Showing papers by "Smithsonian Astrophysical Observatory published in 1976"
TL;DR: In this article, a non-LTE empirical model of the quiet solar photosphere and the temperature-minimum region is presented, and the continuous spectrum computed from this model is in good overall agreement with available disk-center observations throughout the wavelength range from 0.125 to 500 microns.
Abstract: The paper presents a non-LTE empirical model of the quiet solar photosphere and the temperature-minimum region. The continuous spectrum computed from this model is in good overall agreement with available disk-center observations throughout the wavelength range from 0.125 to 500 microns. It is found that (1) absolute-intensity measurements are needed in the range between 1 and 2 microns to establish the structure of the deepest observable layers; (2) absolute-intensity or flux measurements are needed in the range between 20 and 200 microns to determine whether the minimum solar temperature occurring between the photosphere and the chromosphere is as low as indicated by present observations or much higher, as recent theoretical predictions indicate; (3) studies of the far-ultraviolet spectrum based on the assumption of LTE can be substantially in error; and (4) line opacity seems to account for the 'missing opacity' in the ultraviolet.
321 citations
TL;DR: A new type of time variation of cosmic X-ray sources has been found from the Astronomical Netherlands Satellite (ANS) observations of the source 3U 1820-30, associated with the globular cluster NGC 6624 as mentioned in this paper.
Abstract: A new type of time variation of cosmic X-ray sources has been found from the Astronomical Netherlands Satellite (ANS) observations of the source 3U 1820-30, associated with the globular cluster NGC 6624. Two bursts in the 1-30 keV X-ray intensity of this source are reported. Each displayed a rapid rise in flux (no more than 1 s) by a factor of 20-30 followed by an approximately 8-s exponential decay. These bursts appear to be qualitatively different from short time variations previously reported for X-ray sources. Analysis for further source variability, energy spectra, and position is presented. The characteristics of these events may imply the existence of a collapsed core in the globular cluster.
259 citations
TL;DR: In this article, general nuclear constraints are used to show that deuterium is most likely of pregalactic origin, and big-bang nucleosynthesis is the most plausible source for significant amounts of this isotope, but other, more speculative, sources are not ruled out.
Abstract: General nuclear constraints are used to show that deuterium is most likely of pregalactic origin. Big-bang nucleosynthesis is the most plausible source for significant amounts of this isotope, but other, more speculative, sources are not ruled out.
211 citations
TL;DR: In this paper, an analysis of the Prairie Network fireball trajectory data was performed to distinguish between chondritic meteorites and more fragile early-type carbonaceous chondrites, and it was concluded that if the carbonaceous material occurs in reasonable abundance among the fireballs, it has been properly identified by this analysis.
Abstract: An earlier analysis of Prairie Network fireball trajectory data failed to discriminate between the varieties of meteoric materials that are thought to be present. The Lost City meteorite event offers another calibration of the fireball data and prompts a reanalysis intended to distinguish between ordinary chondritic meteorites and the more fragile early-type carbonaceous chondrites. The carbonaceous material is expected to ablate more readily and have shorter trajectories or higher end heights. Observational error and natural dispersion in the structures produce severe smoothing effects, and the end-height diagnostic is, in itself, not entirely convincing. However, after making a tentative division of the data, one can discern differences in orbits and beginning heights among the three end-height groups. These are consistent with expectations of the behavior of chondritic stones (group I), weak carbonaceous chondrites (group II), and still more fragile cometary material (group III). It is concluded that if the carbonaceous material occurs in reasonable abundance among the fireballs, it has been properly identified by this analysis.
198 citations
TL;DR: In this paper, a detailed description of a search procedure for radio meteor streams and reports the detection of 275 streams in a synoptic-year sample of 19,698 radio meteors observed by the radar system of the Radio Meteor Project at Havana, Illinois.
117 citations
TL;DR: The Eastern Rift of Africa runs the gamut of crustal and lithospheric attenuation from undeformed shield through attenuated rift margin to active neo-oceanic spreading zones and is therefore peculiarly well suited to an examination of relationships between volcano spacings and crust/lithosphere thickness as discussed by the authors.
