Search NASA⌕ Search

SEARCH · Search NASA

Results for “Plasma fluctuations”

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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

The Solar Wind as a Laboratory for the Study of Magnetofluid Turbulence

The solar wind is the Sun's exosphere. As the solar atmosphere expands into interplanetary space, it is accelerated and heated. Data from spacecraft located throughout the heliosphere have revealed that this exosphere has velocities of several hundred kilometers/sec, densities at Earth orbit of about 5 particles/cm(exp 3), and an entrained magnetic field that at Earth orbit that is about 5 X 10(exp 5) Gauss. A fascinating feature of this magnetized plasma, which is a gas containing both charged particles and magnetic field, is that the magnetic field fluctuates in a way that is highly reminiscent of "Alfven waves", first defined by Hannes Alfven in 1942. Such waves have the defining property that the fluctuating magnetic fields are aligned with fluctuations in the velocity of the plasma and that, when properly normalized, the fluctuations have equal magnitudes. The observed alignment is not perfect and the resulting mismatch leads to a variety of complex interactions. In many respects, the flow patterns appear to be an example of fully developed magnetofluid turbulence. Recently, the dissipation range of this turbulence has been revealed by Search Coil magnetometer data from the four Cluster spacecraft. This tutorial will describe some of the properties of the large-scale and small-scale turbulence.

Goldstein, Melvyn L.↗
NASA NTRS1987

Plasma instability in the presence of negative ions

Ion density fluctuations are studied in a diffusion-controlled argon plasma containing, as a dilutant, two electron-attaching species, carbon dioxide and sulfur hexafluoride. It is found that the system becomes increasingly unstable as the concentration of the electron-attaching species is increased. Nonlinear mode-mode couplings have been identified and the coupling coefficients for these interactions have been computed. Turbulent fluctuations are observed to have pronounced three-dimensionality with distinct axial and azimuthal behaviors. The power spectra of these fluctuations are composed of many discrete modes. The importance of changes in the total discharge pressure is dependent on the electron-attachment cross section of the dilutant species. From these results it is shown that the ionization instability is probably responsible for the observed phenomenon.

Johnson, J. A., III↗
NASA NTRS1983

Evidence for temporal variations in polar plumes

The variability of the O VI and Mg X emissions from four plumes is studied in order to place empirical constraints on possible plasma heating processes and acceleration mechanisms for mass flows in polar plumes. The observations comprise a 40 min seqeunce of spectroheliograms acquired with a temporal resolution of approximately 2 min. It is found that the Mg X emission from each plume has a strong 'dc' component and that any variations occurred at approximately the 10 percent (of the mean) level or less. This means that temporal variations in the density and/or temperature were 5 percent or less. Some statistically significant variations in the measured Mg X signals are seen. It is noted that two of the plumes decreased in brightness by approximately 10 percent during the 40 min period of the observations. Besides this, two plumes exhibited short-term (few minute) variations that are seen as statistically significant at a confidence level of more than 90 percent. The latter variations (if real) are thought to be due to small (approximately 20,000 K) fluctuations in temperature caused by fluctuations in the local plasma heating rate.

Withbroe, G. L.↗
NASA NTRS1974

Microscale Alfven waves in the solar wind at 1 AU

Analysis of IMP 1 (Explorer 43) plasma and magnetic field fluctuations on a scale of one hour revealed that linearly and circularly polarized Alfven waves are rarely present in the solar wind at 1 AU. The most prevalent microscale fluctuations appeared to be large-amplitude Alfven waves with small but non-zero fluctuations in the magnetic field intensity. These waves are present about 40% of the time and are predominantly propagating away from the sun.

Burlaga, L. F.↗
NASA NTRS1975

Pulsed electromagnetic gas acceleration

Terminal voltage measurements with various cathodes and anodes in a high power, quasi-steady magnetoplasmadynamic (MPD) are discussed. The magnitude of the current at the onset of voltage fluctuations is shown to be an increasing function of cathode area and a weaker decreasing function of anode area. Tests with a fluted cathode indicated that the fluctuations originate in the plasma adjacent to the cathode rather than at the cathode surface. Measurements of radiative output from an optical cavity aligned to examine the current-carrying portion of a two-dimensional, 56 kA magnetoplasmadynamic discharge reveal no lasing in that region, consistent with calculations of electron excitation and resonance radiation trapping. A voltage-swept double probe technique allows single-shot determination of electron temperature and electron number density in the recombining MPD exhaust flow. Current distributions within the cavity of MPD hollow cathodes for various static prefills with no injected mass flow are examined.

