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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 199 records · Page 11
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NASA NTRS1982

Hydromagnetic stability of coronal arcade structures The effects of photospheric line tying

A model is given of the magnetic-field equilibrium and possible dynamic excitations of a solar coronal arcade. Such structures are well observed in the spectral range from H-alpha to X-rays and often give rise to two-ribbon flares. However, the preflare state must be stable to ideal magnetohydrodynamic disturbances, and this problem is treated with particular attention to the necessary foot-point boundary conditions. With reasonably general perturbation set, an energy-principle analysis is used to show the strong stabilizing influence of inertial field-line tying at the photosphere.

Ray, A.↗
NASA NTRS1983

Observations of hydromagnetic turbulence in the solar wind

MHD turbulence is studied by analyzing magnetic field and plasma observations from Helios-1 and -2 at minimum solar activity. The steady conditions in the plasma flows and the magnetic field sector structure in 1975/1976 facilitate an investigation of the radial evolution of the turbulence from 0.29 to 1AU. In high speed streams the fluctuations in the solar wind velocity v and the magnetic field b are highly correlated (the correction coefficient almost being one), which indicates that the turbulence is mainly Alfvenic in high speed plasma. While some general fluctuation properties remain essentially unchanged from 0.29 to 1AU, power spectral analysis reveals a different frequency composition of the Alfvenic turbulence at different heliocentric distances. At 0.3AU much more 'high' frequency fluctuations contribute to the total power in the magnetic field and velocity fluctuations than at 1AU. The contributions of field magnitude fluctuations are found to be distance and frequency dependent. Magnetic field spectra with an extended frequency range up to 470Hz show certain frequency bands, where the steepness of the spectra is independent of the helicocentric distance.

Denskat, K. U.↗
NASA NTRS1983

Theory of hydromagnetic turbulence

The present state of MHD turbulence theory as a possible solar wind research tool is surveyed. The theory is statistical, and does not make statements about individual events. The ensembles considered typically have individual realizations which differ qualitatively, unlike equilibrium statistical mechanics. Most of the theory deals with highly symmetric situations; most of these symmetries have yet to be tested in the solar wind. The applicability of MHD itself to solar wind parameters is highly questionable; yet it has no competitors, as a potentially comprehensive dynamical description. The purpose of solar wind research require sharper articulation. If they are to understand radial turbulent plasma flows from spheres, laboratory experiments and numerical solution of equations of motion may be cheap alternative to spacecraft. If "real life" information is demanded, multiple spacecraft with variable separation may be necessary to go further. The principal emphasis in the theory so far has been on spectral behavior for spatial covariances in wave number space. There is no respectable theory of these for highly anisotropic situations. A rather slow development of theory acts as a brake on justifiable measurement, at this point.

Montgomery, D.↗
NASA NTRS1983

Hydromagnetic vortices. II - Further dawnside events

It is shown that the 11 December 1977 plasma vortex event the subject of a multi-instrument investigation (Saunders et al., 1983) - was neither atypical nor uncommon, by describing the magnetic and plasma characteristics of three further vortices recorded within 3 weeks of, and at similar locations to, the 11 December study. One of the new events has added interest since magnetic pulsations were seen simultaneously on the ground in the vicinity of the satellite magnetic 'footprint'.

Saunders, M. A.↗
NASA NTRS1984

Standing hydromagnetic oscillations in the magnetosphere

The mechanisms for generating Alfven waves in the magnetosphere are determined, on the basis of a comprehensive survey of satellite magnetometer data. The mechanisms are classified in terms of the physical properties of standing Alfven waves such as the direction of the major axis of polarization, harmonic mode, and the azimuthal wave number. In an analysis of the statistical organization of the pulsations, it is shown that the most common transverse waves seem to be azimuthally polarized waves involving the fundamental and several harmonics, with radially polarized waves excited at the second harmonic. The former are probably driven externally, while the latter are likely to be excited internally through particle-wave interactions.

