
NASA NTRS1979

Solar flares

The present paper deals with explosions in a magnetized solar plasma, known as flares, whose effects are seen throughout the electromagnetic spectrum, from gamma-rays through the visible and to the radio band. The diverse phenomena associated with flares are discussed, along with the physical mechanisms that have been advanced to explain them. The impact of solar flare research on the development of plasma physics and magnetohydrodynamics is noted. The rapid development of solar flare research during the past 20 years, owing to the availability of high-resolution images, detailed magnetic field measurements, and improved spectral data, is illustrated.

Rust, D. M.↗
NASA NTRS1995

The origin of the solar wind

The high speed solar wind, which is associated with coronal holes and unipolar interplanetary magnetic field, has now been observed in situ beyond 0.3 a.u. and at latitudes up to 80 degrees. Its important characteristics are that it is remarkably steady in terms of flow properties and composition and that the ions, especially minor species, are favored in terms of heating and acceleration. We have proposed that the high speed wind, with its associated coronal holes, forms the basic mode of solar wind flow. In contrast, the low speed wind is inherently non-stationary, filamentary and not in equilibrium with conditions at the coronal base. It is presumably the result of continual reconfigurations of the force-free magnetic field in the low-latitude closed corona which allow trapped plasma to drain away along transiently open flux tubes. Observations of high speed solar wind close to its source are hampered by the essential heterogeneity of the corona, even at sunspot minimum. In particular it is difficult to determine more than limits to the density, temperature and wave amplitude near the coronal base as a result of contamination from fore- and back-ground plasma. We interpret the observations as indicating that the high speed solar wind originates in the chromospheric network, covering only about 1% of the surface of the sun, where the magnetic field is complex and not unipolar. As a result of small-scale reconnection events in this 'furnace', Alfven waves are generated with a flat spectrum covering the approximate range 10 kHz to 10 Hz. The plasma is likely to be produced as a result of downwards thermal conduction and possibly photoionization at the top of the low density chromospheric interface to the furnace, thus controlling the mass flux in the wind. The immediate source of free (magnetic) energy is in the form of granule-sized loops which are continually carried into the network from the sides. The resulting wave spectrum is such that energy can be efficiently transferred to the ions within a few solar radii of the base of the corona, favoring heavy species and creating stable, fast solar wind.

Axford, W. I.↗
NASA NTRS2002

An X-Ray Reprocessing Model of Disk Thermal Emission in Type 1 Seyfert Galaxies

Using a geometry consisting of a hot central Comptonizing plasma surrounded by a thin accretion disk, we model the optical through hard X-ray spectral energy distributions of the type 1 Seyfert. galaxies NGC 3516 and NGC 7469. As in the model proposed by Poutanen, Krolik, and Ryde for the X-ray binary Cygnus X-1 and later applied to Seyfert galaxies by Zdziarski, Lubifiski, and Smith, feedback between the radiation reprocessed by the disk and the thermal Comptonization emission from the hot central plasma plays a pivotal role in determining the X-ray spectrum, and as we show, the optical and ultraviolet spectra as well. Seemingly uncorrelated optical/UV and X-ray light curves, similar to those which have been observed from these objects can, in principle, be explained by variations in the size, shape, and temperature of the Comptonizing plasma. Furthermore, by positing a disk mass accretion rate which satisfies a condition for global energy balance between the thermal Comptonization luminosity and the power available from accretion, one can predict the spectral properties of the heretofore poorly measured hard X-ray continuum above approximately 50 keV in type 1 Seyfert galaxies. Conversely, forthcoming measurements of the hard X-ray continuum by more sensitive hard X-ray and soft gamma-ray telescopes, such as those aboard the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) in conjunction with simultaneous optical, UV, and soft X-ray monitoring, will allow the mass accretion rates to be directly constrained for these sources in the context of this model.

