SEARCH · Search NASA
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.
Our balloon-borne germain gamma-ray telescope was flown over Alice Springs, Australia, on 1984 November 20 to search for the 511 keV positron annihilation line from the Galactic Center. The measured line flux at Earth was (0.6 + or - 4.4) x 0.001 ph/sq cm/s indicating that the source was still in a low or off state.
The Comet IRAS-Araki-Alcock was observed with the VLA at 6 and 2 cm, when the comet was at geocentric distances of 0.08 and 0.035 AU, respectively, and the results are discussed. The three sigma upper limits to the flux density are 90 and 750 micro-Jy at the two wavelengths, respectively, values fully two orders of magnitude below the flux densities predicted by the icy-grain halo theory as initially developed. The corrected theory also indicates that the icy grain halo theory does not give an adequate description of the cometary environment. It is shown that the halo is either very optically thin, with a filling factor of the order of 10 to the -5th, or that the maximum size of the grain that can be lifted off the nuclear surface is only of the order of 10-100 microns.
Explore the source record for details and available documents.
I describe the qualitative features of a model for gamma-ray bursts assuming that they are produced by galactic neutron stars. The model is based on the assumption that electrons are continuously accelerated high up in the magnetosphere to relativistic energies and large pitch angles. These electrons emit a self-absorbed synchrotron spectrum at energies greater than mc-squared and, as a result, lose all of the energy associated with their motion perpendicular to the magnetic field. They thus slide along the field lines hitting the high density crust where by collision with ions acquire non-zero pitch angle (or are excited to higher Landau levels) and emit either cyclotron or bremsstrahlung photons.
It has been suggested that continuum radiation from Jupiter could provide the source for the 2- to 3-kHz radio emissions detected by Voyagers 1 and 2 in the outer heliosphere. By a fortuitous set of circumstances the Ulysses flyby of Jupiter in early 1992 provided an opportunity to compare the intensity of a strong, new heliospheric radio emission event with the intensity of continuum radiation from Jupiter. These comparisons show no evidence that Jupiter is the source of the heliospheric radio emissions.
The combined effects of interstellar dust absorption and of scattering by hydrogen atoms may give rise to a Ly(alpha) spectral feature of negative equivalent width, as has been observed in several star-forming galaxies. By considering the transfer of Ly(alpha) line radiation and of neighboring stellar continuum radiation within a dusty galaxy, we find that dust absorption has three effects: (1) it reduces the apparent ultraviolet continuum luminosity at all wavelengths; (2) it preferentially decreases the apparent Ly(alpha) line luminosity from H II regions; and (3) it creates an 'attenuation feature' in the continuum spectrum -- centered at the Ly(alpha) rest frequency -- which occurs because the attenuation of the stellar continuum radiation increases as the Ly(alpha) rest frequency is approached, due to the effects of scattering by hydrogen atoms. For plausible values of the galactic dust content and of the disk thickness, these effects can lead to a negative net Ly(alpha) equivalent width, even for galaxies in which the unattenuated spectrum would show a strong Ly(alpha) emission line.
A weak nonthermal continuum, radiated by the earth from energetic electrons in the outer radiation zone, is studied. It is shown that the frequency spectrum of the continuum radiation consists of two components, a trapped component, which is permanently trapped within the magnetosphere at frequencies below the solar wind plasma frequency, and an escaping component which propagates freely away from the earth at frequencies above the solar wind plasma frequency. Direction finding measurements and measurements of the spatial distribution of intensity for both components indicated that the continuum radiation is generated (1) in a broad region which extends through the morning and early afternoon immediately beyond the plasmapause boundary, and (2) over a broad range of latitudes, including the magnetic equator. Possible mechanisms by which this radiation is generated, including gyro-synchrotron radiation from energetic electrons in the outer radiation zone, are discussed.
The GOES X1 flare SOL2014-10-25T17:08:00 was a three-ribbon solar flare observed with the Interface Region Imaging Spectrograph (IRIS)in the near-UV(NUV)and far-UV. One of the flare ribbons crossed a sunspot umbra, producing a dramatic, ∼1000% increase in the NUV continuum radiation. We comprehensively analyze the UV spectral data of the umbral flare brightenings, which provide new challenges for radiative−hydrodynamic modeling of the chromospheric velocity field and the white-light continuum radiation. The emission line profiles in the umbral flare brightenings exhibit redshifts and profile asymmetries, but these are significantly smaller than in another, well-studied X-class solar flare. We present a ratio of the NUV continuum intensity to the FeIIλ2814.45intensity. This continuum-to-line ratio is a new spectral diagnostic of significant heating at high column mass (log m/[gcm−2]>−2) during solar flares because the continuum and emission line radiation originate from relatively similar temperatures but moderately different optical depths. The full spectral readout of these IRIS data also allow for a comprehensive survey of the flaring NUV landscape: in addition to many lines of Fe II and Cr II, we identify a new solar flare emission line, HeIλ2829.91(as previously identified in laboratory and early-type stellar spectra). The Fermi/GBM hard X-ray data provide inputs to radiative−hydrodynamic models (which will be presented in Paper II) in order to better understand the large continuum-to-line ratios, the origin of the white-light continuum radiation, and the role of electron beam heating in the low atmosphere.
Satellite measurements of the natural LF radio emission of the earth's magnetosphere are summarized, and the earth is compared as radio source to such familiar sources as Jupiter, Saturn and the sun. Two principal modes of radio emission from the earth's magnetosphere have been identified: auroral km-wavelength radiation and a continuum radiation. Characteristic spectra of the two modes are displayed. Other modes, not yet studied in much detail, are: dayside km-wavelength radiation, emission upstream of the bow shock, and narrow-bandwidth radio bursts. Emissions from the Jovian moon Io and from pulsars are also drawn upon for comparison. Current views on the sources of these terrestrial radio emissions are summarized.