116 citations
TL;DR: In this article, high-resolution spectroscopy of Mars in the vicinity of 1.27 microns has revealed the presence of emission lines of O2 which are interpreted as the result of production of O 2(1 delta g) in the ultraviolet photolysis of Martian ozone.
Abstract: High-resolution spectroscopy of Mars in the vicinity of 1.27 microns has revealed the presence of emission lines of O2 which are interpreted as the result of production of O2(1 delta g) in the ultraviolet photolysis of Martian ozone. In the equatorial region of Mars, the dayglow intensity implies an ozone abundance near 0.2 microatm, which seems consistent with theoretical models which utilize the measured water-vapor abundance. A much larger ozone abundance exists in the atmosphere at high winter latitude, in agreement with direct measurement of ozone absorption performed by the Mariner spacecraft.
97 citations
TL;DR: The physical processes in comets which involve solar and nuclear radial forces that affect the motions of gases and icy grains, gas-phase chemistry very close to the nuclei of large comets near the sun, dissociation of parent molecules into radicals and of radicals into atoms, and ionization by sunlight and collisions are discussed in this article.
Abstract: The paper discusses physical processes in comets which involve solar and nuclear radial forces that affect the motions of gases and icy grains, gas-phase chemistry very close to the nuclei of large comets near the sun, sublimation of icy grains, dissociation of parent molecules into radicals and of radicals into atoms, and ionization by sunlight and collisions. The composition and dimensions of nuclei are examined along with variations in intrinsic brightness, the nature of volatiles, gas production rates in the coma, characteristics of icy grains in the coma, and the structure of streamers, ion tails, and dust tails. The structure of the coma is described in detail on the basis of spectroscopic observations of several comets. The origin of comets is briefly reviewed together with the relation of comets to earth, the interplanetary complex, and the interstellar medium. Desirable future observations are noted, especially by space missions to comets.
93 citations
TL;DR: In this article, the spin-spin parameter λ is found to vary little with vibrational state near the bottom of the potential well, but overestimate its dependence upon internuclear distance.
Abstract: Laser magnetic resonance spectra at submillimetre wavelengths of 14NH (v = 0 and v = 1), 15NH (v = 0) and 14ND (v = 0 and v = 1) have been measured and analysed Values of the hyperfine parameters for the different isotopic species are derived which are consistent with each other but are not sufficiently precise to show a dependence upon internuclear distance Ab initio calculations of the dipole-dipole hyperfine parameters agree with experiment but calculations by others of the Fermi contact parameters are not supported by these experiments More precise values for the rotational constants and the equilibrium internuclear distance than have been obtained from optical spectroscopy are derived The effective spin-spin parameter λ is found to vary little with vibrational state near the bottom of the potential well Ab initio calculations reproduce the magnitude of λ but overestimate its dependence upon internuclear distance No precise values for the g-factors could be obtained Somewhat larger populations
85 citations
TL;DR: In this paper, a mechanism was proposed to explain the large mean eccentricity and inclination of the early sun by using the second-degree harmonic of the sun's second-order harmonic.
83 citations
TL;DR: In this article, an X-ray detector using microchannel plates as a photocathode surface and imaging photoelectron multiplier, and a crossed wire grid as a two-dimensional position-sensitive detector is described.
Abstract: The paper describes an X-ray detector using microchannel plates as a photocathode surface and imaging photoelectron multiplier, and a crossed wire grid as a two-dimensional position-sensitive detector. The position resolution is 10 microns. The crossed wire grid consists of 100-micron-diam wires on 200-micron centers. Position sensing is accomplished by electronic interpolation to 1/20 of the wire spacing. The quantum efficiency of the microchannel plates varies from 29% at 0.28 keV to 5% at 3 keV. This detector will provide second-of-arc X-ray imaging in the focal plane of the 342.9-cm focal length grazing-incidence telescope being prepared for the HEAO-B observatory. By addition of suitable photocathodes, it can be used for single-photon imaging light detection in the UV, visible, and near-IR-ranges. In all cases, it gives a very low dark counting rate, allows timing of individual events to 1 microsec or less, and can handle counting rates up to 10,000 per sec.
