Search NASA⌕ Search

SEARCH · Search NASA

Results for “bremsstrahlung”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Energy densities and temperature associated with the diffuse X-ray background, galaxies and large scale structures

Model independent evidence indicating numerical coincidences between energy densities and temperatures which indicate possible connections between the diffuse X-ray background and the formation of galaxies and large scale structures is presented. A consideration of characteristic temperatures associated with galaxies, peculiar velocities of galaxies, and voids suggests characteristic energies of emission. Barring complications induced by magnetic fields, these temperatures may also be associated with components of the X-ray background. Thermal bremsstrahlung emmission associated with the formation of large scale structures may be the dominant contributor to the high energy X-ray background, 5 to 200 keV. Thermal bremsstrahlung emission produced by galaxies at the epoch of their formation may contribute significantly to the low energy X-ray background, 0.1 to 3 keV. It is concluded that an important, if not dominant fraction of the X-ray background, was probably emitted by hot gas associated with the formation of large scale structures and galaxies.

Daly, Ruth A.↗
NASA NTRS1988

A new quasi-thermal trap model for solar flare hard X-ray bursts - An electrostatic trap model

A new quasi-thermal trap model of solar flare hard X-ray bursts is presented. The new model utilizes the trapping ability of a magnetic mirror and a magnetic field-aligned electrostatic potential produced by differences in anisotropies of the electron and ion distribution function. It is demonstrated that this potential can, together with the magnetic mirror itself, effectively confine electrons in a trap, thereby enhancing their bremsstrahlung yield per electron. This analysis makes even more untenable models involving precipitation of the bremsstrahlung-producing electrons onto a cold target.

Spicer, D. S.↗
NASA NTRS1989

The behavior of beams of relativistic non-thermal electrons under the influence of collisions and synchrotron losses

For many astrophysical situations, such as in solar flares or cosmic gamma-ray bursts, continuum gamma rays with energies up to hundreds of MeV were observed, and can be interpreted to be due to bremsstrahlung radiation by relativistic electrons. The region of acceleration for these particles is not necessarily the same as the region in which the radiation is produced, and the effects of the transport of the electrons must be included in the general problem. Hence it is necessary to solve the kinetic equation for relativistic electrons, including all the interactions and loss mechanisms relevant at such energies. The resulting kinetic equation for non-thermal electrons, including the effects of Coulomb collisions and losses due to synchrotron emission, was solved analytically in some simple limiting cases, and numerically for the general cases including constant and varying background plasma density and magnetic field. New approximate analytic solutions are presented for collision dominated cases, for small pitch angles and all energies, synchrotron dominated cases, both steady-state and time dependent, for all pitch angles and energies, and for cases when both synchrotron and collisional energy losses are important, but for relativistic electrons. These analytic solutions are compared to the full numerical results in the proper limits. These results will be useful for calculation of spectra and angular distribution of the radiation (x rays, gamma-rays, and microwaves) emitted via synchrotron or bremsstrahlung processes by the electrons. These properties and their relevance to observations will be observed in subsequent papers.

Mctiernan, James M.↗
NASA NTRS1989

The Hard X-rays and Gamma-rays from Solar Flares

Radiation of energies from 10 KeV to greater than 10 MeV has been observed during solar flares, and is interpreted to be due to bremsstrahlung by relativistic electrons. A complete treatment of this problem requires solution of the kinetic equation for relativistic electrons and inclusion of synchrotron energy losses. Using the electron distributions obtained from numerical solutions of this equation the bremsstrahlung spectra in the impulsive x ray and gamma-ray regimes are calculated, and the variation of these spectral indices and directivities with energy and observation angle are described. The dependences of these characteristics of the radiation of changes in the solar atmospheric model, including the convergence of the magnetic field, the injected electron spectral index, and most importantly, in the anisotropy of the injected electrons and of the convergence of the magnetic field are also described. The model results are compared with stereoscopic observations of individual flares and the constraints that this data sets on the models are discussed.

