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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

On scaling the magnetic field strength in interstellar clouds Resolution of the 'B versus n dilemma'

Attention is given to the 'B versus n dilemma' associated with the near constancy of magnetic field strength based on H I Zeeman data over a range of gas densities. The problem is examined in terms of preferential mass flow along magnetic field lines resulting from the low thermal energy of these regions. Approximate relations have been found to scale the magnetic field strength in interstellar clouds. It is noted that the fiducial gas density for scaling the increasing magnetic field strength is 2-3 orders of magnitude above the average interstellar density often used to estimate B.

Fleck, R. C., Jr.↗
NASA NTRS1987

Investigation of plasma contactors for use with orbiting wires

The proposed Shuttle-based short tether experiments with hollow cathodes have the potential for providing important data that will not be obtained in long tether experiments. A critical property for hollow cathode effectiveness as a plasma contactor is the cross magnetic field conductivity of the emitted plasma. The different effects of hollow cathode cloud overlap in the cases of motion-driven and battery-driven operation are emphasized. The calculations presented on the size and shape of the hollow cathode cloud improve the qualitative picture of hollow cathodes in low Earth orbit and provide estimates of time constants for establishing the fully-expanded cloud. The magnetic boundary value problem calculations indicate the way in which the magnetic field will effect the shape of the cloud by resisting expansion in the direction perpendicular to the field. The large-scale interactions of the system were also considered. It was concluded that recent plasma chamber experiments by Stenzel and Urrutia do not model an electrodynamic tether well enough to apply the results to tethered system behavior. Orbiting short tether experiments on hollow cathodes will provide critical information on hollow cathode performance and the underlying physics that cannot be obtained any other way. Experiments should be conducted as soon as funding and a suitable space vehicle are available.

Estes, Robert D.↗
NASA NTRS2010

Interstellar Magnetic Fields and Polarimetry of Dust Emission

Magnetic fields are an important ingredient in the stormy cosmos. Magnetic fields: (1) are intimately involved with winds from Active Galactic Nuclei (AGN) and stars (2) create at least some of the structures observed in the ISM (3) modulate the formation of clouds, cores, and stars within a turbulent medium (4) may be dynamically important in protostellar accretion disks (5) smooth weak shocks (C-shocks).

stormy cosmos↗
NASA NTRS1991

Periodically spaced fragmentation in Orion A

Evidence is found for periodic density structure in the highly fragmented interior of the Orion A molecular cloud from analysis of a large scale map of C(O-18) J = 1 - 0 emission. The spatial wavelength is about 1 parsec, and extends at least over 2 degrees along the main filament of Orion A and is also observed in other filamentary structures in Orion. This periodicity in the clumps is a result of externally triggered and magnetically mediated cloud collapse and fragmentation.

Dutrey, A.↗
NASA NTRS1980

The role of magnetic fields in the collapse of protostellar gas clouds

The paper presents the results of a numerical calculation of the collapse of an idealized protostellar gas cloud including the effects of a 'frozen-in' magnetic field. The 'traditional' picture of magnetic effects on gas clouds and recent observational and theoretical work on the subject are summarized. Attention is given to the method of calculation and the results are interpreted. It is found that the central magnetic field in the collapsing cloud model follows a rho to the 1/2 power relation, and the discussion implies that this is a general result which should hold true for some range of initial conditions around those chosen. In addition, it is found that the outer envelope of the cloud will be held up by tension in the field lines.

Scott, E. H.↗
NASA NTRS1979

'Coronae' of rotating interstellar clouds

This letter considers differential rotation of cool interstellar clouds in the presence of internal magnetic fields, and shows that because of the relative ineffectiveness of field dissipation within the clouds, magnetized gas experiences buoyant forces. The resulting field loops emerge from the cloud and dissipate their energy by field reconnection. The consequent heating is sufficient to produce relatively hot (T approximately 10,000 K) 'coronae' about the clouds.

Rosner, R.↗
NASA NTRS1984

Jupiter's and Saturn's fine-scale magnetic fields

In situ magnetic field data from Jupiter and Saturn are used to interpret earth-based microwave observations for all areas except Branson's hot spot on Jupiter. It is found that Jupiter's field is strongly dipolar but has large high-order moments compared with the magnetic field of the earth. Decametric emissions of Jupiter have a complex rotational pattern which appears to have been stable since 1980. Microwave observations Saturn's radio emissions were strongly asymmetric along the rotational axis, indicating the presence of longitudinal variations in the magnetic fields a thousand kilometers from the cloud tops. The magnetic fields within a few thousand kilmeters of the cloud tops of both Jupiter and Saturn could not be identified.

Warwick, J. W.↗
NASA NTRS1979

Propagation of a Forbush decrease in cosmic ray intensity to 15.9 AU

By observation of the large Forbush decrease in cosmic-ray intensity of April-May 1978 at heliocentric radial distances of 1.01, 6.97, and 15.91 AU, it is found that the causative magnetized plasma cloud moved outward from the sun at an apparent radial speed of about 960 km/s, independent of radial distance over this range. Recovery from the impulsive decrease in intensity was markedly slower at the larger distances. On the basis of this fact and other considerations, several tentative suggestions are made as to the large-scale nature of Forbush decreases and their relationship to the 11-year solar modulation of galactic cosmic-ray intensity.

