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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.

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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Topological constraints and the existence of force-free fields

A fundamental problem in plasma theory is the question of the existence of MHD equilibria. The issue of topological constraints is of crucial importance for the problem of the existence of equilibria. Heuristic methods are used to discuss the coronal wrapping pattern. It is concluded that for a given set of footpoint positions the wrapping pattern in the corona is completely fixed. The topological constraints are included in the boundary conditions on the Euler potentials and impost no additional restrictions on possible equilibria. Although this does not prove that equilibria always exist, it does show that the force-free problem is not overdetermined and that existence of equilibria is still an open question.

Antiochos, S. K.↗
NASA NTRS1988

Modelling of solar magnetic field and prominence structures

Using plasma theory, the interaction is studied between high frequency and magnetohydrodynamic (MHD) waves from which a set of coupling equations resulted. On the basis of this formalism, the modulation instabilities of an electromagnetic soliton in a current sheet are examined, and it is shown that there is a resistive instability at the onset of the magnetic field reconnection. This mechanism could be used to explain the onset of solar flares and prominences. To improve the resolution of vector magnetic fields at the sun's surface, state-of-the-art optics is examined to improve the design and fabrication of a new spaceborne solar vector magnetograph as part of the SAMEX (Solar Active Measurements Experiment) program.

Wu, Shi Tsan↗
NASA NTRS1990

Source of O mode radio emissions from the dayside of Uranus

During the inbound trajectory toward Uranus, the Planetary Radio Astronomy instrument on Voyager 2 observed narrow-band smooth (n-smooth) emission at frequencies centered near 60 kHz and O-mode emission (the dayside source) in a frequency range narrowly confined around 160 kHz. Assuming empirical models of the plasma density for the dayside magnetosphere of Uranus, and using cold plasma theory together with observational constraints, ray-tracing calculations are performed to determine the source location of the O-mode emission. The dayside source appears to originate along magnetic field lines with a footprint near the north magnetic pole. Sources of nightside high-frequency broadband smooth (b-smooth) emission observed by Voyager after encounter are believed to exist near the conjugate footprint of these same field lines. This would indicate that the particle population supplying the free energy source has energies at least as high as a few keV.

Menietti, J. D.↗
NASA NTRS1990

Possible second harmonic gyroemission at Uranus

During the inbound trajectory toward Uranus, the Planetary Radio Astronomy Instrument on board the Voyager 2 spacecraft observed narrow-band smooth (n-smooth) emission at frequencies centered near 60 kHz. By assuming models of the plasma density for the dayside magnetosphere of Uranus and by using cold plasma theory together with stringent observational constraints, ray-tracing calculations were performed to determine the source location and mode of the n-smooth emission. Ray-tracing calculations suggest that the n-smooth emission with sources near the magnetic equator may be fundamental X mode for certain conditions or second harmonic gyroemission. If the emission is second harmonic gyroemission, the fundamental emission at 30 kHz is expected but apparently not observed. These findings are discussed in the context of the most recent developments in the theory of the cyclotron maser instability.

Menietti, J. D.↗
NASA NTRS1990

Auroral kilometric radiation - An example of relativistic wave-particle interaction in geoplasma

The earth's auroral kilometric radiation (AKR) is believed to be produced by the electron-cyclotron maser instability. This instability is the result of a wave-particle interaction in which relativistic effects are crucial. An explanation is given as to how these relativistic effects alter the shape of the resonance curve in velocity space and modify the R - X mode wave dispersion near the electron cyclotron frequency compared to the results obtained in the nonrelativistic limit and from cold-plasma theory. The properties of the cyclotron maser instability in a driven system are illustrated using two-dimensional electromagnetic particle simulations which incorporate a continual flow of primary energetic electrons along the magnetic field.

Pritchett, P. L.↗
NASA NTRS2001

Observational and theoretical analyses of polar cap/auroral region f(sub p) and 2 f(sub p) emissions using the polar and DE 1 satellites

The objective of this proposal was to conduct a comprehensive observational and theoretical investigation, including 2-D numerical simulations of emissions that occur near f(sub p) and 2 f(sub p) as observed by the Dynamics Explorer 1 (DE 1) spacecraft in the mid-altitude polar magnetosphere. Electrostatic electron plasma waves (or Langmuir waves) and associated electromagnetic (ETD) radiation near f(sub p) and 2 f(sub p) have previously been observed associated with the Earth's foreshock and in coronal/solar wind type II and III solar radio bursts. The observations of similar emissions in the terrestrial polar magnetosphere have not been previously reported and are important for a better understanding of the range of generation processes. We have been cataloging the location, electric and magnetic intensity, polarization, bandwidth, etc. of the emissions. The theoretical analysis included linear and nonlinear plasma theory. The goal was to fully describe the generation processes for both the f(sub p) (PF component) and 2 f(sub p) (H component) emissions. Although this goal was not completely accomplished, we have made significant advances in our understanding of these emissions.

