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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Electric field evidence for tailward flow at substorm onset

Electric field observations made near the neutral sheet of the magnetotail provide additional support for the view that reconnection occurs in the near-earth region of the tail. Southward turnings of the magnetic field that start at, or shortly after, substorm onsets are accompanied by enhancements in the dawn-to-dusk electric field, resulting in a tailward E x B drift velocity. Both the magnetic and the electric fields in the tailward-flowing plasma are nonuniform and vary with inferred spatial scales of several earth radii in the events examined in this paper. These nonuniformities may be the consequence of the tearing-mode process. The E x B flow was also towards the neutral sheet and away from midnight in the events studied.

Nishida, A.↗
NASA NTRS1984

Magnetic structure of the distant geotail from -60 to -220 earth radii - ISEE-3

ISEE-3 magnetic-field measurements in the region of the geomagnetic tail from -80 to -220 earth radii are reported and discussed. A well-ordered field structure is found, comprising two 7-8-nT lobes separated by a plasma sheet, an embedded neutral sheet with significant By fields, and an intermittent plasma-sheet boundary layer with 5-nT-amplitude (peak-to-peak) electromagnetic waves. The plasma-sheet Bz distribution changes from principally northern orientation near the earth to an approximately equal north-south distribution at 200-220 earth radii. These findings are considered to be in general agreement with magnetic-reconnection models of the magnetosphere, with reconnection either throughout the region observed (in tearing-mode or plasmoid-formation models) or at a constant (about 220-earth-radii) or variable (40-80 to 220-earth-radii) X line (in X-line models).

Tsurutani, B. T.↗
NASA NTRS1982

A worldwide photographic network for wide-field observations of Halley's Comet in 1985-1986

A global network of ground-based observatories for the study of Halley's Comet in 1985/1986 is discussed. Recommendations are made with respect to improving coordination between reporting observatories, in order to ensure detailed imaging of such fast-generating cometary phenomena as plasma-tail knots, helices, disconnected tails, rays and condensations. A method for calibrating telescopes is considered by which well-studied objects will be photographed to provide references for images of Halley's Comet. This procedure is expected to reduce errors to approximately 0.05 mag. A coordinated study of Halley's Comet will provide important data on the physical properties of the Comet. Examples of the topics of study related to the plasma physics of the Comet's tail include: magnetic reconnection, rippling and tearing modes, kink instability, Kelvin-Helmholtz instability, and the flute instability.

Niedner, M. B., Jr.↗
NASA NTRS1984

Tearing instability in an anisotropic neutral sheet

A study is made of the collisionless tearing-mode stability properties of a field-reversed plasma layer whose temperature distribution is anisotropic. The plasma is confined by its self magnetic field with no external field. A kinetic description is used for both ions and electrons. The effects of the axis-crossing and nonaxis-crossing orbits are discussed. It is found that the conventional technique of matching the inner and outer asymptotic solutions at the electron inner-region is inadequate for the anisotropic case. An intermediate region in which the axis-crossing ion orbits are important is identified. The eigenvalue equation is solved using both analytic approximations and numerical methods to obtain the eigenmode structure and the linear dispersion relation. Previously announced in STAR as N84-14920

Chen, J.↗
NASA NTRS1984

Nonlinear computations of a solar flare model

A cylindrical axisymmetric tearing mode model for solar flares is investigated numerically. Large magnetic energy release only occurs when there are at least two mode rational surfaces in the current-carrying plasma.

Strauss, H.↗
NASA NTRS1985

Explosive tail reconnection - The growth and expansion phases of magnetospheric substorms

In the current 'conceptual' model of magnetospheric substorms, the growth phase terminates and the expansion phase commences with the onset of rapid reconnection at a new, near-earth X-type neutral line. Physical concepts developed in the analysis of the collisionless tearing mode and the flow of collisionless plasma in weakly magnetized, thin current sheets are combined to construct a model of purely collisionless, time-dependent, ion-dominated reconnection. Formulated in the context of time-dependent magnetospheric convection, the model describes the reconnection collapse of the initially thick plasma sheet. In the nonlinear phase the reconnection rate grows explosively in time and saturates into a steady collisionless reconnection flow when the initial magnetic flux in the current sheet has reconnected; at saturation the reconnection rate is comparable to the maximum Petschek rate. The time scale and dynamics of the explosive reconnection model are broadly consistent with observations of substorm growth phase and expansion phase onset. For typical plasma sheet parameters the explosive reconnection electromotive force across the tail approaches 1 MV at saturation.

