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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Alternative formulations of magnetospheric plasma electrodynamics

The fundamental equations of magnetospheric plasma electrodynamics are considered from a theoretical standpoint that stresses the basic equivalence of various seemingly different formal representations. The mathematical properties of vector fields are reviewed, and their implications in electrodynamics are studied. The irrotational and solenoidal parts of the electric field are associated with two physically distinct types of sources. Relativistic covariance and gauge invariance in electromagnetic theory are reviewed and discussed in the context of an approach in which the mathematical properties of vector fields are taken as primary concepts. Special attention is given to the use and interpretation of the Coulomb gauge potential functions. This choice of gauge is sometimes regarded with undue suspicion, possibly because of a certain paradox concerning causality. The paradox is discussed and resolved. Useful properties of the Coulomb gauge are identified. These need not be limited to the case of slow time variations and can extend beyond the limits of validity of ideal MHD theory.

Cragin, B. L.↗
NASA NTRS1983

Modeling of transient disturbances in coronal-streamer configurations

Numerical simulations of the formation and propagation of mass ejection, loop transients in coronal streamers are discussed. The simulations are accomplished with numerical solutions of the single fluid, ideal MHD equations of motion in the meridional plane. The streamer is produced by simulating the relaxation of an initially radial hydrodynamic flow coupled with a dipole magnetic field. The simulated transient then results from an energy release at the base of the streamer. The legs of the loop transient produced remain essentially stationary while the loop expands mainly in the radial direction with velocities of 400 to 750 km s-1. Once the leading edge of the transient has passed out of the lower corona, the initial streamer configuration is restored after 15 to 24 hours. A second energy release 2 hours later than, and with an energy release identical to, the first does not produce a significant coronal disturbance.

Steinolfson, R. S.↗
NASA NTRS1986

Magnetic energy storage and conversion in the solar atmosphere

According to the approach employed in this investigation, particularly important simple configurations of magnetic field and plasma are identified, and it is attempted to achieve an understanding of the large-scale dynamic processes and transformations which these systems can undergo. Fundamental concepts are discussed, taking into account aspects of magnetic energy generation, ideal MHD theory, non-MHD properties, the concept of 'anomalous' resistivity, and global electrodynamic coupling. Questions of magnetically controlled energy conversion are examined, giving attention to magnetic modifications of plasma transport, the transition region structure and flows, channeling and acceleration of plasma, channeling and dissipation of MHD waves, and anomalous dissipation of field-aligned currents. A description of the characteristics of magnetohydrodynamic energy conversion is also provided, and outstanding questions are discussed.

Spicer, D. S.↗
NASA NTRS1985

On the formation of coronal cavities

The formation of a coronal cavity and its relation to a quiescent prominence is studied theoretically. The stability of condensation modes of a plasma in the coronal streamer model (Steinolfson et al., 1982) is considered using a two-dimensional time-dependent ideal MHD numerical simulation. It is found that a plasma with beta = 0.5 is unstable but one with beta = 4 is stable because the density enhancement of the plasma trapped by the closed fields increases with the strength of the magnetic field. The means by which condensation modes can produce a coronal cavity and/or initiate the formation of a prominence (depending on the field configuration) are discussed. It is argued that prominence and cavity material is all supplied from the chromospheric level in the form of spicules.

An, C.-H.↗
NASA NTRS1986

Condensation modes in magnetized plasmas

Condensation modes in magnetized cylindrical plasmas, with concentration on how magnetic field affects the stability were studied. It is found that the effects of magnetic field (shear, twist, and strength) on the condensation modes are different depending on the wave vector. For modes whose wave vector is not perpendicular to magnetic field lines the plasma motion is mainly along the field lines; the effects of magnetic field on the modes are negligible except on the heat flow parallel to the field line. For a mode which is localized near a surface where the wave vector is perpendicular to the field line, the plasma moves perpendicular to the line carrying the field line into the condensed region; magnetic field affects the mode by building up magnetic pressure in the condensed region. The stability of condensation modes strongly depends on how density and temperature vary with field twist. The stable nature of global quiescent prominence magnetic configurations implies that prominences form for low field twist for which ideal MHD modes are stable; plasma temperature should increase with field twist for stable prominence formation.