Menietti, J. D.↗
NASA NTRS1974

A theoretical and experimental investigation of cylindrical electrostatic probes at arbitrary incidence in flowing plasma

The theory for calculating the current collected by a negatively biased cylindrical electrostatic probe at an arbitrary angle of attack in a weakley ionized flowing plasma is presented. The theory was constructed by considering both random and directed motion simultaneous with dynamic coupling of the flow properties and of the electric field of the probe. This direct approach yielded a theory that is more general than static plasma theories modified to account for flow. Theoretical calculations are compared with experimental electrostatic probe data obtained in the free stream of an arc-heated hypersonic wind tunnel. The theoretical calculations are based on flow conditions and plasma electron densities measured by an independent microwave interferometer technique. In addition, the theory is compared with laboratory and satellite data previously published by other investigators. In each case the comparison gives good agreement.

Jenkins, R. V.↗
NASA NTRS1972

Reformulation of quasi-linear theory.

Standard plasma quasi-linear theory is reformulated on the basis of a classical quantum derivation proceeding from the Vlasov equation and dealing only with frequency, wavenumber, and velocity. The wave amplitudes are assumed to be weakly time-dependent, and no distinction is made between growing and decaying waves. The proposed method leads to no negative diffusivity of 'fake' diffusion. By appropraite treatment of nonresonant interaction, expressions are obtained for wave energy and momentum.

Kaufman, A. N.↗
NASA NTRS1981

Anomalous heating of the polar E region by unstable plasma waves. II - Theory

It is found that anomalous electron temperatures in the disturbed high-latitude E region can be quantitatively explained in terms of heating by unstable plasma waves. The electron temperatures at 110 km have been measured to be as high as 1500 K instead of the expected value of about 300 K. It is shown that by using quasi-linear theory there is an ample source of heat in the unstable waves and that the measured electron temperature profiles have a shape very similar to what is expected from plasma wave heating by the modified two-stream instability. It is found that there is even more heating going to the ion gas, but that the resulting effect on the ion temperature may be difficult to measure. The best estimate of the wave heating rates leads to the conclusion that wave heating can be as much as 50% of the Joule heating for dc electric field strengths of the order of 45 mV/m or greater.

St.-Maurice, J. P.↗
NASA NTRS1983

Nonlinear scattering from a plasma column. I - Theory. II Special cases

The scattered signal excited by nonlinear mixing of two plane waves normally incident on an infinitely long column of plasma is investigated. A general solution is obtained for the polarization in which the electric field vectors of the waves are perpendicular to the column axis and the column is assumed to be radically inhomogeneous. This general theory is then applied to the special cases of the inhomogeneous column in the long-wavelength limit, and the homogeneous column both for the general case and in the long-wavelength limit. It is determined that dipole and quadrupole components should predominate in the polar radiation pattern for the long-wavelength case. The special case of second harmonic generation due to a single incident wave is analyzed in detail. Nonlinear scattering coefficients are computed, and the corresponding polar radiation patterns are determined. The findings of this study are employed to evaluate the feasibility of observing nonlinear scattering from meteor trails.

Crawford, F. W.↗
NASA NTRS1971

Theory of the unmagnetized plasma.

The Vlasov mathematical model of a plasma, which has come to be thought more useful than any other in describing the dynamical behavior of the majority of plasmas of interest, is first examined. Macroscopic variables and moment equations; linear electrostatics solutions; plasma oscillations, ion acoustic waves, and linear instabilities are treated, as well as external fields, 'test' charges, and nonlinear Vlasov phenomena. Plasmas are statistically described, and attention is given to the kinetic theory of the stable, uniform plasma and the Balescu-Lenard equation; two-time ensemble averages and fluctuation spectra in stable plasmas; the kinetic theory of the unstable plasma; and ensembles of Vlasov plasmas. Some illustrative experiments are described. Four appendixes deal with the electrostatic approximation and transverse waves; solution of the linearized Vlasov equation in a magnetic field; estimates of correlation functions from thermal equilibrium; and equivalence of spatially uniform BBGKY and Klimontovich correlations.

Montgomery, D. C.↗