Coroniti, F. V.↗
NASA NTRS1985

Unstable current systems and plasma instabilities in astrophysics; Proceedings of the 107th Symposium, University of Maryland, College Park, August 8-11, 1983

Among the topics discussed are: magnetic field reconnection in cosmic plasmas; energy dissipation mechanisms in the solar corona; and the acceleration of runaway electrons and Joule heating in solar flares. Consideration is also given to: the nonlinear evolution of the resistive tearing mode; anomalous transport in current sheets; equilibrium and instability in extragalactic jets; and magnetic field reconnection in differentially rotating accretion disks. Among additional topics discussed are: the creation of high energy electron tails by lower hybrid waves and its connection with type-II and type-III bursts; beam current systems in solar flares; and the spatio-temporal features of microwave emissions of active regions and flares.

Kundu, M. R.↗
NASA NTRS1986

Collisional tearing instability in the current sheet with a low magnetic Lundquist number

An MHD model is used to calculate the growth rate of the collisional tearing mode instability in a current layer with a Lundquist number in the range 3-100,000. The maximum growth rate is found to be a function of the Alfven transit time in the current sheet and the Lundquist number. When the Lundquist number is below 50, the growth rate asymptotically approaches a value of about -0.075/transit time. The discovery of the dependence of the asymptotic value is considered to be of use in studies of the current sheet in the diffusion region of a magnetic reconnection configuration, where the Lundquist number will be in the range 2-100.

Lee, L. C.↗
NASA NTRS1986

Magnetohydrodynamic instability

There have been major advances in the theory of magnetic reconnection and of magnetic instability, with important implications for the observations, as follows: (1) Fast and slow magnetic shock waves are produced by the magnetohydrodynamics of reconnection and are potential particle accelerators. (2) The impulsive bursty regime of reconnection gives a rapid release of magnetic energy in a series of bursts. (3) The radiative tearing mode creates cool filamentary structures in the reconnection process. (4) The stability analyses imply that an arcade can become unstable when either its height or twist of plasma pressure become too great.

Priest, E. R.↗
NASA NTRS1986

ISEE-1 and -2 observations of magnetotail flux ropes - FTEs in the plasma sheet?

ISEE-1 and 2 observations from about 20 Re down the near-earth magnetotail indicate the presence of magnetic flux ropes in the neutral sheet. Magnetic and electric field and fast plasma data show that these structures convect across the spacecraft at speeds of 200-600-km/s, and have scale sizes of roughly 3 5-Re. The rope axis orientation is approximately cross-tail. Their magnetic structure is similar to Venus ionospheric flux ropes, and to flux transfer events at the dayside magnetopause. These structures may arise from patchy reconnection or tearing mode reconnection within the plasma sheet.

Elphic, R. C.↗
NASA NTRS1986

The lower atmosphere of solar flares; Proceedings of the Solar Maximum Mission Symposium, Sunspot, NM, Aug. 20-24, 1985

The topics discussed by the present conference encompass the chromospheric flare phenomenon, white light flares, UV emission and the flare transition region, the flare corona and high energy emissions, stellar flares, and flare energy release and transport. Attention is given to radiative shocks and condensation in flares, impulsive brightening of H-alpha flare points, the structure and response of the chromosphere to radiation backwarming during solar flares, the interpretation of continuum emissions in white light flares, and the radiation properties of solar plasmas. Also discussed are EUV images of a solar flare and C III intensity, an active region survey in H-alpha and X-rays, dynamic thermal plasma conditions in large flares, the evolution of the flare mechanism in dwarf stars, the evidence concerning electron beams in solar flares, the energetics of the nonlinear tearing mode, macroscopic electric fields during two-ribbon flares, and the low temperature signatures of energetic particles.

Neidig, Donald F.↗
NASA NTRS1988

Nonlinear 3-D evolution of bounded kinetic Alfven waves due to shear flow and collisionless tearing instability

The three-dimensional nonlinear simulation of the evolution of a kinetic Alfven wave is presented for simple boundary conditions that resemble those of a discrete auroral breakup arc. The model used is a variant of the reduced megnetohydrodynamic description that includes, in addition, the important dispersive effect of electron inertia. It is found that with a 'generator' boundary condition corresponding to a current generator and a fully conducting 'load' boundary, the wave evolves three-dimensionally through a combination of shear flow and collisionless tearing mode instability.