Chiang, James↗
NASA NTRS1987

Simultaneous measurements of energetic ion (50 keV and above) and electron (220 keV and above) activity upstream of earth's bow shock and inside the plasma sheet - Magnetospheric source for the November 3 and December 3, 1977 upstream events

Simultaneous observations of energetic ions and electrons by the IMP 7 and 8 spacecraft are used here to separate temporal variations from spatial variations during the upstream ion events observed on December 3, 1977 and November 2-3, 1977, in order to determine the source of these particles. Analysis of the observations and comparison with theory shows that: (1) for each of the observed upstream enhancements, energetic ions and electrons were simultaneously present inside the plasma sheet; (2) the low-energy ion intensity profile inside the plasma sheet was relatively flat, while at higher energies there was considrable variability; (3) relativistic electron bursts were seen inside the plasma sheet and also upstream of the shock but at substantially reduced intensities; (4) the ion energy spectrum for the December 3 event, extended to energies of about 2 MeV, was identical in form with the plasma sheet and upstream of the shock; (5) ion anisotropies exhibited typically large dawn-dusk or dusk-dawn gradients and large field-aligned streaming away from the bow shock.

Sarris, E. T.↗
NASA NTRS1977

Investigation of possible lower hybrid emission from the NASA Lewis bumpy torus plasma

Radio frequency emission has been detected near the power hybrid frequency of a bumpy torus plasma by using a responsive detection system that consists of a spectrum analyzer and a 50 ohm miniature coaxial antenna concentrically located in a re-entrant quartz tube. The frequency shift of a broad emission peak was monitored as a function of background pressure, electrode voltage, and the strength of the dc magnetic field. Simultaneous measurements of the average plasma density were made with a polarization diplexing microwave interferometer. The information derived from the experiment is discussed with particular reference to the following: (1) whether the emissions are dominated by atomic or molecular species of deuterium; (2) the strength of the dc magnetic field in the emitting region; (3) the geometric location of the emitting region of the plasma; (4) comparison of the lower hybrid plasma density with the average plasma density; and (5) relation of ion spoke geometry to lower hybrid emission.

Mallavarpu, R.↗
NASA NTRS1971

Radio astronomy Explorer-1 observations of the Gum nebula

Complicating factors in the spectrum analysis of the Gum nebula are discussed. These include accounting for the spectrum of supernova remnants in the direction of the nebula, the different absorption laws for radiation from beyond and within the nebula, and the Razin effect. This last results in a low frequency cutoff to the spectrum of synchrotron radiation by particles in a thermal plasma. These factors cause the observer to overestimate the amount of absorption occurring in the nebula. Data from the Explorer 38 satellite are presented for 3.93 and 6.55 MHz. Average optical depth for the nebula at 4 MHz was calculated.

Alexander, J. K.↗
NASA NTRS1972

Computer study of convection of weakly ionized plasma in a nonuniform magnetic field.

A weakly ionized plasma in a strong and nonuniform magnetic field exhibits an instability analogous to the flute instability in a fully ionized plasma. The instability sets in at a critical magnetic field. To study the final state of the plasma after the onset of the instability, the plasma equations are integrated numerically assuming a certain initial spectrum of small disturbances. In the regime studied, numerical results indicate a final steadily oscillating state consisting of a single finite amplitude mode together with a time-independent modification of the original equilibrium. These results agree with the analytic results obtained by Simon in the slightly supercritical regime. As the magnetic field is increased further, the wavelength of the final oscillation becomes nonunique. There exists a subinterval in the unstable wave band. Final stable oscillation with a wavelength in this subinterval can be established if the initial disturbance has a sufficiently strong component at the particular wavelength.

Shiau, J. N.↗
NASA NTRS1971

X-ray structure of the Cygnus Loop.

