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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Spectral anomalies in low dispersion SWP images

Physical interpretation of IUE spectra obtained with the SWP camera may be significantly affected by artificial spectral features of several types. In low dispersion large aperture SWP exposures of sources with nominally featureless spectra, a spectral imprint which alters the shape of the continuum, several spurious "emission features" which recur at fixed locations on the camera target, and fixed pattern noise which can result in illusory emission and absorption features were identified. The anomalies appear in spectra extracted from line by line files generated by IUESIPS at all epochs, regardless of the ITF or rectification scheme used for processing.

Hackney, R. L.↗
NASA NTRS1981

Revised age estimates of Brunhes palaeomagnetic events - Support for a link between geomagnetism and eccentricity

Revisions in the dates of reported geomagnetic excursions during the Brunhes Epoch are proposed in light of possible correlations between a section at Gioia Tauro, Italy, deep-sea cores, a core from Lake Biwa, Japan, and some lava flows. The anomalously long, double Blake Event reported at Gioia Tauro is here correlated with the Blake Event (approximately 110 kyr) and the Biwa 1 event (180 plus or minus 5 kyr); an hiatus may be present in the section between these two events. The alpha event at Gioia Tauro is correlated with the Biwa 2 event at about 295 kyr; the beta event with the 'Biwa 3' event at about 400 kyr; the gamma event with the Snake River event at 480 plus or minus 50; and the delta event, not recorded elsewhere, is estimated to have occurred at approximately 620 kyr. These proposed refinements in the age estimates of the excursions suggest an approximately 100 kyr cyclicity. If the events are real and the revised dating is correct, the timing of the geomagnetic events seems to coincide with times of peak eccentricity of the earth's orbit, suggesting a causal connection.

Rampino, M. R.↗
NASA NTRS1981

Intergalactic shells at large redshift

The intergalactic shells produced by galactic explosions at large redshift, whose interiors cool by inverse Compton scattering off the cosmic background radiation, have a characteristic angular size of about 1 arcmin at peak brightness. At z values lower than 2, the shells typically have a radius of 0.5 Mpc, a velocity of about 50 km/sec, a metal abundance of about 0.0001 of cosmic values, and strong radiation in H I(Lyman-alpha), He II 304 A, and the IR fine-structure lines of C II and Si II. The predicted extragalactic background emission from many shells, strongly peaked toward the UV, sets an upper limit to the number of exploding sources at z values of about 10. Shell absorption lines of H I, C II, Si II, and Fe II, which may be seen at more recent epochs in quasar spectra, may probe otherwise invisible explosions in the early universe.

Shull, J. M.↗
NASA NTRS1981

318-MHz variability of complete samples of extragalactic radio sources

It is found by a low-frequency variability survey, involving two- and three-epoch, 318-MHz observations of extragalactic sources in samples complete to 3 Jy at 1400 MHz and 1 Jy at 5000 MHz, that steep-spectrum sources do not seem to vary while all flat-spectrum sources exhibit low-frequency variability greater than 8% over about 5 yr. It is also found that the flat-spectrum sources with inverted spectra show the largest fractional variations, and that there is a correlation between the incidence of low-frequency variability and the determination that a source is an optically violent variable. These statistical properties are consistent with models which invoke radio and optical emission relativistic beaming.

Dennison, B.↗
NASA NTRS1982

Interplanetary magnetic clouds at 1 AU

Statistical characteristics of magnetic clouds observed in the interplanetary medium near earth are investigated. Forty-five magnetic clouds are identified in which one component of B changes direction by rotating parallel to a plane, while the component of B normal to that plane is either constant or zero. Three classes of clouds are identified: clouds preceded by a shock, clouds followed by a stream interface, and clouds associated with a CME. In superposed epoch plots, the maximum field strength is found to be approximately the same for each class of clouds. Temperatures are all low, densities are somewhat high, and the pressure is higher that the ambient pressure in all three types of clouds. The physical characteristics of the magnetic clouds and their rate of occurrence suggests that many or all of the clouds may be related to coronal mass eject elements observed in white light data.

Klein, L. W.↗
NASA NTRS1982

Performance analysis for the expanding search PN acquisition algorithm

An approach is described for approximating the cumulative probability distribution of the acquisition time of the serial pseudonoise (PN) search algorithm. The results are applicable to both variable and fixed dwell time systems. The theory is developed for the case where some a priori information is available on the PN code epoch (reacquisition problem or acquisition of very long codes). Also considered is the special case of a search over the whole code. The accuracy of the approximation is demonstrated by comparisons with published exact results for the fixed dwell time algorithm.

Braun, W. R.↗
NASA NTRS1982

The effects of transmitter/receiver clock time-base instability on coherent communication system performance

The purpose of this paper is to present a model and associated analysis of the deleterious effects which both transmitter and receiver data clock time-base instability have on bit-error probability. The types of time-base instability modeled and analyzed include what is commonly referred to as bit jitter (epoch jitter), bit-jitter rate (time interval jitter), and data asymmetry. The bit-error probability is determined as a function of the statistical characteristics of the time-base instability, the data-transition density, the signal-to-noise ratio in the bit-rate bandwidth and the mechanizations of both the data-bit detector and bit synchronizer. The latter is manifested through the bit synchronizer's natural frequency and loop-damping parameters. Numerical results which indicate the sensitivity of the system bit-error rate to various parameters of the model are provided.

