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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
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NASA NTRS1992

Coronal magnetic structures observing campaign. IV - Multiwaveband observations of sunspot and plage-associated coronal emission

Results of simultaneous observations of an active region located near the central meridian obtained on December 18, 1987, are presented. An asymmetric looplike structure connects the strong leading sunspot with a nearby region of opposite polarity. Both 6- and 20-cm emission lie along this structure, rather than over the sunspot, with higher frequency emission originating closer to the footpoint inside the sunspot. The 20-cm emission is due to a superposition of second- and third-harmonic gyroemission, where the field strength is 160-300 G, while the 6-cm emission is due to third-harmonic gyroemission from a region where the magnetic field strength ranges from 547 to 583 G. X-ray data associated with an area of trailing plage are used to predict the brightness temperature structure due to thermal bremsstrahlung emission in the 6- and 20-cm wavebands.

Brosius, Jeffrey W.↗
NASA NTRS1992

The synchrotron bays of the Crab Nebula - A magnetic structure associated with a presupernova circumstellar disk

We investigate the nature of two large conical indentations in the Crab Nebula's synchrotron emission, commonly known as the east and west 'bays'. Optical images spanning nearly nine decades (1899-1988) reveal the bays to be long-lasting remnant features. Proper motions of the bays away from the remnant's center of expansion are consistent with their locations near the SN progenitor site in A.D. 1054. Based on their morphologies and polarization properties, the bays appear to be portions of a torus encircling the remnant's center of expansion. The nebular magnetic field wrapped around this torus blocks the pulsar's relativistic particles, resulting in the observed lack of synchrotron emission. We propose that this torus is the result of a presupernova disklike ejection not less than 0.1 solar mass by the Crab progenitor, similar to the ring around SN 1987 A. A 0.5 solar mass disk of He-rich filaments along the equatorial plane is probably also related to presupernova mass loss, which might explain the wide range of abundances seen among filaments.

Fesen, Robert A.↗
NASA NTRS1998

Study of Magnetic Structure in the Solar Photosphere and Chromosphere

This grant funded an observational and theoretical program to study the structure and dynamics of the solar photosphere and low chromosphere, and the spectral signatures that result. The overall goal is to learn about mechanisms that cause heating of the overlying atmosphere, and produce variability of solar emission in spectral regions important for astrophysics and space physics. The program exploited two new ground-based observational capabilities: one using the Swedish Solar Telescope on La Palma for very high angular resolution observations of the photospheric intensity field (granulation) and proxies of the magnetic field (G-band images); and the other using the Near Infrared Magnetograph at the McMath-Pierce Solar Facility to map the spatial variation and dynamic behavior of the solar temperature minimum region using infrared CO lines. We have interpreted these data using a variety of theoretical and modelling approaches, some developed especially for this project. Previous annual reports cover the work done up to 31 May 1997. This final report summarizes our work for the entire period, including the period of no-cost extension from 1 June 1997 through September 30 1997. In Section 2 we discuss observations and modelling of the photospheric flowfields and their consequences for heating of the overlying atmosphere, and in Section 3 we discuss imaging spectroscopy of the CO lines at 4.67 mu.

Noyes, Robert W.↗
NASA NTRS1982

Magnetic structure of a flaring region producing impulsive microwave and hard X-ray bursts

Using VLA observations of the 1B/M1 flare of June 25, 1980, 6-cm 'snapshot' maps are synthesized. The spatial and temporal resolutions during the 9 minutes of the impulsive phase were, respectively, 1 arcsec x 2 arcsec and 10 s. Some displacement is noted between the locations of the burst source and the preflare loop structures seen in the preflare map. The burst peak occurred on the neutral line of the preflare polarization map, between the two oppositely polarized microwave 'loop' structures approximately 40 arcsec long. Concurrent hard X-ray observations were made of the burst, although these had no spatial resolution. The 6-cm maps show the locations of a number of the X-ray burst spikes. The 6-cm burst was fully resolved into at least eight components, many of which were bipolar.