An, Chang-Hyuk↗
NASA NTRS1986

Numerical simulations of comets - Predictions for Comet Giacobini-Zinner

Simulations of Comet Giacobini-Zinner's interaction with solar wind are described and results are presented. The simulations are carried out via the numerical solution of the ideal MHD equations as an initial value problem in a uniform solar wind. The calculations are performed on a Cartesian mesh centered at the comet. Results reveal that the first significant modifications of the solar wind along the ISEE/ICE trajectory will occur 100,000 km from the solar wind comet axis.

Fedder, J. A.↗
NASA NTRS1988

Numerical simulation of mass injection for the formation of prominence magnetic field configurations. I - Asymmetric injection

A two-dimensional ideal MHD numerical model is used to investigate chromospheric mass injection into an overlying coronal dipole magnetic field. Such injection is shown to produce magnetic field deformations conducive to the formation of active region prominences. The results support a model in which an absorptive strand is formed by chromospheric asymmetric mass injection into the overlying coronal magnetic field. A necessary condition for the accumulation of the strands is that the mass injection forms a Kippenhahn-Schluter-type (1957) field configuration.

An, Chang-Hyuk↗
NASA NTRS1989

Plasma conductivity for Comet Halley ionosphere

Observational as well as semitheoretical magnetic field profiles have been used to derive self-consistently the plasma conductivity profiles for the ionosphere of Comet Halley. The characteristic diffusion length for the field, according to the present model, is about 28 km; this is in very good agreement with the Giotto spacecraft observations. It is shown that ideal MHD as well as constant conductivity models are not appropriate for the study of dynamical structure of the Halley's ionosphere.

Buti, B.↗
NASA NTRS1989

Steady state magnetic field configurations for the earth's magnetotail

A two-dimensional, force-balance magnetic field model is presented. The theoretical existence of a steady state magnetic field configuration that is force-balanced and consistent with slow, lossless, adiabatic, earthward convection within the limit of the ideal MHD is demonstrated. A numerical solution is obtained for a two-dimensional magnetosphere with a rectangular magnetopause and nonflaring tail. The results are consistent with the convection time sequences reported by Erickson (1985).

Hau, L.-N.↗
NASA NTRS1989

The theory of magnetohydrodynamic wave generation by localized sources. II - Collisionless dissipation of wave packets

The dispersion equation of Barnes (1966) is used to study the dissipation of asymptotic wave packets generated by localized periodic sources. The solutions of the equation are linear waves, damped by Landau and transit-time processes, in a collisionless warm plasma. For the case of an ideal MHD system, most of the waves emitted from a source are shown to cancel asympotically through destructive interference. The modes transporting significant flux to asymptotic distances are found to be Alfven waves and fast waves with theta (the angle between the magnetic field and the characteristics of the far-field waves) of about 0 and about pi/2.

Collins, William↗
NASA NTRS1989

Current sheet formation in a sheared force-free-magnetic field

This paper presents the results of a study showing how continuous shearing motion of magnetic footpoints in a tenuous, infinitely conducting plasma can lead to the development of current sheets, despite the absence of such sheets or even of neutral points in the initial state. The calculations discussed here verify the earlier suggestion by Low and Wolfson (1988) that extended current sheets should form due to the shearing of a force-free quadrupolar magnetic field. More generally, this work augments earlier studies suggesting that the appearance of discontinuities - current sheets - may be a necessary consequence of the topological invariance imposed on the magnetic field geometry of an ideal MHD system by virtue of its infinite conductivity. In the context of solar physics, the work shows how the gradual and continuous motion of magnetic footpoints at the solar photosphere may lead to the buildup of magnetic energy that can then be released explosively when finite conductivity effects become important and lead to the rapid dissipation of current sheets. Such energy release may be important in solar flares, coronal mass ejections, and other eruptive events.

Wolfson, Richard↗
NASA NTRS1990

Transient flux tubes in the terrestrial magnetosphere

Small, isolated density depletions are postulated to be introduced into the magnetotail by an uneven plasma loading process in the far tail. Such bubbles would be displaced earthward by an interchange process, and a significant depletion would move faster than either the gradient-curvature drift speed or the average convection speed. The dissolution of a bubble into the background results in a local reduction in plasma density, thereby violating ideal MHD and the subsequent conservation of plasma content. If a sufficient amount of magnetic flux in the far tail is contained in bubbles, then the net effect may be a reduction of plasma pressure in the near tail sufficient to resolve the pressure balance inconsistency.

