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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Investigation of plasma instabilities in the polar cusp

During the last six months, considerable progress was made in studying the excitation of electromagnetic waves in the whistler frequency range by an anisotropic or gyrating electron beam. A paper entitled 'Electron Cyclotron Wave Generation by Relativistic Electrons' was published in the Journal of Geophysical Research. Another paper entitled 'Electron Beam Excitation of Upstream Waves in the Whistler Mode Frequency Range' was submitted for publication in Journal of Geophysical Research. This paper is in collaboration with Dr. C. W. Smith at Bartol Research Institute. In this paper, it was shown that an anisotropic electron beam (or gyrating electron beam) is capable of generating both left-hand and right-hand polarized electromagnetic waves in the whistler frequency range. Our earlier paper 'Electromagnetic Components of Auroral Hiss and Lower Hybrid Waves in the Polar Magnetosphere' was accepted for publication in the AGU Chapman Conference on Micro and Meso Scale Phenomena in Space Plasmas. Electromagnetic waves in the lower hybrid and whistler waves regime were identified and a mechanism of how these waves are generated was suggested.

Wong, H. K.↗
NASA NTRS1974

Further remarks on plasma instabilities produced by ions born in the solar wind

Whistler and Alfven wave instabilities produced in the solar wind by newly born ions of planetary or interstellar origin are examined, and it is shown that the dispersion equation considered by Wu and Davidson (1972) contains another class of complex roots which imply stronger instabilities than previously noted. The applicability of the proposed theoretical models is discussed together with the implications of the results for the interaction of planetary and interstellar He+ ions with the solar wind.

Wu, C. S.↗
NASA NTRS1975

Current-driven plasma instabilities at high latitudes

Earlier Ogo 5 discussions of high-latitude magnetospheric wave-particle interaction phenomena associated with field aligned current systems are extended by considering some September 7, 1968, out-bound measurements and corresponding March 11, 1969, observations, when the spacecraft was traversing auroral L shells in midafternoon. It is shown that during moderate magnetospheric disturbances the most intense waves and currents were detected near sharp boundaries in the density of polar cleft electrons, and it is possible that local wave-particle interactions produced anomalous resistivity. Some observed changes in the electron distribution functions might be explained in terms of local acceleration, but significant causal questions about the dynamics remain open if one can use only local data from a single spacecraft in this region.

Scarf, F. L.↗
NASA NTRS1981

On plasma instabilities in the high-latitude ionospheric E region

The use of the Farley-Buneman instability in the high-latitude E region of the earth's ionosphere as a diagnostic tool for ionospheric and solar wind electric fields, and the effect of Farley-Buneman waves on cosmic radio noise events observed on riometers, are discussed. Data are analyzed and presented in support of the hypothesis, suggested by Olesen (1972), that the Slant E condition in polar cap ionograms is a manifestation of the Farley-Buneman instability in the E region. Detailed descriptions are given of the experimental apparatus employed in these investigations.

Dangelo, N.↗
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 NTRS1985

Quenching of the beam-plasma instability by large-scale density fluctuations in 3 dimensions

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 are addressed by a model based on the existence of large scale density fluctuations capable of sufficiently diffusing small-k beam-unstable Langmuir waves in phase space. The model is also informed by the presence of a significant isotropic nonthermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The model is able to predict various levels of Langmuir waves, depending on the parameters; calculations indicate that, for realistic parameters, the most unstable small k modes are fully stabilized, while some oblique mode with higher k and lower growth rate may remain unstable.