Takahashi, K.↗
NASA NTRS1986

Coupled hydromagnetic wave excitation and ion acceleration upstream of the Jovian bow shock

We extend the Lee (1982) self-consistent theory of upstream wave excitation and particle energization to address observations by Voyager 2 of sunward propagating MHD waves and diffuse suprathermal particle populations upstream of the Jovian bow shock. Two new ideas are incorporated into the theory. First, the interplanetary seed wave population is taken to be an equal admixture of waves propagating both toward and away from the shock parallel to the interplanetary magnetic field. Second, finite connection times are incorporated approximately into the theory in an effort to understand whether the particle spectra at high energy are limited by particle escape or finite connection time. It is found that finite connection times dominate the particle distribution at energies above 40 keV. In this manner the suprathermal proton distributions can be accounted for by a multiple reflection, shock acceleration theory. We find that the theory can also account for the low-frequency waves observed upstream of the shock in conjunction with the suprathermal ions.

Smith, C. W.↗
NASA NTRS1986

On the behavior of hydromagnetic surface waves

The behavior of velocity, magnetic field, and pressure perturbations about a continuously varying interface in pressure equilibrium is investigated in detail within ideal incompressible magnetohydrodynamics. A specific initial value problem is solved in quadrature for a thin interface and compared with the solution for a discontinuous interface. The unattenuated surface wave about a discontinuous interface is replaced at a thin interface by a collective surface disturbance which decays, with the associated energy density flowing into local oscillations within the interface. At long times the envelope of the local oscillations is concentrated within a small fraction of the thin interface (gradients within the envelope increase linearly with time, eventually resulting in a breakdown of the linearized ideal theory). Thus, the derived decay rate of the surface disturbance gives a mode-conversion rate rather than a heating rate. In applications to the propagation and dissipation of surface waves in the solar corona, this rate cannot in general be interpreted as a coronal heating rate.

Lee, M. A.↗
NASA NTRS1986

Strong hydromagnetic turbulence associated with Comet Giacobini-Zinner

The turbulence surrounding Comet Giacobini-Zinner reached intensities three orders of magnitudes above that of 'intermediate' solar wind conditions, was characterized by Delta B/B0 almost equal to one, and extended to cometocentric distances beyond one-million km. The strong spectral peak at 0.01 Hz and the general intensity profile of the turbulence are in excellent agreement with generation by plasma instabilities associated with the pickup of H2O(+) group ions. The total energy of the magnetic turbulence near 0.01 Hz is estimated to be 5 x 10 to the 15th joules and that of ion energization due to solar wind pickup 6 x 10 to the 13th watts. It thus takes a minimum of 100 s to build up the magnetic turbulence. This scale is approximately the linear growth rate of the ion pickup instability, giving a consistent picture of the energy source of the turbulence.

Tsurutani, B. T.↗
NASA NTRS1987

Hydromagnetic constraints on deep zonal flow in the giant planets

A simple model of the equatorial zonal jet in the giant planets is studied in which the flow is assumed uniform on cylinders concentric with the spin axis, and viscous and magnetic torques on each cylinder are balanced. This 'Taylor constraint' is solved simultaneously with the dynamo equation to obtain the velocity and magnetic field in the equatorial plane. The model is used to reproduce the widely differing jet widths of Jupiter and Saturn eddy viscosity of 2500 sq cm/s. For Saturn, the model has a large magnetic Reynolds number where the Chandrasekhar number Q = 1, and hence exhibits substantial axisymmetrization of the field in the equatorial plane.

Kirk, R. L.↗
NASA NTRS1987

Hydromagnetic wave excitation by ionised interstellar hydrogen and helium in the solar wind

The modification of the interplanetary MHD wave spectrum due to the pickup and isotropization of interstellar hydrogen and helium in the solar wind is predicted on the basis of quasi-linear and (for hydrogen) WKB theory. Results indicate that pickup helium has a very small (unobservable) effect on the solar wind wave spectrum, but that pickup hydrogen results in substantial modifications of cyclotron resonant frequencies (about 0.01 Hz at about 7 AU). It is noted that the predicted modifications beyond about 5 AU are substantial and could be observable at spacecraft frequencies greater than about 0.005 Hz if not degraded by turbulent wave-wave interactions or stochastic ion acceleration.

