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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
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Quenching of the beam-plasma instability by 3-D spectra of large scale density fluctuations

A model is presented to explain the highly variable yet low level of Langmuir waves measured in situ by spacecraft when electron beams associated with Type III solar bursts are passing by; the low level of excited waves allows the propagation of such streams from the Sun to well past 1 AU without catastrophic energy losses. The model is based, first, on the existence of large scale density fluctuations that are able to efficiently diffuse small k beam unstable Langmuir waves in phase space, and, second, on the presence of a significantly isotropic nonthermal tail in the distribution function of the background electron population, which is capable of stabilizing larger k modes. The strength of the model lies in its ability to predict various levels of Langmuir waves depending on the parameters. This feature is consistent with the high variability actually observed in the measurements.

Muschietti, L.↗
NASA NTRS1991

Multi-wave band SMM-VLA observations of an M2 flare and an associated coronal mass ejection

Results are presented of observations of an M2 flare and an associated coronal mass ejection CME by instruments on the SMM as well as by the VLA and other ground-based observatories on September 30, 1988. The multiwave band data show a gradual slowly changing event which lasted several hours. The microwave burst emission was found to originate in compact moderately circularly polarized sources located near the sites of bright H-alpha and soft X-ray emission. These data are combined with estimates of an electron temperature of 1.5 x 10 to the 7th K and an emission measure of about 2.0 x 10 to the 49th/cu cm obtained from Ca XIX and Fe XXV spectra to show that the microwave emission can be attributed to thermal gyrosynchrotron radiation in regions where the magnetic field strength is 425-650 G. The CME acceleration at low altitudes is measured on the basis of ground- and space-based coronagraphs.

Willson, Robert F.↗
NASA NTRS1995

A case study of transient event motion in the magnetosphere and in the ionosphere

We present a case study of the magnetospheric and ionospheric response to a series of repetitive (7-8 min) solar wind dynamic pressure and interplanetary magnetic field (IMF) strength variations generated at the bow shock. During a period of duskward and antisunward IMF orientation, the magnetospheric and ionospheric observations indicate transient events moving dawnward and duskward away from a point of origin at or after local noon. In contrast, during a subsequent period of dawnward and antisunward IMF orientation, the observations indicate events moving duskward away from a point of origin at or prior to 0900 LT. We consider explanations in terms of pressure pulse driven riplets on the magnetopause or directly driven bursts of antiparallel merging.

Korotova, G. I.↗
NASA NTRS2008

Relativistic Particle-In-Cell Simulations of Particle Accleration in Relativistic Jets

Highly accelerated particles are observed in astrophysical systems containing relativistic jets and shocks, e.g., active galactic nuclei (AGNs), microquasars, and Gamma-Ray Bursts (GRBs). Particle-In-Cell (PIC) simulations of relativistic electron-ion and electron-positron jets injected into a stationary medium show that efficient acceleration occurs downstream in the jet. In collisionless relativistic shocks particle acceleration is due to plasma waves and their associated instabilities, e.g., the Buneman instability, other two-stream instabilities, and the Weibel (filamentation) instability. Simulations show that the Weibel instability is responsible for generating and amplifying highly non-uniform, small-scale magnetic fields. The instability depends on strength and direction of the magnetic field. Particles in relativistic jets may be accelerated in a complicated dynamics of relativistic jets with magnetic field. We present results of our recent PIC simulations.

Nishikawa, K.-I.↗
NASA NTRS1972

Information from Jupiter's decametric radiation

Radiation associated with decametric bursts are observed from about 39.5 MHz downwards, and its occurrence probability depends partly on the central meridian longitude of Jupiter. Three central meridian longitude regions of emission are designated as Sources A, B, and C. Emissions from B and C regions are also dependent on Io's phase. The frequency band of noise storms drifts slowly up or down the spectrum. Pulses with durations of 20 microseconds display drifts at the rate of 1000 MHz/sec. Decametric data are used to estimate Jupiter's surface magnetic field strength to be 10 gauss; however, if the maximum emission, about 10 MHz, is the electron gyrofrequency at Io's orbit, the surface field strength would be about 800 gauss. A model of the emission beam control exercised by Io is summarized. Data are presented which indicate that the width of source A emissions results from the changes in the angle of Jovicentric declination of the earth. One of two recent theories places the plasmapause at least 50 Jupiter radii from the surface on the sunward side, and the other places it within Io's orbit.

