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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 181 records · Page 10
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NASA NTRS1987

Simultaneous measurements of energetic ion (50 keV and above) and electron (220 keV and above) activity upstream of earth's bow shock and inside the plasma sheet - Magnetospheric source for the November 3 and December 3, 1977 upstream events

Simultaneous observations of energetic ions and electrons by the IMP 7 and 8 spacecraft are used here to separate temporal variations from spatial variations during the upstream ion events observed on December 3, 1977 and November 2-3, 1977, in order to determine the source of these particles. Analysis of the observations and comparison with theory shows that: (1) for each of the observed upstream enhancements, energetic ions and electrons were simultaneously present inside the plasma sheet; (2) the low-energy ion intensity profile inside the plasma sheet was relatively flat, while at higher energies there was considrable variability; (3) relativistic electron bursts were seen inside the plasma sheet and also upstream of the shock but at substantially reduced intensities; (4) the ion energy spectrum for the December 3 event, extended to energies of about 2 MeV, was identical in form with the plasma sheet and upstream of the shock; (5) ion anisotropies exhibited typically large dawn-dusk or dusk-dawn gradients and large field-aligned streaming away from the bow shock.

Sarris, E. T.↗
NASA NTRS2009

Simulation of Relativistic Shocks and Associated Radiation from Turbulent Magnetic Fields

Plasma instabilities (e.g., Buneman, Weibel and other two-stream instabilities) excited in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a new 3-D relativistic particle-in-cell code, we have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic electron-positron jet propagating into an unmagnetized electron-positron plasma. The simulation has been performed using a long simulation system in order to study the nonlinear stages of the Weibel instability, the particle acceleration mechanism, and the shock structure. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic (HD) like shock structure. In the leading shock, electron density increases by a factor of <_ 3.5 in the simulation frame. Strong electromagnetic fields are generated in the trailing shock and provide an emission site. We discuss the possible implication of our simulation results within the AGN and GRB context. We have calculated the time evolution of the spectrum from two electrons propagating in a uniform parallel magnetic field to verify the technique. The same technique will be used to calculate radiation from accelerated electrons (positrons) in turbulent magnetic fields generated by Weibel instability.

Nishikawa, K.↗
NASA NTRS2004

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Shocks

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (m) code, we have investigated particle acceleration associated with a relativistic jet front propagating through an ambient plasma with and without initial magnetic fields. We find only small differences in the results between no ambient and weak ambient parallel magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates particles perpendicular and parallel to the jet propagation direction. New simulations with an ambient perpendicular magnetic field show the strong interaction between the relativistic jet and the magnetic fields. The magnetic fields are piled up by the jet and the jet electrons are bent, which creates currents and displacement currents. At the nonlinear stage, the magnetic fields are reversed by the current and the reconnection may take place. Due to these dynamics the jet and ambient electron are strongly accelerated in both parallel and perpendicular directions.

Nishikawa, Ken-IchiI.↗
NASA NTRS2005

Particle Acceleration, Magnetic Field Generation in Relativistic Shocks

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating through an ambient plasma with and without initial magnetic fields. We find only small differences in the results between no ambient and weak ambient parallel magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates particles perpendicular and parallel to the jet propagation direction. New simulations with an ambient perpendicular magnetic field show the strong interaction between the relativistic jet and the magnetic fields. The magnetic fields are piled up by the jet and the jet electrons are bent, which creates currents and displacement currents. At the nonlinear stage, the magnetic fields are reversed by the current and the reconnection may take place. Due to these dynamics the jet and ambient electron are strongly accelerated in both parallel and perpendicular directions.

Nishikawa, Ken-Ichi↗
NASA NTRS1991

Relativistic motion in gamma-ray bursts

Three fundamental problems affect models of gamma-ray bursts, i.e., the energy source, the ability of high-energy photons to escape the radiation region, and the comparative weakness of X-ray emission. It is indicated that relativistic bulk motion of the gamma-ray-emitting plasma generically provides a solution to all three of these problems. Results show that, if the plasma that produces gamma-ray bursts has a bulk relativistic velocity with Lorentz factor gamma of about 10, several of the most troubling problems having to do with gamma-ray bursts are solved.