Kundu, M. R.↗
NASA NTRS1986

Average dimension and magnetic structure of the distant Venus magnetotail

The first major statistical investigation of the far wake of an unmagnetized object embedded in the solar wind is reported. The investigation is based on Pioneer Venus Orbiter magnetometer data from 70 crossings of the Venus wake at altitudes between 5 and 11 Venus radii during reasonably steady IMF conditions. It is found that Venus has a well-developed-tail, flaring with altitude and possibly broader in the direction parallel to the IMF cross-flow component. Tail lobe field polarities and the direction of the cross-tail field are consistent with tail accretion from the solar wind. Average values for the cross-tail field (2 nT) and the distant tail flux (3 MWb) indicate that most distant tail field lines close across the center of the tail and are not rooted in the Venus ionosphere. The findings are illustrated in a three-dimensional schematic.

Saunders, M. A.↗
NASA NTRS1986

Preferential site occupation and magnetic structure of Nd2(Co/x/Fe/1-x/)14B systems

Rietveld analyses of room-temperature neutron diffraction data for seven Nd2(Co/x/Fe/1-x/)14B alloys (x = 0,0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. Throughout the entire composition range the Nd2Fe14B-type tetragonal crystal structure is maintained, with the lattice constants decreasing significantly as the Co concentration x increases. It is found that the J2-type transition-metal sites are preferentially occupied by Fe ions in the pseudoternary systems, a result which is analogous to the preferential Fe occupation of c sites previously observed in hexagonal Nd2(Co/x/Fe/1-x/)17 alloys.

Herbst, J. F.↗
NASA NTRS1987

The acceleration of winds by MHD waves in open magnetic structures

It has become increasingly clear in recent years that the available models describing the acceleration of high speed streams in solar coronal holes are inadequate. Thermal models do not result in winds with the proper thermal speed and density. In addition models which assume acceleration by Alfven waves, though promising, are not self-consistent when applied low in coronal holes where the acceleration of the wind takes place. An Alfven wave in the corona with a period of a few hundred seconds will have a wavelength of the order of 10 cm. This is of the order of the diameter of a coronal hole in the low corona, and therefore it is expected that the MHD waves are guided in some sense by the coronal hole in which they propagate. This basic idea led to the development of a leaky waveguide theory to describe MHD wave propagation in coronal holes. Recent efforts were concentrated in two areas: the development of numerical codes to properly describe the hydrodynamics of the flow, and more realistic models of the coronal hole properties such as density gradients and spherical divergence of the flux tube. Some of the important aspects of the theory are discussed, including the effect of density inhomogeneities and smooth transverse density gradients. In addition it is hoped that some early results from the numerical models will be presented. The acceleration of cool winds is a problem of interest in cool supergiants also. Some work was done on the development of wind models for these stars as well.

Davila, Joseph M.↗
NASA NTRS1989

Magnetic structure in cool stars. XVI - Emissions from the outer atmosphere of M-type dwarfs

Consideration is given to emission from the outer atmospheres of M-type dwarfs in several spectral lines originating from the chromosphere, the transition-region, and the soft X-ray emission from the corona. It is shown that M-type dwarfs systematically deviate from relations between flux densities in soft X-rays and chromospheric and transition-region emission lines. The quantitative relation between the equivalent width of H-alpha and the Ca II, H, and K emission index is determined. It is suggested that the emission in the Balmer spectrum may result from back heating by coronal soft X-rays.

Rutten, R. G. M.↗
NASA NTRS2008

Diagnosis of Magnetic Structures and Intermittency in Space Plasma Turbulence using the Method of Surrogate Data

Several observations in space plasmas have reported the presence of coherent structures at different plasma scales. Structure formation is believed to be a direct consequence of nonlinear interactions between the plasma modes, which depend strongly on phase synchronization of those modes. Despite this important role of the phases in turbulence, very limited work has been however devoted to study the phases as a potential tracers of nonlinearities in comparison with the wealth of literature on power spectra of turbulence where phases are totally missed. We present a method based on surrogate data to systematically detect coherent structures in turbulent signals. The new method has been applied successfully to magnetosheath turbulence (Sahraoui, Phys. Rev. E, 2008, in press), where the relationship between the identified phase coherence and intermittency (classically identified as non Gaussian tails of the PDFs) as well as the energy cascade has been studied. Here we review the main results obtained in that study and show further applications to small scale solar wind turbulence. Implications of the results on theoretical modelling of space turbulence (applicability of weak/wave turbulence, its validity limits and its connection to intermittency) will be discussed.

Sahraoui, Fouad↗