TL;DR: In this paper, a relativistic version of the random phase approximation (RPA) is used to study allowed and forbidden radiative transitions in atoms, and the theory is applied to the He isoelectronic sequence to test its utility.
Abstract: A relativistic version of the random phase approximation (RPA) is used to study allowed and forbidden radiative transitions in atoms. The theory is applied to the He isoelectronic sequence to test its utility. Precise numerical solutions to the relativistic RPA equations are obtained describing the transitions $1^{1}S_{0}\ensuremath{\rightarrow}2^{3}S_{1} (M1)$, $1^{1}S_{0}\ensuremath{\rightarrow}2^{1,3}P_{1} (E1)$, and $1^{1}S_{0}\ensuremath{\rightarrow}2^{3}P_{2} (M2)$. The resulting excitation energies and transition probabilities are in good agreement with accurate nonrelativistic calculations for low-$Z$ elements. For intermediate- and high-$Z$ elements where relativistic effects are more important, the results are expected to be very accurate also. Extensive comparison shows good agreement of the calculated forbidden transition rates with available beamfoil measurements and the calculated transition energies with several lines from solar corona for high-$Z$ ($Z\ensuremath{\sim}25$) elements.
TL;DR: In this article, a detailed semianalytic algorithm is presented for calculating radiation-pressure perturbations in the Keplerian elements, which is also made to hold when e = 0, i = 0 or both.
Abstract: On the basis of expressions derived by Kozai (1961) and those developed in this paper, a detailed semianalytic algorithm is presented for calculating radiation-pressure perturbations in the Keplerian elements. Through some simple modifications, the algorithm is also made to hold when e = 0, i = 0, or both. The perturbations are obtained by summing over the sunlit segment of the satellite's orbit during each revolution or part thereof. The end points of this segment are evaluated numerically once per revolution. The effect of the inherent uncertainties in the boundaries of the earth's shadow is discussed. The algorithm is tested by means of numerical integration of the equations of motion and through comparisons with observations of the balloon satellite 1963 30D during a 200-day interval.
TL;DR: In this article, extreme-ultraviolet (EUV) observations of two polar transient features ('macrospicules') are described. These features appear to be caused by jets of chromospheric material that shoot upward to a height of 35,000 km above the limb and then fall back into the chromosphere, reaching terminal velocities of about 140 km/s.
Abstract: Extreme-ultraviolet (EUV) observations of two polar transient features ('macrospicules') are described. These features appear to be caused by jets of chromospheric material that shoot upward to a height of 35,000 km above the limb and then fall back into the chromosphere, reaching terminal velocities of about 140 km/s. On the basis of a model developed from the EUV measurements, it is found that the energy required to produce each event is about 3 by 10 to the 26th power ergs, about two orders of magnitude more than that required to produce an ordinary spicule. This indicates that macrospicules may be an important factor in the energy balance of the chromosphere and corona.
TL;DR: In this paper, the authors compared the prominence-corona interface (PC) with the chromospherecorona transition region (CC) in nine hedgerow prominences observed at the limb of the solar disk in order to compare the intensities of 52 optically thin EUV emission lines formed at temperatures of 350,000 to 2.2 million K.