Seyler, Charles E., Jr.↗
NASA NTRS1988

Quasi-static MHD processes in earth's magnetosphere

An attempt is made to use the MHD equilibrium theory to describe the global magnetic field configuration of earth's magnetosphere and its time evolution under the influence of magnetospheric convection. To circumvent the difficulties inherent in today's MHD codes, use is made of a restriction to slowly time-dependent convection processes with convective velocities well below the typical Alfven speed. This restriction leads to a quasi-static MHD theory. The two-dimensional theory is outlined, and it is shown how sequences of two-dimensional equilibria evolve into a steady state configuration that is likely to become tearing mode unstable. It is then concluded that magnetospheric substorms occur periodically in earth's magnetosphere, thus being an integral part of the entire convection cycle.

Voigt, Gerd-Hannes↗
NASA NTRS1988

Spectral analysis of turbulent effects on resistivity and the tearing instability

A spectral numerical-simulation code and ancillary diagnostics are used here to analyze the behavior of resistive magnetic tearing in the presence of short-wavelength turbulence. It is found that, in general, the anomalous resistivity defined by Biskamp and Welter (1983) due to short-wavelength turbulence tends to return quickly towards zero even when artificially supported away from zero, and that its effect on tearing-mode behavior is not consistent with its interpretation as a resistivity. The behavior reported by Biskamp and Welter and the behavior observed here are analyzed. It is argued that, while not meaningful as a true resistivity, the anomalous resistivity parameter is a useful diagnostic showing the energy balance of the system.

Deeds, D.↗
NASA NTRS1989

Magnetic reconnection in partially ionized gases

Magnetic field lines in a plasma reconnect at a rate scaled by the Alfven speed. In a partially ionized gas there are two natural Alfven speeds: one determined by the ionized mass density alone, which applies when ion-neutral friction is negligible, and one determined by the total mass density, which applies when ion-neutral friction is strong. When the ionization fraction is low, as in a dense molecular cloud, these two speeds differ by several orders of magnitude. Both time-dependent tearing modes and steady-state magnetic reconnection in partially ionized gas are considered, and the regimes in which the charged and neutral components are strongly, intermediately, and weakly coupled are delineated. Molecular clouds are probably in the intermediate regime, while reconnection in solar prominences probably has strong ion-neutral coupling. Reconnection proceeds more rapidly when coupling is not strong.

Zweibel, Ellen G.↗
NASA NTRS1989

Collisionless reconnection in a quasi-neutral sheet near marginal stability

Particle simulations are used to investigate the process of collisionless reconnection in a magnetotail configuration which includes a pressure gradient along the tail axis and tail flaring. In the absence of electron stabilization effects, the tearing mode is stabilized when the ion gyrofrequency in the normal field exceeds the growth rate in the corresponding one-dimensional current sheet. The presence of a low-frequency electromagnetic perturbation in the lobes can serve to destabilize a marginally stable current sheet by producing an extended neutral-sheet region which can then undergo reconnection. These results help to explain how X-type neutral lines, such as those associated with the onset of magnetospheric substorms, can be formed in the near-earth plasma sheet.

Pritchett, P. L.↗
NASA NTRS1990

A mathematical model of the structure and evolution of small-scale discrete auroral arcs

A three-dimensional fluid model for the structure and evolution of small-scale discrete auroral arcs originating from Alfven waves is developed and used to study the nonlinear macroscopic plasma dynamics of these auroral arcs. The results of simulations show that stationary auroral arcs can be unstable to a collisionless tearing mode which may be responsible for the observed transverse structuring in the form of folds and curls. At late times, the plasma becomes turbulent having transverse electric field power spectra that tend toward a universal k exp -5/3 spectral form.

Seyler, Charles E.↗
NASA NTRS1990

Plasma sheet convection and the stability of the magnetotail

Particle simulations are used to investigate the effects of plasma sheet convection into regions of increasing tail lobe magnetic field strength on the stability of a magnetotail equilibrium. The self-consistent treatment of convection first drives B(z) to zero on axis in the region of the strongest lobe field B(x) and then causes the equilibrium to break due to the rapid growth of tearing modes driven by the induced temperature anisotropy. The net result is the evolution of the inward-convecting plasma sheet into a slowly moving or stagnant plasmoid. The time scale for this process is much more rapid than that associated with particle drift losses across the tail. These results support the suggestion that steady convection within the plasma sheet may not be possible.

Pritchett, P. L.↗