Muschietti, L.↗
NASA NTRS1988

Plasma instabilities stimulated by HF transmitters in space

Diffuse incoherent signal returns are often observed on Alouette and ISIS topside ionograms in addition to coherent echoes of electromagnetic and electrostatic waves. These diffuse signals, which at times can be the dominant features on topside ionograms, have been attributed to sounder-induced temperature anisotropies which drive the Harris instability. Previous theoretical investigations were based on the electrostatic approximation to the dispersion equation. The present paper will present calculations indicating that when the electromagnetic terms are retained in the dispersion equation and when the sounder-stimulated perpendicular electron temperature approaches 1 keV, then the whistler mode can have a temporal growth rate larger than the electrostatic electron cyclotron harmonic wave mode central to the diffuse resonance problem. Present sounders lack the power and antenna lengths to generate whistler mode waves in this manner. In addition, such waves would have large group velocities and would quickly leave the vicinity of the sounder. Experiments to investigate the wave growth, propagation, and damping of such stimulated waves are planned for the 1990s using a highly flexible sounder on the Space Shuttle and a receiver on a subsatellite.

Benson, Robert F.↗
NASA NTRS1995

An assessment of plasma instabilities or planetary lightning as a source for the VLF bursts detected at Venus

Very low frequency (VLF) signals detected in the nightside ionosphere of Venus have generally been attributed to atmospheric lightning. However it has recently been suggested that these bursts could be generated by either whistler-mode or lower hybrid drift instabilities. It has previously been shown that the growth rate for whistler-mode instabilities in the nightside ionosphere is too small for appreciable growth at altitudes less than 200 km, where the VLF burst rate is highest. We show that the bursts are usually observed in regions of low electron beta, where whistler-mode attenuation is small. We further show that many of the bursts are detected in regions of high collision frequency, which stabilizes the lower hybrid drift instability. Lastly, the waves are also detected in regions where the wavelength required for Doppler-shift of lower hybrid waves to 100 Hz is shorter than the electron Larmor radius, which also argues against a lower hybrid drift instability. Planetary lightning is consequently a more likely source for the VLF bursts.

Strangeway, R. J.↗
NASA NTRS1981

Boundary conditions which lead to excitation of instabilities in plasma simulations

Two examples of two-dimensional electrostatic particle-code simulations are shown in which one exhibits characteristics of a stable plasma while the other exhibits unstable, long wavelength plasma oscillations. The only difference between the two simulations is a change in the boundary condition on the electrostatic potential. An energy theorem is derived which shows that the rate of change of field and particle energy within a closed volume is related to a surface integral involving the electrostatic potential and the normal component of the electric current. An analytic theory is developed for a one-dimensional plasma to show how boundary effects can excite spurious plasma instabilities. The theory is tested with a series of one-dimensional plasma simulations. Finally, practical considerations on means of avoiding the non-physical instabilities in simulation plasmas are given.

Swift, D. W.↗
NASA NTRS1977

HF excited instabilities in space plasmas

It has now become technologically feasible to construct ground-based HF radar systems which can deliver RF energy to the ionospheric plasma, with power densities sufficient to alter the ionospheric electron thermal budget and plasma characteristics. Effects produced by a ground-based transmitter of a power aperture of the order of 10,000 Mw sq m in the frequency range from 4 to 12 MHz are shown in a graph. Another graph presents a profile of electron gas temperature enhancement due to energy deposition by a high power HF transmitter. The conduction of high-power HF ionospheric plasma experiments is discussed. Attention is given to plasma instabilities, wave-particle acceleration effects, spacial and time scales, parametric instabilities, spread-F, and the possibility of placing high power-aperture radars on an orbiting space platform.

Carlson, H. C., Jr.↗
NASA NTRS1991

Cometary plasma waves and instabilities

Various plasma waves and instabilities that have been observed near comets are discussed. Observational results are ordered by plasma parameters and compared with specific instabilities. The variation in the cometary ion and electron beam densities and velocities are studied by examining regions far from and near the comet nucleus. Spacecraft observations relevant to nonlinear wave evolution, wave cascading, and the development of turbulence are reviewed. The implications of the results for wave-particle interactions - linear, nonlinear, resonant, and nonresonant - and their effects on stochastic particle acceleration are addressed. Higher-frequency ELF/VLF wave observations are also reviewed, and comparisons between the various measurements are made.

Tsurutani, Bruce T.↗