Mctiernan, James M.↗
NASA NTRS1989

U. radio emission from quiescent filaments

Full-disk Very Large Array (VLA) synthesis maps of the quiet Sun indicate that filaments can be seen in emission at 91.6 cm wavelength; they are detected in absorption at shorter microwave wavelengths. The 91.6 cm emission has a brightness temperature of T sub B = 3 x 10(exp 5) K. It is hotter, wider and longer than the underlying filament detected at H alpha wavelengths, but the similarity between the shape, position, elongation and orientation of the radio and optical features suggests their close association. The 91.6 cm emission is attributed to the thermal-bremsstrahlung of a hot transition sheath that envelopes the H alpha filament and acts as an interface between the cool, dense H alpha filament and the hotter, rarefied corona. The transition sheath is seen in emission because of the lower optical depth of the corona at 90 cm wavelength, and the width of this sheet is 10(exp 9) cm. A power law gradient in pressure provides a better match to the observations than a constant pressure model; definitive tests of theoretical models await simultaneous multi-wavelength studies of filaments at different observing angles. When the thermal bremsstrahlung is optically thin, the magnetic field strength in the transition sheath can be inferred from the observed circular polarization. Variable physical parameters of the sheath, such as width, electron density, and electron temperature, can explain controversial reports of the detection of, or the failure to detect, the meter-wavelength counterpart of H alpha filaments.

Lang, Kenneth R.↗
NASA NTRS1990

The behavior of beams of relativistic nonthermal electrons under the influence of collisions and synchrotron losses

For many astrophysical situations, such as in solar flares or cosmic gamma-ray bursts, continuum gamma rays with energies up to hundreds of MeV were observed, and can be interpreted to be due to bremsstrahlung radiation by relativistic electrons. The region of acceleration for these particles is not necessarily the same as the region in which the radiation is produced, and the effects of the transport of the electrons must be included in the general problem. Hence it is necessary to solve the kinetic equation for relativistic electrons, including all the interactions and loss mechanisms relevant at such energies. The resulting kinetic equation for non-thermal electrons, including the effects of Coulomb collisions and losses due to synchrotron emission, was solved analytically in some simple limiting cases, and numerically for the general cases including constant and varying background plasma density and magnetic field. New approximate analytic solutions are presented for collision dominated cases, for small pitch angles and all energies, synchrotron dominated cases, both steady-state and time dependent, for all pitch angles and energies, and for cases when both synchrotron and collisional energy losses are important, but for relativistic electrons. These analytic solutions are compared to the full numerical results in the proper limits. These results will be useful for calculation of spectra and angular distribution of the radiation (X rays, gamma-rays, and microwaves) emitted via synchrotron or bremsstrahlung processes by the electrons. These properties and their relevance to observations will be observed in subsequent papers.

Mctiernan, James M.↗
NASA NTRS1990

The hard X-rays and gamma rays from solar flares

Radiation of energies from 10 KeV to greater than 10 MeV has been observed during solar flares, and is interpreted to be due to bremsstrahlung by relativistic electrons. A complete treatment of this problem requires solution of the kinetic equation for relativistic electrons and inclusion of synchrotron energy losses. Using the electron distributions obtained from numerical solutions of this equation the bremsstrahlung spectra in the impulsive X-ray and gamma ray regimes are calculated, and the variation of these spectral indices and directivities with energy and observation angle are described. The dependences of these characteristics of the radiation of changes in the solar atmospheric model, including the convergence of the magnetic field, the injected electron spectral index, and most importantly, in the anisotropy of the injected electrons and of the convergence of the magnetic field are also described. The model results are compared with stereoscopic observations of individual flares and the constraints that this data sets on the models are discussed.