Van Allen, J. A.↗
NASA NTRS1990

Collisionless coupling processes in AMPTE releases

An evaluation is made of results obtained to date by in situ measurements, numerical simulations, and theoretical considerations of Active Magnetospheric Particle Tracer Explorer chemical releases bearing on the nature of collisionless coupling processes. It is noted that both laminar and turbulent forces act to couple the solar wind momentum and energy to the release cloud; the magnetic field compression formed in this interaction plays an important intermediary role in coupling the two plasmas, and the intense electrostatic turbulence generated enhances the interaction. A scenario accounting for several features in the observed evolution of the December 27, 1984 artificial comet release is presented.

Lui, A. T. Y.↗
NASA NTRS1994

Effect of surface conductivity on the peak magnetic field radiated by first return strokes in cloud-to-ground lightning

The effect of surface conductivity on the peak magnetic field radiated by the first return stroke in cloud-to-ground lightning was investigated by comparing the peak magnetic fields from return strokes that struck water with those that struck land. The data were obtained from a network of three gated, wideband magnetic direction finders (DFs) at the NASA Kennedy Space Center during the summer of 1985. Two geographical areas that were equidistant from two of the direction finders were compared where the flash distances ranged from approximately 40 to 60 km. An unbiased data set was obtained by correcting site errors, equalizing differences in sensor gain, eliminating directional biases in DF triggering, and keeping differences in signal attenuation over the two surfaces to a minimum. When a statistical analysis was performed on the frequency distributions of the signal amplitudes, there was no statistically significant difference in the peak amplitudes of first return strokes over land (lambda = 8.2 x 10(exp -3) mho/m) and over water (lambda = 4 mho/m). Therefore we infer that the conductivity of the underlying surface does not significantly affect the magnitude of the peak magnetic field, and hence the peak current, in the first return stroke of a cloud-to-ground lightning flash.

Tyahla, Lori J.↗
NASA NTRS1990

The fuelling of active galactic nuclei

Accretion mechanisms for powering the central engines of active galactic nuclei (AGN) and possible sources of fuel are reviewed. It is a argued that the interstellar matter in the main body of the host galaxy is channeled toward the center, and the problem of angular momentum transport is addressed. Thin accretion disks are not a viable means of delivering fuel to luminous AGN on scales much larger than a parsec because of the long inflow time and effects of self-gravity. There are also serious obstacles to maintaining and regulating geometrically thick, hot accretion flows. The role of nonaxisymmetric perturbations of the gravitational potential on galactic scales and their triggers is emphasized. A unified model is outlined for fueling AGN, in which the inflow on large scales is driven by gravitational torques, and on small scales forms a mildly self-gravitating disk of clouds with inflow driven by magnetic torques or cloud-cloud collisions.

Shlosman, Isaac↗
NASA NTRS2002

Chandra Observations of a Young Embedded Magnetic B Star in the p Ophiuchus Cloud

This paper reports the analysis of two Chandra X-ray observations of the young magnetic B star rho Ophiuchus S1. X-ray emission from the star was detected in both observations. The average flux is almost the same in both, but during each observation the flux shows significant time variations by a factor of two on timescales of 20-40 ksec. Each spectrum can be fit by either an absorbed power law model with a photon index of approx. -3 or a thin-thermal plasma model with a temperature of approx. 2 keV and an extremely low metal abundance (approx. less than 0.1 solar). The spectrum of the first observation has an apparent line feature at about 6.8 keV, which likely corresponds to highly ionized iron K alpha. In contrast, the spectrum of the second observation shows an anomalous edge absorption component at E approx. 1 keV. The continuum emission and log (L(sub X)/L(sub bol)) approx. -6 are similar to those of young intermediate-mass stars (Herbig Ae/Be stars) although the presence of the magnetic field inferred from the detection of non-thermal radio emission has drawn an analogy between rho Ophiuchus S1 and magnetic chemically peculiar (MCP) stars. If the X-ray emission is thermal, the highest plasma temperature observed is too high to be explained by the conventional theories of magnetic stars, and favors some kind of magnetic dynamo activity, while if the emission is nonthermal, it might be related to mass infall. The 6.8 keV line and 4 keV edge features are marginal but they give important information near the stellar body if they are real. Their physical interpretation is discussed.

Hamaguchi, Kenji↗
NASA NTRS1980

Analytic solutions for single and multiple cylinders of gravitating polytropes in magnetostatic equilibrium

Exact analytic solutions for the static equilibrium of a gravitating plasma polytrope in the presence of magnetic fields are presented. The means of generating various equilibrium configurations to illustrate directly the complex physical relationships between pressure, magnetic fields, and gravity in self-gravitating systems is demonstrated. One of the solutions is used to model interstellar clouds suspended by magnetic fields against the galactic gravity such as may be formed by the Parker (1966) instability. It is concluded that the pinching effect of closed loops of magnetic fields in the clouds may be a dominant agent in further collapsing the clouds following their formation.

Lerche, I.↗
NASA NTRS1987

Mass outflows associated with young stellar objects

Properties of optical and molecular outflows associated with young stellar objects are discussed with emphasis placed on new results concerning outflow energetics, collimating structures, and the relationship between the outflow properties and the magnetic field geometry characterizing the host molecular clouds. Particular consideration is given to the IRAS observations of YSO mass outflows, which reveal extended far-IR emission associated with high-velocity molecular gas and in which a number of disk-like structures associated with YSO outflow sources were resolved. The disk axes appeared to lie along the direction of molecular outflows or stellar jets. The mass outflows showed a remarkable tendency to align along the direction of the magnetic fields which thread their host molecular clouds, suggesting that the cloud magnetic field must play an important role in determining the flattening (and perhaps the rotation) of protostellar structures.

Strom, Stephen E.↗