Krolik, Julian H.↗
NASA NTRS1992

The dynamics of low-beta plasma clouds as simulated by a three-dimensional, electromagnetic particle code

A 3D electromagnetic and relativistic particle simulation code is used to investigate the dynamics of low-beta plasma clouds moving perpendicular to an ambient magnetic field in vacuum and in a background plasma. The simulations show the formation of the space charge sheaths at the sides of the cloud with the associated polarization electric field which facilitate the cross-field propagation, as well as the sheaths at the front and rear end of the cloud caused by the larger ion Larmor radius, which allows ions to move ahead and lag behind the electrons as they gyrate. It is found that, in a background plasma, electron and ion sheaths expand along the magnetic field at the same rate, whereas in vacuum the electron sheath expands much faster than the ion sheath. Sheath clouds are accelerated up to relativistic energies, which indicates that artificial plasma clouds released in the ionosphere or magnetosphere may generate optical emissions as energetic sheath electrons scatter in the upper atmosphere.

Neubert, T.↗
NASA NTRS2012

Radiation from Shock-Accelerated Particles

Plasma instabilities excited in collisionless shocks are responsible for particle acceleration, generation of magnetic fields , and associated radiation. We have investigated the particle acceleration and shock structure associated with an unmagnetized relativistic jet propagating into an unmagnetized plasma. Cold jet electrons are thermalized and slowed while the ambient electrons are swept up to create a partially developed hydrodynamic-like shock structure. The shock structure depends on the composition of the jet and ambient plasma (electron-positron or electron-ions). Strong electromagnetic fields are generated in the reverse , jet shock and provide an emission site. These magnetic fields contribute to the electron's transverse deflection 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. The detailed properties of the radiation are important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jet shocks, and supernova remnants

Nishikawa, Ken-ichi↗
NASA NTRS1978

Origin and evolution of the radio emission from immediate postoutburst supernovae

Several models for the radio emission from immediate postoutburst supernovae are examined under the assumption that the expanding remnant consists of a homogeneously mixed distribution of relativistic particles, magnetic field, and thermal plasma. The evolutionary models are: (1) an adiabatic expansion model; (2) a model incorporating the existence of a central pulsar; and (3) variations on the first two models in which relativistic electrons are accelerated either instantaneously or over an extended period of time and in which ionization, bremsstrahlung, synchrotron, Compton, and expansion losses are explicitly included. The character of the radio emission expected from these models is quite dissimilar. Whereas in adiabatic expansion models the emission is expected to increase slowly and become most intense at high frequencies, in models involving a central pulsar the emission should increase rapidly with a maximum flux density that is the same at all frequencies. The theoretical evolution of the radio emission for each model is compared with observations of SN 1970g.

Marscher, A. P.↗
NASA NTRS1986

Wave propagation in pulsar magnetospheres - Refraction of rays in the open flux zone

The propagation of waves through a relativistically outflowing electron-positron plasma in a very strong dipolar magnetic field, conditions expected in pulsar magnetospheres, is investigated. Halmilton's equations is derived for the propagation of rays through a plasma which is inhomogeneous in density, magnetic field directions, and Lorentz factor. These equations are solved for rays propagating through the plasmas outflowing along the 'open' dipolar field lines in which the density decreases inversely as the radius cubed and in the case where gradients transverse to the radial direction exist. In the radial case, the effects of refraction on pulse profiles, spectrum, and polarization are examined, and the effects of a transverse gradient are indicated. Attention is given to models in which the observed broad bandwidth in the radio emission has its origin in a radius to frequency map. Models with broad-band emission at a single radius are also studied. These are compared to observations of pulse width and pulse component separation as a function of frequency. The origin of 'orthogonal modes' is discussed.

Barnard, J. J.↗
NASA NTRS1989

Physics of the central engine

Physical processes which may be important in the central engines of AGNs are discussed. Black hole accretion models have been elaborated during the past few years, and provide a plausible framework for jet and spectrum formation. However, the specific mechanisms responsible for producing the observed continuum radiation remain uncertain. Consideration is given to nonthermal models for the IR continuum, constraints on thermal plasma, and the production of relativistic electrons.