X-ray emission from the Cygnus Loop was observed in the energy region around 0.2 to 1 keV with a collector that focused X-rays along one dimension while scanning across the nebula. The total integrated intensity is 13 nanoerg per square centimeter per second. The one-dimensional X-ray structure has the same angular size - about 3 degrees - as the outermost boundaries of the optical filaments. There is no increase in X-ray emission at the center of the nebula nor at the strong feature that is seen in certain radio maps. The X-ray spectrum is consistent with thermal radiation from a hot plasma at a temperature of about 4 million K.

Gorenstein, P.↗
NASA NTRS1977

Radio and X-ray variability of the nucleus of Centaurus A (NGC 5128)

Centaurus A was observed at radio frequencies of 10.7, 31.4, 85.2, and 89 GHz and at X-ray energies greater than 20 keV. The source exhibits significant variability in all the observed radio frequencies. The observed radio and X-ray intensities show some concurrent variations but do not track one another throughout the observations. A model of the source in which X-rays are produced by inverse Compton scattering of blackbody photons by relativistic electrons is proposed to explain these observations. The observed variations in the electromagnetic spectrum are consistent with adiabatic expansion of a trapped plasma in conjunction with turbulent accelerations of the relativistic electrons.

Beall, J. H.↗
NASA NTRS1980

Detection of a new extended soft X-ray source H1538-32 - A possible old supernova remnant

The discovery in the Lupus region of a new, extended soft X-ray source, H1538-32, is reported, having a distance of approximately 340 pc, and a luminosity of 1 to 2 x 10 to the 34th ergs/sec. The observed energy spectrum of the source is well fitted either by a thermal bremsstrahlung spectrum with Gaunt factor but without line emission, or by a coronal plasma model which includes the X-ray emission lines of various elements and the continuum as outlined by Raymond and Smith (1977). On the basis of the extended nature of the source and its thermal spectrum, it is suggested that H1538-32 may be an old supernova remnant.

Riegler, G. R.↗
NASA NTRS1981

Modulational instability in a plasma with suprathermal electrons

The consequences of the presence of suprathermal electrons on the linear stage of modulational instabilities are investigated for a range of parameters appropriate to both laboratory and astrophysical plasmas. Substantial modification to the growth rate and secondary spectrum are found to occur for instabilities driven by large amplitude Langmuir waves in the dipole limit due to the kinetic effects of the suprathermal electrons. In particular, for reasonable choices of pump amplitude and suprathermal energy density, additional modes become unstable.

Freund, H. P.↗
NASA NTRS1984

Mean excitation energies for stopping powers in various materials using local plasma oscillator strengths

The basic model of Lindhard and Scharff, known as the local plasma model, is used to study the effects on stopping power of the chemical and physical state of the medium. Unlike previous work with the local plasma model, in which individual electron shifts in the plasma frequency were estimated empirically, he Pines correction derived for a degenerate Fermi gas is shown herein to provide a reasonable estimate, even on the atomic scale. Thus, the model is moved to a complete theoretical base requiring no empirical adjustments, as characteristic of past applications. The principal remaining error is in the overestimation of the low-energy absorption properties that are characteristic of the plasma model in the region of the atomic discrete spectrum, although higher-energy phenomena are accurately represented, and even excitation-to-ionization ratios are given to fair accuracy. Mean excitation energies for covalent-bonded gases and solids, for ionic gases and crystals, and for metals are calculated using first-order models of the bonded states.

Wilson, J. W.↗
NASA NTRS1985

Departures from LTE Populations Versus Anomalous Abundances: the Effects of Heating

The deposit of nonradiative heat and momentum in the mantle of a hot star affects the interpretation of the stellar spectrum in two ways. First, a superheated and moving plasma should be considered when doing the analysis, and second, a model atom which is appropriate for the physical state of the line forming regions. Some examples are presented for H and He showing how the changes in the electron temperature affect the solution of the equations of statistical equilibrium. The observed spectra of the Wolf-Raynet stars HD 191765, HD 192103, and HD 192163 are compatible with a normal H/He abundance ratio.

Underhill, A. B.↗