Lee, Martin A.↗
NASA NTRS1988

Hydromagnetic wave heating of the low-density interstellar medium

A simple model for supernova remnant sources of MHD waves is used to calculate the energy spectrum of waves in the intercloud medium and the heating rate resulting from their dissipation. Models of thermal phases of interstellar gas in ionization and thermal equilibrium are then constructed, and it is demonstrated that wave dissipation can be an important heating mechanism which can account for the observed high H I temperatures in low-density (intercloud) neutral gas.

Ferriere, Katia M.↗
NASA NTRS1988

Hydromagnetic wave heating of low density interstellar gas

The origin of the observed wave spectrum for hot gas in the ISM is considered theoretically. The governing equations for the generation, propagation, and dissipation of compressive waves are reviewed, and particular attention is given to the heating of warm neutral gas and the implications for radio-wave scattering. It is shown that little power from interactions between SN shocks and hot coronal gas reaches short wavelengths, and that scintillation probably does not originate in a warm weakly ionized gas.

Zweibel, Ellen G.↗
NASA NTRS1989

Steady hydromagnetic flows in open magnetic fields. II - Global flows with static zones

A theoretical study of an axisymmetric steady stellar wind with a static zone is presented, with emphasis on the situation where the global magnetic field is symmetrical about the stellar equator and is partially open. In this scenario, the wind escapes in open magnetic fluxes originating from a region at the star pole and a region at an equatorial belt of closed magnetic field in static equilibrium. The two-dimensional balance of the pressure gradient and the inertial, gravitational, and Lorentz forces in different parts of the flow are studied, along with the static interplay between external sources of energy (heating and/or cooling) distributed in the flow and the pressure distribution.

Tsinganos, K.↗
NASA NTRS1992

The effects of radiation drag on radial, relativistic hydromagnetic winds

The effects of drag on an idealized relativistic MHD wind of radial geometry are studied. The astrophysical motivation is to understand the effects of radiation drag on the dynamics of a jet or wind passing through the intense radiation field of an accreting compact object. From a critical point analysis, it is found that a slow magnetosonic point can appear in a dragged flow even in the absence of gravitational force, as a result of a balance between the drag force and the combination of thermal pressure and centrifugal forces. As in the undragged case, the Alfven point does not impose any constraints on the flow. Although it is formally possible for a dragged flow to possess more than one fast magnetosonic point, it is shown that this is unlikely in practice. In the limit of a 'cold', centrifugally driven flow, it is shown that the fast magnetosonic point moves to infinite radius, just as in the drag-free case. For a given mass flux, the total energy output carried to infinity, and the final partition between the kinetic energy and the Poynting flux, are the same for the dragged and the drag-free flows. The main effects of radiation drag are to increase the amount of energy and angular momentum extracted from the source and to redistribute the regions where acceleration occurs in the flow. This is accomplished through the storage and release of magnetic energy, as a result of additional winding and compression of the field caused by the action of the drag. For a relativistic wind, the dissipated energy can exceed the final kinetic energy of the flow and may be comparable to the total flow energy (which is dominated by Poynting flux). The energy lost to radiation drag will appear as a Doppler-boosted beam of scattered radiation, which could dominate the background radiation if the flow is well-collimated.

Li, Zhi-Yun↗
NASA NTRS1995

Hydromagnetic conditions near the core-mantle boundary

The main results of the grant were (1) finishing the manuscript of a proof of completeness of the Poincare modes in an incompressible nonviscous fluid corotating with a rigid ellipsoidal boundary, (2) partial completion of a manuscript describing a definition of helicity that resolved questions in the literature about calculating the helicities of vector fields with complicated topologies, and (3) the beginning of a reexamination of the inverse problem of inferring properties of the geomagnetic field B just outside the core-mantle boundary (CMB) from measurements of elements of B at and above the earth's surface. This last work has led to a simple general formalism for linear and nonlinear inverse problems that appears to include all the inversion schemes so far considered for the uniqueness problem in geomagnetic inversion. The technique suggests some new methods for error estimation that form part of this report.

Backus, George E.↗
NASA NTRS1972

Model for energy transfer in the solar wind: Formulation of model

The two-fluid solar-wind model is extended by including the collisionless dissipation of hydromagnetic waves originating at the sun. A series of solar wind models is generated, parameterized by the total energy flux of hydromagnetic waves at the base of the model. The resulting properties of propagation and dissipating of hydromagnetic waves on this model are presented.

Hartle, R. E.↗