Mctiernan, James M.↗
NASA NTRS1990

Comparison of thermal and nonthermal hard X-ray emission in electron-heated solar flares

Direct nonthermal bremsstrahlung and electronbeam-energized hot plasma thermal bremsstrahlung calculations are presently conducted on the basis of numerical simulation results for solar atmospheric response to heating during solar flares by nonthermal electron beams. It is established that while emission is dominated by the thermal component below a certain 'cross-over' energy, the direct brehmsstrahlung component becomes more important at higher energies. The dependence of the cross-over energy on the parameters of the electron energy input is also investigated, and the minimum photon energy which should be observable with future instruments in order that the nonthermal component not be swamped by the self-consistent thermal counterpart created by beam heating.

Li, Peng↗
NASA NTRS1991

Cosmic X-ray background from hot gas

This paper considers constraints on models of the cosmic X-ray background (XRB) in which the XRB is produced by optically thin thermal bremsstrahlung from hot gas. It is shown that models in which the gas is gravitationally confined in a spherical configuration and is heated only once are contradicted by the observed number of gravitationally lensed quasars together with the lower limit on the number of XRB sources required by limits on fluctuations in the XRB and the cosmic microwave background. In addition, it is shown that, for models in which the gas is not gravitationally confined, the expansion time of the gas is much shorter than the radiative cooling time, so that such models cannot explain the XRB. It is concluded that thermal bremsstrahlung models cannot account for the XRB if the emitting gas is heated only once.

Rogers, R. D.↗
NASA NTRS1992

Aeronomical modeling of the outer planets

The purpose is twofold: to report the programmatic issues of the project as to accomplishments and anticipated work statement for the next six months and to report the technical findings of the project. From the programmatic point of view the study was a clear success. Four talks were given on the work: (1) a talk on electron Bremsstrahlung X ray production at the annual American Astronomical Society, Division of Planetary Studies meeting in October of 1990 in Charlottesville, Virginia; (2) a poster on proton precipitation at the May, 1991 American Geophysical Union in Baltimore, Maryland; (3) an invited paper on multispectral time dependent modeling at the International Union of Geodosy and Geomagnetism in Vienna in August of 1991; and (4) a paper at the annual Division of Planetary Studies of the American Astronomical Society held in Palo Alto in November of 1991 entitled 'Jovian Bremsstrahlung X Rays: A Ulysses Prediction' which predicted the Jovian auroral X ray flux that should be measured by the Ulysses Gamma Ray Burst experiment during the Ulysses spacecraft's closest encounter with Jupiter in February 1992. The Ulysses' prediction paper generated much interest from people studying the Jovian aurora and from experimenters on the Ulysses spacecraft. As a result of this interest two things happened: (1) a paper of the same title was submitted and accepted for publication in the January issue of the Geophysical Research Letters; and (2) a massive observing campaign was organized to provide supporting ultraviolet and infrared observations at the time of the Ulysses encounter. This observational interest allowed personnel at the Southwest Research Institute (SwRI) (Alan Stern, PI; Hunter Waite Co-I) to obtain director's discretionary time on the Hubble Space Telescope to support the Ulysses encounter by obtaining ultraviolet observations. Once the multiple wavelength observations are obtained in February, the auroral model developed under this project will be used to analyze the data with hopes of providing new insight into the physical processes responsible for the Jovian aurora.

Waite, J. Hunter, Jr.↗
NASA NTRS1990

Solar accelerated particles - Comparisons of abundances and energy spectra from particle and gamma-ray observations

Gamma-ray observations of the April 27, 1981 flare have been used to derive elemental abundances of both the ambient gas and the accelerated particle which interact with this gas to produce gamma rays. The results show that the abundances of the accelerated particles which produce the gamma rays are much more consistent with first-class particle abundances than with large proton-flare abundances. Observed bremsstrahlung spectral from 16 flares are compared with calculated bremsstrahlung spectra. It is concluded that the electrons observed in interplanetary space and the electrons responsible for gamma-ray production cannot be drawn from the same accelerated particle population without the former being affected by reacceleration or energy-dependent escape.