Carr, T. D.↗
NASA NTRS1983

Comparison of Photon-photon and Photon-magnetic Field Pair Production Rates

Neutron stars were proposed as the site of gamma-ray burst activity and the copious supply of MeV photons admits the possibility of electron-positron pair production. If the neutron star magnetic field is sufficiently intense ( 10 to the 12th power G), both photon-photon (2 gamma) and photon-magnetic field ( gamma) pair production should be important mechanisms. Rates for the two processes were calculated using a Maxwellian distribution for the photons. The ratio of 1 gamma to 2 gamma pair production rates was obtained as a function of photon temperature and magnetic field strength.

Burns, M. L.↗
NASA NTRS1983

Comparison of photon-photon and photon-magnetic field pair production rates

Neutron stars were proposed as the site of gamma-ray burst activity and the copious supply of MeV photons admits the possibility of electron-positron pair production. If the neutron star magnetic field is sufficiently intense (10 to the 12th power G), both photon-photon (2 gamma) and photon-magnetic field (gamma) pair production should be important mechanisms. Rates for the two processes were calculated using a Maxwellian distribution for the photons. The ratio of 1 gamma to 2 gamma pair production rates was obtained as a function of photon temperature and magnetic field strength.

Burns, M. L.↗
NASA NTRS2012

The Swift Burst Alert Telescope Detected Seyfert 1 Galaxies: X-Ray Broadband Properties and Warm Absorbers

We present results from an analysis of the broadband, 0.3-195 keV, X-ray spectra of 48 Seyfert 1-1.5 sources detected in the very hard X-rays with the Swift Burst Alert Telescope (BAT). This sample is selected in an all-sky survey conducted in the 14-195 keV band. Therefore, our sources are largely unbiased toward both obscuration and host galaxy properties. Our detailed and uniform model fits to Suzaku/BAT and XMM-Newton/BAT spectra include the neutral absorption, direct power-law, reflected emission, soft excess, warm absorption, and narrow Fe I K[alpha] emission properties for the entire sample. We significantly detect O VII and O VIII edges in 52% of our sample. The strength of these detections is strongly correlated with the neutral column density measured in the spectrum. Among the strongest detections, X-ray grating and UV observations, where available, indicate outflowing material. The ionized column densities of sources with O VII and O VIII detections are clustered in a narrow range with Nwarm [approx] 1021 cm-2, while sources without strong detections have column densities of ionized gas an order of magnitude lower. Therefore, we note that sources without strong detections likely have warm ionized outflows present but at low column densities that are not easily probed with current X-ray observations. Sources with strong complex absorption have a strong soft excess, which may or may not be due to difficulties in modeling the complex spectra of these sources. Still, the detection of a flat [Gamma] [approx] 1 and a strong soft excess may allow us to infer the presence of strong absorption in low signal-to-noise active galactic nucleus spectra. Additionally, we include a useful correction from the Swift BAT luminosity to bolometric luminosity, based on a comparison of our spectral fitting results with published spectral energy distribution fits from 33 of our sources.

Winter, Lisa M.↗
NASA NTRS1975

Fatigue performance of metal-lined graphite/epoxy pressure vessels

Using an ultrahigh-strength graphite fiber, a program was started to develop a thin metal-lined fiber/epoxy pressure vessel that would have a fatigue life of over 1000 cycles. First, the performance factor of the fiber/epoxy composite was found to be 351 kN m/kg from the average of 18 rubber-lined pressure vessels. Then, both aluminum- and titanium-lined vessels were filament wound with the graphite fiber in an epoxy matrix. Several of these metal-lined vessels were subjected to hydraulic fatigue testing to about 50% of their expected burst pressures. The average fatigue life of the aluminum-lined vessels was 462 cycles; the average for the titanium-lined vessels was 2190 cycles.

Hamstad, M. A.↗
NASA NTRS1992

Gamma ray monitoring of a AGN and galactic black hole candidates by the Compton Gamma Ray Observatory

The Compton Gamma-Ray Observatory's Burst and Transient Source Experiment (BATSE) has a powerful capability to provide nearly uninterrupted monitoring in the 25 keV-10 MeV range of both active galactic nuclei (AGN) and galactic black hole candidates (GBHC) such as Cygnus X-1, using the occultation of cosmic sources by the Earth. Since the Crab is detected by the BATSE Large Area Detectors with roughly 25(sigma) significance in the 15-125 keV range in a single rise or set, a variation by a factor of two of a source having one-tenth the strength of Cygnus X-1 should be detectable within a day. Methods of modeling the background are discussed which will increase the accuracy, sensitivity, and reliability of the results beyond those obtainable from a linear background fit with a single rise or set discontinuity.