Abstract: The intensities of 52 optically thin EUV emission lines formed at temperatures of 350,000 to 2.2 million K in nine hedgerow prominences observed at the limb are compared with the intensities of the same lines formed within network cells at the center of the solar disk in order to compare the prominence-corona interface (PC) with the chromosphere-corona transition region (CC). It is found for all nine prominences that the ratio of the intensity of a line measured in a cell to that in a prominence decreases with increasing temperature approximately as the -0.6 power of temperature. This ratio is used as the basis for comparing the PC with the CC in the framework of two different geometries wherein the prominence consists of one or more identical fully resolved slabs or threads in the line of sight or contains one or more identical unresolved cylindrical threads. It is concluded that three effects may contribute to the systematic difference between the PC and the CC: (1) the pressure within the PC might increase outward; (2) the temperature gradient within the PC might increase more slowly with temperature than in the CC; and (3) the unresolved internal geometry of a prominence can directly explain some, but not all, of the systematic difference.
TL;DR: In this paper, a model potential method was used to calculate the potential energy curves and the wavefunctions of the lowest six electronic states of the molecular ion Na 2 +, and an analysis of the long range behaviour of all the six states was presented.
TL;DR: Theoretical calculations of the equilibrium photoelectron fluxes in the upper atmosphere are reported in this article, which employ an accurate description of the photo-electron production processes and take explicit account of the discrete nature of the electron energy losses.
TL;DR: In this paper, the authors analyzed the densities of N2, O, Ar, and He at a height of 280 km, obtained from gas analyzer measurements aboard Esro-4, to establish a global pattern for variations that are associated with geomagnetic induced disturbances.
Abstract: Densities of N2, O, Ar, and He at a height of 280 km, obtained from gas analyzer measurements aboard Esro-4, were analyzed to establish a global pattern for variations that are associated with geomagnetically induced disturbances. It is shown that during longer periods of sustained geomagnetic activity, thermospheric temperature increases with geomagnetic latitude, reaching a maximum in the region of the magnetic pole, while during short-lived magnetic storms, the maximum temperature may be reached in the auroral zones. The observed variations of composition in middle and high latitudes can be explained by assuming that temperature variations are accompanied by variations in the height of the homopause. In the equatorial region the four gases studied vary in phase with similar amplitudes during a magnetic storm, indicating a density wave proceeding from higher latitudes.
TL;DR: In this paper, it is suggested that a hot optically thin corona along both faces of the accretion disk can produce the unpulsed component of emission observed during both the high and low-intensity states.
Abstract: X-ray emission from Her X-1 has been observed during the 23-day low state of the 35-day cycle. During this time, all the activity normally present during the high state was seen, including regular eclipses, absorption 'dips,' and 1.24-s pulsations. The present observations are compared with the models of Her X-1 derived by Roberts (1974), Gerend and Boynton (1976), Katz (1973), and Petterson (1975). It is suggested that a hot optically thin corona along both faces of the accretion disk can produce the unpulsed component of emission observed during both the high- and low-intensity states.
TL;DR: In this article, a theory of the probability of encounter of the sun with an interstellar comet at a distance comparable to the earth-sun distance is formulated, and a general expression is derived establishing the relationship among the influx rate of interstellar comets, the perihelion distance, the space density of the comets and the Maxwellian distribution of comet velocities.
TL;DR: In this paper, an analysis of photoelectric light curves of 91 mutual eclipses and occultations of the Galilean satellites that occurred from June to December 1973 is presented, which reduces all the data provided by these mutual events in one body, diminishes the latitude and longitude residuals of the unrevised theory by as much as a factor of about 10.
Abstract: This paper presents an analysis of photoelectric light curves of 91 mutual eclipses and occultations of the Galilean satellites that occurred from June to December 1973. From the deepest curves, radii and standard errors are deduced for J2, J3, and J4. It is shown that the substantial differences between the depths and midtimes observed and those predicted by Sampson's theory can be used in a clear and definite way to revise the orbital constants of that theory. The present analysis, which reduces all the data provided by these mutual events in one body, diminishes the latitude and longitude residuals of the unrevised theory by as much as a factor of about 10. The resulting overall consistency and the demonstrated inadequacy of an explanation based solely on surface-brightness variations argue that these corrections do represent the dominant effects required to account for the light curves. In order that these mutual events can be used with other astrometric data, the separations between a satellite pair at midevent are given. A small systematic (with solar phase angle) and unexplained difference in the longitude corrections is found for Io and Europa as obtained from nearly simultaneous pairs of occultations and eclipses.