Ramaty, R.↗
NASA NTRS1992

Properties of quasi-periodic oscillations in accreting magnetic white dwarfs

Previous studies of time-dependent accretion onto magnetic white dwarfs, in which the cooling was assumed to be due to bremsstrahlung emission, have shown that the accretion shock undergoes oscillations. However, when cyclotron cooling is also included, the oscillations are damped for sufficiently strong magnetic fields. Here we demonstrate that the oscillations can be sustained by accretion-fluctuation-induced excitations. The frequency of the QPOs are shown to increase quadratically with the magnetic field strength. We interpret the oscillations as a two-phase process in which bremsstrahlung cooling dominates in one half-cycle and cyclotron cooling in the other. Such a process may have very different consequences compared to a single-phase process where the functional form of the cooling is essentially the same throughout the cycle. If in the two-phase process damping occurs mainly in the cyclotron cooling half-cycle, there will be a universal effective damping factor which tends to suppress all oscillation modes indiscriminately. The oscillations of the accretion shock also could be a limit cycle process in which the system vacillates between two branches.

Wu, Kinwah↗
NASA NTRS1992

Noise-driven radiative shocks. II - Further implications for the quasi-periodic oscillations of the AM Herculis objects

The driven-shock model by Wolff et al. (1992) is extended and used to model the low-amplitude quasiperiodic oscillations (QPOs) driven by accretion noise in AM Her objects. The driven-shock model is discussed qualitatively with calculational details of the revised version. QPO-like features in the optical and X-ray are shown to be produced by bremsstrahlung-dominated shocks from accretion onto white dwarfs. These features are found to be consistent with the range of properties associated with the AM Her QPO sources. The bremsstrahlung-dominated shocks are shown to have linear responses for the conditions associated with QPO sources, and a luminosity expression for QPO sources is derived. The present analysis supports the shock-oscillation model for describing the optical QPOs from these objects.

Wood, Kent S.↗
NASA NTRS1992

Coronal Magnetic Structures Observing Campaign. II - Magnetic and plasma properties of a solar active region

Simultaneous soft X-ray, microwave, and photospheric magnetic field observations were taken during the Coronal Magnetic Structures Observing Campaign. The plasma electron temperatures and emission measures determined from the X-ray data are used to predict the intensity and structure of the thermal bremsstrahlung emission at 20 and 6 cm. Comparing these predictions with the microwave observations, it is found that the 20 cm structure is very similar to that expected from the X-rays, but a substantial amount of the 6 cm emission was resolved out. The predicted 20 cm brightness temperatures are higher than the observed, requiring cool absorbing material (not greater than 500,000 K) between the hot X-ray plasma and the observer. The absorption mechanism in the cool plasma at 20 cm is most likely thermal bremsstrahlung, requiring coronal magnetic fields as high as 150 G. 'Coronal Magnetograms', made by extrapolating the photospheric longitudinal field using the Sakurai code, show that appropriate values of the total field are reached at heights of 6000-10,000 km above the photosphere (at many but not all locations).

Schmelz, J. T.↗
NASA NTRS1993

Diffuse Galactic low energy gamma ray continuum emission

We investigate the origin of diffuse low-energy Galactic gamma-ray continuum down to about 30 keV. We calculate gamma-ray emission via bremsstrahlung and inverse Compton scattering by propagating an unbroken electron power law injection spectrum and employing a Galactic emmissivity model derived from COSB observations. To maintain the low energy electron population capable of producing the observed continuum via bremsstrahlung, a total power input of 4 x 10 exp 41 erg/s is required. This exceeds the total power supplied to the nuclear cosmic rays by about an order of magnitude.