Skelton, R. T.↗
NASA NTRS1992

Observations of mode coupling in the solar corona and bipolar noise storms

High-spatial-resolution observations of the sun which reflect on the role of mode coupling in the solar corona, and a number of new observations are presented. It is shown that typically, polarization inversion is seen at 5 GHz in active region sources near the solar limb, but not at 1.5 GHz. Although this is apparently in contradiction to the simplest form of mode coupling theory, it remains consistent with current models for the active region emission. Microwave bursts show no strong evidence for polarization inversion. Bipolar noise storm continuum emission is discussed in some detail, utilizing recent VLA observations at 327 MHz. It is shown that bipolar sources are common at 327 MHz. Further, the trailing component of the bipole is frequently stronger than the leading component, in apparent conflict with the 'leading-spot' hypothesis. The observations indicate that, at 327 MHz, mode coupling is apparently strong at all mode-coupling layers in the solar corona. The 327 MHz observations require a much weaker magnetic field strength in the solar corona to explain this result than did earlier lower-frequency observations: maximum fields are 0.2 G. This is a much weaker field than is consistent with current coronal models.

White, S. M.↗
NASA NTRS2012

Radiation from Accelerated Particles in Shocks and Reconnections

Plasma instabilities are responsible not only for the onset and mediation of collisionless shocks but also for the associated acceleration of particles. We have investigated particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. In the leading shock, electron density increases by a factor of about 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. These magnetic fields contribute to the electrons transverse deflection and, more generally, relativistic acceleration behind the shock. We have calculated, self-consistently, the radiation from electrons accelerated in the turbulent magnetic fields. We found that the synthetic spectra depend on the Lorentz factor of the jet, its thermal temperature and strength of the generated magnetic fields. Our initial results of a jet-ambient interaction with anti-parallelmagnetic fields show pile-up of magnetic fields at the colliding shock, which may lead to reconnection and associated particle acceleration. We will investigate the radiation in a transient stage as a possible generation mechanism of precursors of prompt emission. In our simulations we calculate the radiation from electrons in the shock region. The detailed properties of this radiation are important for understanding the complex time evolution and spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Nishikawa, K. I.↗
NASA NTRS2005

Discovery of a Neutron Star with Spin Frequency 530 Hz in A1744-361

We report the detection with the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) of 530 Hz burst oscillations in a thermonuclear (Type I) burst from the transient X-ray source A1744-361. This is only the second burst ever observed from this source, and the first to be seen in any detail. Our results confirm that A1744-361 is a low mass X-ray binary (LMXB) system harboring a rapidly rotating neutron star. The oscillations are first detected along the rising edge of the burst, and show evidence for frequency evolution of a magnitude similar to that seen in other burst sources. The modulation amplitude and its increase with photon energy are also typical of burst oscillations. The lack of any strong indication of photospheric radius expansion during the burst suggests a 9 kpc upper limit of the source distance. We also find energy dependent dips, establishing A1744-361 as a high inclination, dipping LMXB. The timescale between the two episodes of observed dips suggests an orbital period of approx. 97 min. We have also detected a 2 - 4 Hz quasi-periodic-oscillation (QPO) for the first time from this source. This QPO appears consistent with approx. 1 Hz QPOs seen from other high inclination systems. We searched for kilohertz QPOs, and found a suggestive 2.3 sigma feature at 800 Hz in one observation. The frequency, strength and quality factor are consistent with that of a lower frequency kilohertz QPO, but the relatively low significance argues for caution, so we consider this a tentative detection requiring confirmation.