Begelman, Mitchell C.↗
NASA NTRS2012

Radio Detections During Two State Transitions of the Intermediate-Mass Black Hole HLX-1

Relativistic jets are streams of plasma moving at appreciable fractions of the speed of light. They have been observed from stellar-mass black holes (approx. 3 to 20 solar masses) as well as supermassive black holes (approx.. 10(exp 6) to 10(exp 9) Solar Mass) found in the centers of most galaxies. Jets should also be produced by intermediate-mass black holes (approx. 10(exp 2) to 10(exp 5) Solar Mass), although evidence for this third class of black hole has, until recently, been weak. We report the detection of transient radio emission at the location of the intermediate-mass black hole candidate ESO 243-49 HLX-1, which is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole to be between approx. 9 × 10(exp 3) Solar Mass and approx. 9 × 10(exp 4) Solar Mass.

transitions↗
NASA NTRS2004

Particle Acceleration, Magnetic Field Generation, and Emission in Relativistic Pair Jets

Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., Buneman, Weibel and other two-stream instabilities) created in collisionless shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating into an ambient plasma. We find that the growth times depend on the Lorenz factors of jets. The jets with larger Lorenz factors grow slower. Simulations show that the Weibel instability created in the collisionless shock front accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The small scale magnetic field structure generated by the Weibel instability is appropriate to the generation of "jitter" radiation from deflected electrons (positrons) as opposed to synchrotron radiation. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks.

Nishikawa, K. I.↗
NASA NTRS1981

Hot relativistic winds and the Crab Nebula

Efforts to formulate a self-consistent model of pulsar magnetospheres which links the particle source near the pulsar to the outflowing relativistic wind and couples the wind to the surrounding nebula are reviewed. The use of a relativistic MHD wind is recommended to account for global photon emission and the invisibility of the method of plasma transport. Consideration of a magnetic monopole relativistic wind due to an axially symmetric aligned rotator is combined with calculations of the initial velocity of the wind to show that the flow velocity in such a model will never exceed Mach 1. Extending the solution to the case of a hot relativistic wind at supersonic speeds is noted to yield results consistent with observations of the Crab Nebula

Fujimura, F. S.↗
NASA NTRS1985

Prompt acceleration of ions by oblique turbulent shocks in solar flares

Solar flares often accelerate ions and electrons to relativistic energies. The details of the acceleration process are not well understood, but until recently the main trend was to divide the acceleration process into two phases. During the first phase elctrons and ions are heated and accelerated up to several hundreds of keV simultaneously with the energy release. These mildly relativistic electrons interact with the ambient plasma and magnetic fields and generate hard X-ray and radio radiation. The second phase, usually delayed from the first by several minutes, is responsible for accelerating ions and electrons to relativistic energies. Relativistic electrons and ions interact with the solar atmosphere or escape from the Sun and generate gamma ray continuum, gamma ray line emission, neutron emission or are detected in space by spacecraft. In several flares the second phase is coincident with the start of a type 2 radio burst that is believed to be the signature of a shock wave. Observations from the Solar Maximum Mission spacecraft have shown, for the first time, that several flares accelerate particles to all energies nearly simultaneously. These results posed a new theoretical problem: How fast are shocks and magnetohydrodynamic turbulence formed and how quickly can they accelerate ions to 50 MeV in the lower corona? This problem is discussed.

Decker, R. B.↗
NASA NTRS2003

Particle Acceleration and Emission in Relativistic Jets

Shock wave acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating through an ambient plasma with and without initial magnetic fields. We find only small differences in the results between no ambient and weak ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates particles perpendicular and parallel to the jet propagation direction. While some Fermi acceleration may occur at the jet front, the majority of electron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that this instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields, which contribute to the electron's transverse deflection behind the jet head. The 'jitter' radiation from deflected electrons has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Nishikawa, K.↗
NASA NTRS2003

Particle Acceleration and Radiation associated with Magnetic Field Generation from Relativistic Collisionless Shocks

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas. Plasma waves and their associated instabilities (e.g., the Buneman instability, two-streaming instability, and the Weibel instability) created in the shocks are responsible for particle (electron, positron, and ion) acceleration. Using a 3-D relativistic electromagnetic particle (REMP) code, we have investigated particle acceleration associated with a relativistic jet front propagating through an ambient plasma with and without initial magnetic fields. We find only small differences in the results between no ambient and weak ambient magnetic fields. Simulations show that the Weibel instability created in the collisionless shock front accelerates particles perpendicular and parallel to the jet propagation direction. While some Fermi acceleration may occur at the jet front, the majority of electron acceleration takes place behind the jet front and cannot be characterized as Fermi acceleration. The simulation results show that this instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields, which contribute to the electron s transverse deflection behind the jet head. The "jitter" radiation from deflected electrons has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

Nishikawa, K.↗