Skibo, J. G.↗
NASA NTRS1993

Radio observations of peculiar emission-line Algol binary stars

Twelve peculiar emission-line Algol (PELA) binaries were observed at cm wavelengths. Six of these objects, WW And (tentative), KX And, AM Aur, V644 Mon, DN Ori, and AY Per were detected at the 100 to 400 micro-Jy level, corresponding to luminosity densities of order 10 exp 17 erg/s/Hz. These luminosity densities, combined with nearly flat cm spectra and high brightness temperatures, are consistent with the nonthermal gyrosynchrotron radiation model proposed as the result of observations from the previous PELA survey. Also, the star V367 Cyg was reobserved at thirteen different orbital phases; no variability was found within the errors. V367 Cyg's lack of variability and well-determined average spectral index, however, imply the possibility of a different emission mechanism, thermal bremsstrahlung, although the nonthermal model cannot be entirely ruled out. The characteristic brightness temperature of thermal bremsstrahlung is only about 10,000 K, which means that the radius of the emission region would have to be much larger than previously assumed, about 10 AU. Suggestions for further observations of all previously observed PELAS are presented in order to determine the correct emission mechanism.

Elias, Nicholas M., II↗
NASA NTRS1995

Plasma properties and magnetic field structure of the solar corona, based on coordinated Max 1991 observations from SERTS, the VLA, and magnetographs

The purposes of this investigation are to use existing, calibrated, coaligned sets of coordinated multiwaveband observations of the Sun to determine the coronal magnetic field strength and structure, and interpret the collective observations in terms of a self-consistent model of the coronal plasma and magnetic field. This information is vital to understanding processes such as coronal heating, solar wind acceleration, pre-flare energy storage, and active region evolution. Understanding these processes is the central theme of Max '91, the NASA-supported series of solar observing campaigns under which the observations acquired for this work were obtained. The observations came from NASA/GSFC's Solar EUV Rocket Telescope and Spectrograph (SERTS), the Very Large Array (VLA), and magnetographs. The technique of calculating the coronal magnetic field is to establish the contributions to the microwave emission from the two main emission mechanisms: thermal bremsstrahlung and thermal gyroemission. This is done by using the EUV emission to determine values of the coronal plasma quantities needed to calculate the thermal bremsstrahlung contribution to the microwave emission. Once the microwave emission mechanism(s) are determined, the coronal magnetic field can be calculated. A comparison of the coronal magnetic field derived from the coordinated multiwaveband observations with extrapolations from photospheric magnetograms will provide insight into the nature of the coronal magnetic field.

Brosius, Jeffrey W.↗
NASA NTRS1994

Concurrent X-ray and optical observations of two dwarf novae during eruption

The dwarf novae SU UMa and RU Peg were each observed with the ROSAT Observatory during outburst. SU UMa was also observed twice between outbursts. The spectrum of SU UMa during quiescence is well fitted by a 2.1 keV thermal bremsstrahlung model with a Gaussian line at 0.97 keV absorbed by a column density of 1.4 x 10(exp 20)/sq cm. During the outburst the X-ray count rate dropped by a factor of 3. The outburst spectrum is well fitted to a similar spectrum with a significantly larger absorption. In outburst the mass transfer rate, dot-M, approaches the critical value for the optically thin/thick transition predicted by theoretical models of the boundary layer while the X-ray emission remains optically thin. The best-fit model to the outburst spectrum of RU Peg is a 4.9 keV thermal bremsstrahlung plus a line at 0.89 keV absorbed by a column density of 2.8 x 10(exp 20)/sq cm. The column density is consistent with the upper limit determined for interstellar absorption. The boundary layer at outburst is expected to be optically thick based on the value of dot-M determined from the IUE spectra. There are no ROSAT observations of RU Peg between outbursts, but the flux detected by ROSAT is a factor of 10 lower than that seen with the Einstein Observatory, probably during quiescence. For both CVs the X-ray flux from the boundary layer significantly decreases at the same time that the optical-UV flux greatly increases. We suggest that during outburst most of the boundary layer energy is carried away as kinetic energy of the wind.

Silber, A.↗