The rate of heat transfer to the heat shield of a Jupiter probe has been estimated to be one order of magnitude higher than any previously experienced in an outer space exploration program. More than one-third of this heat load is due to an emission of continuum radiation from atoms and ions. The existing computer code for calculating the continuum contribution to the total load utilizes a modified version of Biberman's approximate method. The continuum radiation absorption cross sections of a C - H - O - N ablation system were examined in detail. The present computer code was evaluated and updated by being compared with available exact and approximate calculations and correlations of experimental data. A detailed calculation procedure, which can be applied to other atomic species, is presented. The approximate correlations can be made to agree with the available exact and experimental data.
Investigations of auroral radiation; terrestrial nonthermal continuum radiation; magnetospheric electrostatic emissions; ELF-VLF wave observations in the ionosphere and magnetosphere; wave-particle interactions; plasma wave propagation; plasma parameters; and ground-based observations of the magnetosphere during the International Magnetospheric Study are summarized.
Spectral observations of the central region of the Eta Carinae nebula at 4 microns have measured the Brackett-alpha hydrogen recombination-line intensity. The bulk of the line and continuum radiation arises in a small central source which is slightly narrower in the line radiation and is barely resolved by the employed beamwidth of 3.4 arcsec. The continuum radiation is well-fitted by an exponential relation to a distance of 10 arcsec from the central source. The line intensity requires a central source which emits 10 to the 49th power ionizing photons per sec into a region of electron density greater than 1 million per cu cm.
Plasma wave measurements from the outbound passes of Voyager 1 and 2 are used to study the plasma density and structure of the Jovian magnetotail. Two principal types of plasma waves are observed in the magnetotail, continuum radiation and narrowband emissions near the electron gyrofrequency. The low frequency cutoff of the continuum radiation can be used to determine the local electron density. Profiles of the electron density from the outbound passes of Voyager 1 and 2 provide evidence of a broad region of nearly uniform plasma density between the magnetopause and the inner corotating portion of the magnetosphere. We refer to this region as the boundary layer. Comparisons are made with other experimental and theoretical evidence for the existence of such a boundary layer inside the Jovian magnetosphere.
Results are reported for an investigation of the intensity, energy spectrum, and spatial distribution of the diffuse gamma radiation detected by SAS 2 away from the galactic plane in the energy range above 35 MeV. The gamma-ray data are compared with relevant data obtained at other wavelengths, including 21-cm emission, radio continuum radiation, and the limited UV and radio information on local molecular hydrogen. It is found that there are two quite distinct components to the diffuse radiation, one of which shows a good correlation with the galactic matter distribution and continuum radiation, while the other has a much steeper energy spectrum and appears to be isotropic at least on a coarse scale. The galactic component is interpreted in terms of its implications for both local and more distant regions of the Galaxy. The apparently isotropic radiation is discussed partly with regard to the constraints placed on possible models by the steep energy spectrum, the observed intensity, and an upper limit on the anisotropy.
The contribution of negative atomic ions to continuum radiation in nitrogen plasma is discussed. It is shown that both unstable N(-)(3P) and metastable N(-)(1D) ions have a significant effect on the total production of the continuum radiation at electron temperatures below 12,000 K.
Observations of locally trapped nonthermal continuum radiation and distant encounters with the Jovian magnetotail are reported. An attempt is made to deduce information on the morphology of the distant tail of Jupiter. It is noted that the observations of continuum radiation trapped in low-density regions of the solar wind suggest that Voyager may at times be connected to the distant tail by a low-density trough which acts as a wave guide and allows radiation from the tail to reach the spacecraft.
We have modeled the fluorescent pumping of electronic and vibrational emissions of molecular hydrogen (H2) within diffuse molecular clouds that are illuminated by ultraviolet continuum radiation. Fluorescent line intensities are predicted for transitions at ultraviolet, infrared, and red visible wavelengths as functions of the gas density, the visual extinction through the cloud, and the intensity of the incident UV continuum radiation. The observed intensity in each fluorescent transition is roughly proportional to the integrated rate of H2 photodissociation along the line of sight. Although the most luminous fluorescent emissions detectable from ground-based observatories lie at near-infrared wavelengths, we argue that the lower sky brightness at visible wavelengths makes the red fluorescent transitions a particularly sensitive probe. Fabry-Perot spectrographs of the type that have been designed to observe very faint diffuse Ha emissions are soon expected to yield sensitivities that will be adequate to detect H2 vibrational emissions from molecular clouds that are exposed to ultraviolet radiation no stronger than the mean radiation field within the Galaxy. Observations of red H2 fluorescent emission together with cospatial 21 cm H I observations could serve as a valuable probe of the gas density in diffuse molecular clouds.
Results are presented for Voyager 2 plasma wave and plasma measurements, obtained during mid-1980 to August 1981, which indicate the existence of clear signatures of Jovian nonthermal continuum radiation while the spacecraft was in the general downstream direction from Jupiter up to distances of approximately 4.5 AU. A periodicity indicating some solar wind control was indicated by the increasing magnitude and duration of the events as Voyager 2 approached the nominal aberrated tail position in the spring of 1981. It is found that each event shows characteristics suggestive of electromagnetic radiation trapped within a low-density cavity. Several of the events are characterized by a broad, moderately low density region surrounding a well-defined, very low density core. It is concluded that this series of continuum radiation events is best interpreted as the passage of Voyager 2 through the extended magnototail and wake of Jupiter. In addition, the various allowed tail configurations consistent with the observations are examined.