Jahn, R. G.↗
NASA NTRS1975

Statistical properties of the interplanetary microscale fluctuations

Results are reported for a statistical study of short-period fluctuations in the interplanetary plasma velocity and magnetic field. The data base used consists of measurements of the interplanetary plasma and magnetic field by Pioneer 6 with a time resolution of 72 sec and by Mariner 5 with a resolution of 5 min. The analysis is conducted to characterize the parent population from which all individual microscale events on these time scales are drawn. The microscale changes are grouped according to their energy densities relative to the energy density of the background magnetic field, and it is found that each grouping exhibits certain statistical properties within the limits of observational uncertainty. It is noted that these statistical properties are purely observational and independent of any physical model which may be used to interpret them. The results are discussed in the context of MHD discontinuity theory for a thermally anisotropic plasma.

Belcher, J. W.↗
NASA NTRS1979

Pitch-angle diffusion by whistler mode waves near the Io plasma torus

As Voyager 1 traversed the inner radiation belt of Jupiter, wave-particle interactions involving energetic electrons and whistler mode turbulence were strongly affected by the presence of the Io plasma torus. Within the high density torus the resonant electron energy was low and the associated high index of refraction yielded high B-to-E ratios for the wave fields, leading to very strong pitch-angle scattering. It is shown that significant spatial and temporal variations in plasma conditions produced large fluctuations in local scattering times, and the problems associated with the evaluation of precipitation lifetime are discussed.

Scarf, F. L.↗
NASA NTRS1980

Infrared heterodyne receivers with IF responses approaching 5 GHz

Specialized coherent 10.6 micrometer infrared receivers were developed with infrared frequency responses approaching 5 GHz. Carbon dioxide lasers were employed for these applications because of their availability, stability, and high average power levels. Heterodyne receivers which employ extrinsic photoconductive Ge:Cu(Sb) mixers were developed for measurements of carbon dioxide laser scattering to determine: (1) the drive lower hybrid wave density fluctuations; and (2) the plasma ion temperature of the dense Tokamak plasmas.

Wolczok, J. M.↗
NASA NTRS1981

Constance mirror program: Progress and plans

The current state of the mechanics of the Constance II experiment, the physics results gathered, the motivation background, and future plans for the Constance II experiment are reviewed. Several improvements have been made and several experimental investigations have been completed. These include the construction/installation/testing of: (1) liquid-nitrogen cooled, Ioffe bars installed, (2) a diverter coil (3) the 100 kW ICRF generator, (4) the data acquisition system, and (5) the optimum hot-iron operation of the machine with Titanium and pulsed-gas plasma guns. Measurements were made of the density, temperature, and radius of the plasma. Ion-cyclotron fluctuations were observed, their bandwidth measured, and data collected demonstrating resonance heating. New X-ray diagnostics were designed and purchased, and progress on the Thomson scattering was made. Finally, a new hot cathode gun was designed and constructed.

Klinkowstein, R. E.↗
NASA NTRS1981

Anomalous resistivity on auroral field lines

Consideration is given to a possible explanation for the one-dimensional anomalous resistivity and breakdown of runaway acceleration observed in the auroral zones. It is shown that under conditions existing along auroral field lines, where the cyclotron frequency is greater than the plasma frequency and density fluctuations exceed 0.25, trapping of a major portion of the electron distribution function by large-amplitude ion cyclotron waves can inhibit the free acceleration of runaways and act to violate the conservation of the first adiabatic invariant for electrons above a certain threshold velocity. Particle simulations combining the effects of finite ion cavities and pitch angle scattering at the anomalous Doppler resonance are then presented which demonstrate the formation of a runaway distribution upon the application of a constant dc field and the effects of the instability at the anomalous Doppler resonance in preventing it. The necessity for further large-scale macrophysics simulations to gain a fuller understanding of the total system dynamics is pointed out.