Bhattacharyya, Sudip↗
NASA NTRS1982

The low energy spectra of gamma-ray bursts

The implications of observed gamma-ray burst spectra for the physical conditions and geometries of the sources are examined. It is noted that an explanation of the continua in terms of optically thin thermal bremsstrahlung requires a relatively large area but a fairly shallow depth. On the other hand, a spectrum similar to that observed could be produced by rapid flickering of sources with less extreme geometries if each flicker emits a Comptonized thermal spectrum. Either field inhomogeneities or plasma motions are required to interpret the low energy features as cyclotron extinction. An alternative explanation is photoelectric absorption by heavy atoms; this requires a field strength high enough to make one-photon electron positron annihilation possible. Observational tests of these possibilities are proposed

Bussard, R. W.↗
NASA NTRS1990

Burst prediction by acoustic emission in filament-wound pressure vessels

Acoustic emission in 51-cm diameter graphite/epoxy pressure vessels was monitored during pressurization (hydrotesting). Several vessels were subjected to impact by a blunt impactor, but only after the vessels had been proofed; that is, pressurized to 80 percent of nominal burst pressure as determined from control (unimpacted) vessels. AE activity was then monitored throughout a series of successively higher pressure cycles ranging from 10 to 60 percent of ultimate. Each cycle included a ramp up to pressure followed by a 4-min hold period and then pressure unload. The event rate was high, and especially modified AE analyzers had to be used to acquire the data. This paper presents the AE event count versus pressure history of these tests and demonstrates the ability of the AE technique to monitor the growth of damage and to estimate the effect on ultimate strength. The number of events that occurred during pressure holds proved to be a reasonable estimator of vessel performance.

Gorman, Michael R.↗
NASA NTRS1973

Mass motion in solar flares

Mass motions in solar flares are here considered in terms of a previously proposed model. Particle acceleration occurs during reconnection of a current sheet located at coronal heights. The downward component of the particle flux produces an impulsive hard X-ray burst and heats the upper layers of the chromosphere sufficiently to lead to explosive evaporation. Some of the evaporated gas remains trapped in newly closed magnetic field lines and is responsible for the soft thermal component of X-ray emission. Gas which flows along open magnetic field lines subsequently forms a plasmoid which is ejected by magnetic stresses into interplanetary space and may subsequently cause a geomagnetic storm. Analysis of a highly simplified model leads to formulas for the density, temperature, and other parameters of the flare-produced plasma in terms of a length scale and mean magnetic field strength for the flare.

Sturrock, P. A.↗
NASA NTRS1989

VLA observations of the coronal plasma

VLA observations at 20 cm wavelength specify the brightness temperature and magnetic structure of plasma constrained within coronal loops in solar active regions. Comparisons with simultaneous SMM observations at soft x ray wavelengths lead to measurements of physical parameters like electron density, electron temperature and magnetic field strength. Such comparisons also indicate coronal loops can be detected at either radio or x ray wavelengths while remaining invisible in the other spectral domain, and that the dominant radiation mechanisms can be thermal bremsstrahlung or thermal gyroresonance radiation. VLA observations at the longer 90 cm wavelength reveal the thermal emission of a hot transition sheath enveloping a cooler, underlying H alpha filament seen in absorption. The 20 cm VLA observations indicate that the precursor, impulsive and post-flare components of solar flares originate in spatially separated and resolved sources. The 90 cm VLA data indicate that time-correlated radio bursts can occur in active regions on opposite sides of the solar equator. These regions are apparently linked by large scale, trans-equatorial magnetic loops at least 2.6 x 10(exp 5) km (or 6 feet) long; these loops act as magnetic conduits for relativistic electrons moving at one-third the velocity of light.

Lang, Kenneth R.↗
NASA NTRS1990

VLA observations of the coronal plasma

VLA observations at 20 cm wavelength specify the brightness temperature and magnetic structure of plasma constrained within coronal loops in solar active regions. Comparisons with simultaneous SMM observations at soft X-ray wavelengths lead to measurements of physical parameters like electron density, electron temperature and magnetic field strength. Such comparisons also indicate coronal loops can be detected at either radio X-ray wavelengths while remaining invisible in the other spectral domain and that the dominant radiation mechanisms can be thermal bremsstrahlung or thermal gyroresonance radiation. VLA observations at the longer 90 cm wavelength reveal the thermal emission of a hot transition sheath enveloping a cooler, underlying H alpha filament seen in absorption. The 20 cm VLA observations indicate that the precursor, impulsive and post-flare components of solar flares originate in spatially separated and resolved sources. The 90 cm VLA data indicate that time-correlated radio bursts can occur in active regions on opposite sides of the solar equator. These regions are apparently linked by large scale, trans-equatorial magnetic loops at least 2.6 x 10(exp 5) km (or 6 feet) long; these loops act as magnetic conduits for relativistic electrons moving at one-third the velocity of light.

Lang, Kenneth R.↗