TL;DR: Using hyperspherical coordinates and a Born-Oppenheimer-type expansion, the potential curves of hydrogen that converge to the $n = 1$ and $n=2$ thresholds of hydrogen were obtained in this paper.
Abstract: Using hyperspherical coordinates and a Born-Oppenheimer-type expansion, the potential curves of ${\mathrm{H}}^{\ensuremath{-}}$ that converge to the $n=1$ and $n=2$ thresholds of hydrogen are obtained. From the computed potential curves, the properties of bound states, Feshbach resonances, and shape resonances are conveniently studied.
TL;DR: In this article, the role of metastable species is examined, and theoretical O((2D) densities are compared to infer global variation of the solar ionizing flux, which provides an adequate basis for the calculation of solar ultraviolet photoionization rates.
Abstract: We have analyzed daytime composition measurements in the thermosphere from Atmosphere Explorer-C by use of a theory of the ion chemistry. Predicted and measured N2(+) densities are compared to infer global variation of the solar ionizing flux. Results indicate that the solar flux of Hinteregger measured on Atmosphere Explorer-C provides an adequate basis for the calculation of solar ultraviolet photoionization rates. The role of metastable species is examined, and theoretical O+(2D) densities are presented.
TL;DR: In this article, a model potential description for three valence transitions in Li2+ and Na2+ has been used to compute oscillator strengths and photodissociation cross sections.
TL;DR: In this paper, the occurrence times were fitted by least squares to a linear sequence with an rms deviation of 621 sec, corresponding to a phase jitter of 3.9%.
Abstract: Observational data are presented on ten brief X-ray bursts, recurrent at nearly equal time intervals, recorded during a two-day observation of 3U 1820-30 in the globular cluster NGC 6624 with the wide-field modulation collimator detectors on the SAS-3 X-ray observatory. The occurrence times were fitted by least squares to a linear sequence with an rms deviation of 621 sec, corresponding to a phase jitter of 3.9%. The bursts were observed in two energy channels. For a sequence of five of the most clearly observed bursts, the 2-6 keV rate rose to its peak value in about 1 sec and then decayed to preburst level in about 10 sec. The 6-11 keV rate rose more slowly and decayed similarly. The smooth decay of their averaged light curve suggests the temporal evolution may have a geometrical origin, where Compton scattering in a surrounding cloud of hot plasma stretches the pulse out and causes a progressive hardening of the spectrum.
TL;DR: A swarm of dikes formed the core of the Sagatu Ridge, a 70 km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau as mentioned in this paper.
TL;DR: In this paper, the authors used EUV and X-ray images of the Sun as tracers of the magnetic field structure in the Solar atmosphere in order to study the existence of currents in active regions.
Abstract: EUV and X-ray images of the Sun are used as tracers of the magnetic field structure in the Solar atmosphere in order to study the existence of currents in active regions. Criteria are suggested for comparing the data with theoretical extrapolations of magnetic field lines above the level of magnetograph observations. Analysis of the data presented in conjunction with force-free field calculations suggests the existence of currents flowing in opposite directions in nearby sets of loops in the active regions shown. Some simple qualitative implications of opposed current structures are suggested.
01 Jan 1976
TL;DR: In this article, the authors focused on cosmic dust measurements made by sensors on Pioneers 8 and 9 in Earth-like orbits and concluded that the particles are largely "β-meteoroids", interplanetary impact debris expelled by solar radiation pressure.
Abstract: Attention is centered on cosmic dust measures made by sensors on Pioneers 8 and 9 in Earth-like orbits. The conclusion follows Zook and Berg that the particles are largely “β-meteoroids,” interplanetary impact debris expelled by solar radiation pressure. An analysis of periodic comet orbits and comets observed during the missions failed to yield correlations, except possibly for debris from Comet Encke. A treatment of β-meteoroids from this stream is presented.