Rowland, H. L.↗
NASA NTRS1984

Submilliarcsecond VLBI Using Compact Close Pairs of Radio Sources: Error Analysis

The potential accuracy attainable for (delta) VLBI positional measurements (submilliarc second level) is reached by simultaneously observing pairs of compact radio sources whose angular separation are smaller than the beamwidth of each antenna. Simultaneous (delta) VLBI (SVLBI) enables significant cancellation of measurement errors. Solar plasma is the dominant fluctuating error source in SVLBI positional measurements since there is enhanced cancellation of the troposphere and ionosphere, and complete cancellation of oscillator instabilities. Of the nonfluctuating error sources, errors due to universal time predominate. By performing SVLBI experiments over several years with many different close pairs of radio sources, limits can be placed on reference frame stability. Intrinsic properties of the sources, such as source structure and proper motion, will limit measurements. The SVLBI differential phase and corruptive noise sources will be discussed here along with estimated results.

Morabito, D. D.↗
NASA NTRS1984

Quenching of the beam-plasma instability by 3-D spectra of large scale density fluctuations

A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with Type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large scale density fluctuations that are able to efficiently diffuse small k beam unstable Langmuir waves in phase space, and, second, on the presence of a significantly isotropic nonthermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements.

Muschietti, L.↗
NASA NTRS1984

Detailed examination of a plasmoid in the distant magnetotail with ISEE 3

Details of an investigation of a lone plasmoid encountered by the ISEE-3 spacecraft in February 1983 are reported. ISEE-3 was then at 217 earth radii distance, in the magnetosheath, and had entered the magnetotail. Intense energetic particle, plasma and magnetic field fluctuations were detected, indicating a large electron flux moving tailward, a suprathermal proton flux, a jump in electron temperature, and inverse signatures of an increasing magnetic field. Closed magnetic field lines were also detected as were IMF field lines around the plasmoid and open field lines connecting the earth with space. The hot particles in the plasmoid are concluded to have been magnetically confined in a boundary one earth radius thick. An earthward directed beam was also detected and was found to balance the tailward flux. The origin and characteristics of the beam were not discerned.

Hones, E. W., Jr.↗
NASA NTRS1984

Thermal fluctuations from an artificial ion beam injection into the ionosphere

Simulations of argon beam experiments flown on two sounding rocket flights (ARCS 1, 2) reveal two bands of electrostatic wave emissions, one at high frequencies around the upper hybrid mode, the other at low frequencies around the lower hybrid mode. The latter is significantly enhanced by the argon beam in the simulations, and the enhancement was clearly observed on ARCS 2. This enhancement at phase velocities greater than the beam velocity is suggested to be due to an increase in the thermal fluctuation level of the plasma when the argon beam is present. Inclusion of electron dynamics and oblique angles of wave propagation with respect to B allows investigation of electron heating. Electron tail heating is observed parallel to B for k-parallel/k-perpendicular proportional to the square root of m(e)/m(0), while background ions are heated perpendicular to B.

Hudson, M. K.↗
NASA NTRS1986

Clumpy Langmuir waves in type III solar radio bursts

A model is developed for type III radio emission in the interplanetary medium based on recent data on Langmuir waves, associated with ion sound waves, density fluctuations in the interplanetary plasma, streaming electrons and radio emission. In this model, Langmuir wave growth is suppressed by refraction in field-aligned density irregularities except near density minima where clumps of Langmuir waves form. Quasi-linear relaxation limits the growth of the Langmuir waves in the clumps. The radio emission, which is attributed to coalescence of the Langmuir waves with associated ion sound waves, saturates at a brightness temperature equal to the effective temperature of the Langmuir waves, estimated to be between 10 to the 15th and 10 to the 16th K from obserrvational data. The model is consistent with all the relevant data on type III events. In particular, it accounts naturally for observed brightness temperatures of type III bursts.

Melrose, D. B.↗
NASA NTRS1998

Interstellar Optics

We discuss the effects of finite source size on the diffraction pattern produced by scattering in a thin screen, particularly as applied to radio-wave scattering, by density fluctuations in the interstellar plasma.

Interstellar Optics Scattering techniques telescop↗