Pontius, D. H., Jr.↗
NASA NTRS1990

Comments on compressible effects on Alfven normal modes in nonuniform plasmas

The paper discusses the regime of validity of the theory of dissipative Alfven normal modes presented by Mok and Einaudi (1985) and Einaudi and Mok (1985), which was based on the incompressible closure of the system of ideal MHD equations. Some simple extensions of the earlier results to the compressible case are described. In addition, certain misunderstandings of this work, which have appeared in other papers, are clarified.

Mok, Y.↗
NASA NTRS1990

The kink instability in infinite cylindrical flux tubes - Eigenvalues for power-law twist profiles

Simple, accurate methods of calculating ideal MHD instability eigenvalues for infinitely long cylindrical tubes with twist function T(r) are developed. The results show that the most rapidly growing and energetic instabilities occur in the Gold-Hoyle v = 0 field, with the instability progressively weakening with increasing v. However, the maximum force eigenvalue is always small, so that even in the Gold-Hoyle case only a small proportion of the available magnetic energy can be released in the linear phase. The results also confirm that the linear pinch is remarkably weak yet relatively resistant to line-tying. It is shown that the weakness of the force eigenvalue implies that the influence of uniform gas pressure on stability is negligible. Implications for the energy-release mechanism in solar flares are discussed.

Craig, I. J. D.↗
NASA NTRS1990

Numerical simulation of a catastrophe model for coronal mass ejections

Van Tend and Kuperus (1978) proposed a simple catastrophe model for magnetically driving coronal mass ejections, prominence eruptions, and two-ribbon flares. Their model, which is based on simple circuit concepts, suggests that a stable configuration containing a current filament will lose equilibrium when the filament current exceeds a critical value. Here, a two-dimensional numerical simulation is used to test how the Van Tend-Kuperus model works in an ideal MHD fluid. The simulation exhibits the expected loss of mechanical equilibrium near the predicted critical value, but the current filament moves only a short distance upward before coming to rest at a new equilibrium. However, this new equilibrium contains a current sheet which is resistively unstable to magnetic reconnection, and if magnetic reconnection occurs rapidly, the filament can continue to move upward at Alfvenic speeds.

Forbes, T. G.↗
NASA NTRS1990

Statistical properties of ideal three-dimensional magnetohydrodynamics

Classical Gibbs ensemble methods are used to study the spectral structure of three-dimensional ideal MHD in periodic geometry. In this paper the equilibrium ensemble incorporates constraints of total energy, magnetic helicity, and cross helicity. Several new results are proven for ensemble averages, including the constraint that magnetic energy equal or exceed kinetic energy, and that cross helicity represents a constant fraction of magnetic energy across the spectral domain, for arbitrary size systems. Two zero-temperature limits are considered in detail, emphasizing the role of complete and partial condensaiton of spectral quantities to the longest wavelength states. The ensemble predictions are compared to direct numerical solution using a low-order truncation Galerkin spectral code. Implications for spectral transfer of nonequilibrium, dissipative turbulent MHD systems are discussed.

Stribling, T.↗
NASA NTRS1990

Resonant Alfven wave heating of the plasma sheet boundary layer

The exchange of energy between the plasma mantle and the plasma sheet boundary layer (PSBL) is examined with a one-dimensional magnetotail model. The energy exchange occurs via Poynting flux generated by the localized mode conversion of a surface wave to an Alfven wave. This Poynting flux propagates through the lobe and into the PSBL where it is absorbed by two processes. The first arises from a gradient in the plasma beta causing a smooth absorption of Poynting flux. The second process results from the localized mode conversion of the decaying surface wave to an Alfven wave, causing a localized absorption of energy. A numerical solution of the linearized ideal MHD equations is obtained by assuming an adiabatic equation of state.

Harrold, B. G.↗
NASA NTRS1990

Theory of electromagnetic cyclotron wave growth in a time-varying magnetoplasma

The effect of a time-dependent perturbation in the magnetoplasma on the wave and particle populations is investigated using the Kennel-Petchek (1966) approach. Perturbations in the cold plasma density, energetic particle distribution, and resonance condition are calculated on the basis of the ideal MHD assumption given an arbitrary compressional magnetic field perturbation. An equation is derived describing the time-dependent growth rate for parallel propagating electromagnetic cyclotron waves in a time-varying magnetoplasma with perturbations superimposed on an equilibrium configuration.

Gail, William B.↗