Chie, C. M.↗
NASA NTRS1980

Open-loop nanosecond-synchronization for wideband satellite communications

A synchronization technique for use with an onboard processing satellite communication system is discussed. The satellite oscillator is used both as the system time reference and as the frequency source for all downlink carriers and data clocks. Downlink timing is established at each system earth terminal through a combination of carrier and data-clock tracking and a downlink timing epoch signal consisting of one bit per TDMA data burst. Uplink timing is established by an open-loop range prediction process using precision ephemerides calculated and distributed by the central control station. Overall timing accuracy of the uplink signal at the satellite receiver of + or - 7 nanoseconds permits unambiguous identification of each data bit position in a 128 Mbps TDMA burst. This is accomplished by means of simple, inexpensive terminal hardware using available crystal oscillators for time/frequency references and digital synthesis techniques that may be implemented in digital LSI chips.

Holmes, W. M., Jr.↗
NASA NTRS1981

Remote sensing of the asteroids

Knowledge of the compositions of the asteroids is obtained by the remote sensing of reflected and emitted radiation from what are essentially star-like points of light. Since asteroids are a remnant population of planetesimals that were never accreted into the larger planets, their compositions and properties can provide insight into the nature of planetary matter in early epochs, before most of it was physically and chemically modified by geological processes within the planets. The progress made during the past decade in learning about asteroids through remote sensing is reviewed.

Chapman, C. R.↗
NASA NTRS1982

Galactic mass loss - A mild evolutionary correction to the angular size test

Galactic mass loss due to gas production by an evolving stellar population and removal due to supernovae or some other energy source can alter the size of a galaxy over a long time through the adiabatic invariants of the orbits. Numerical and analytic results show that if the mass profile changes homologously, the scale size of the galaxy does also. Under conventional assumptions about galaxy age and initial mass function, this bloating is smaller than the differences between open and flat cosmologies at z = 0.5. If, however, some elliptical galaxies maintain star formation until recent epochs and then lose a (correspondingly) larger fraction of their mass, the effect may be important. It may also be important in decreasing the size of cD galaxies as a result of gas accretion.

Richstone, D. O.↗
NASA NTRS1982

Comparison of starting values for iterative solutions to a universal Kepler's equation

General starting values for the iterative numerical solution of a universal Kepler's equation for position in a conic orbit at a specified time are investigated. Three starting values based on recent refinements of previously obtained bounds on the solution are derived and tested numerically. Of these, a simple starting value based on a cubic approximation to Kepler's equation provides the most rapid convergence using both first and second order Newton algorithms. The performance of the starting values are compared with similar studies which used the restricted case of elliptical orbits with the initial epoch at periapse.

Bergam, M. J.↗
NASA NTRS1982

The magnetic earth as seen from Magsat, initial results

Initial results of the Magsat mission are presented, in keeping with the categories of earth core field, the crustal or anomaly field of the earth, and external fields. It is noted that one major advantage of orbital magnetic field surveys is their ability to obtain a global set of magnetic field measurements of uniform precision and accuracy at a single epoch, aiding the derivation of the core field models that are a prerequisite for further data analysis. The ability to isolate fields from crustal sources was unexpected, because of the low amplitudes of such fields by comparison with those of the core and those external to the earth. The most significant external-field finding has been an east-west component variation near the dip equator.

Langel, R. A.↗
NASA NTRS1982

Evaluation of high latitude disturbances with Magsat - The importance of the Magsat geomagnetic field model

Studies of Birkeland current systems have been conducted primarily with low altitude satellite magnetometer data whereas electrojet current signatures have been usually analyzed with data from ground-based magnetometer chains. However, the Magsat magnetometer data set allows magnetic disturbances due to both field-aligned and electrojet currents to be examined simultaneously. This is mainly due to Magsat's approximately 400 km altitude, thus providing data closer to the electrojet current system than has been previously available from satellite measurements. Of additional importance are the superior accuracy, both in sensitivity and altitude, of the magnetic field measurements obtained by Magsat and last, but not least, the availability of an accurate magnetic field model of the concurrent epoch (based on the Magsat data set) that allows one to obtain a better baseline than previous models. The aforementioned points are emphasized and both Birkeland and electrojet currents have been analyzed in a case study for November 13, 1979.

Zanetti, L. J.↗
NASA NTRS1982

Cosmological constraints on hot plasma in a closed universe

A hot uniform collisionally heated intergalactic medium (IGM) at closure density is reconsidered. Optical depth measurements, plasma cooling and ionic abundances during the recombination epoch are discussed. The data are used to calculate X-radiation at critical energies. The results indicate a present temperature from 160,000 to 4,200,000 K if the reheating of the IGM ceased at the latest possible time. As cessation of heating moves to earlier times, the range of present plasma temperature narrows until at the earliest possible cessation time only about 160,000 K is acceptable. Relevant formulae are provided to modify these numbers as new data become available. This IGM avoids any hard X-ray production and thus assumes that discrete sources alone constitute the entire cosmic X-ray background.

Sherman, R. D.↗
NASA NTRS1981

Theoretical models of X-ray emission from rich clusters of galaxies

After assessing the theory of cluster gas with polytropic equations of state, and discussing the prediction of local polytropic indices in the gas distribution, attention is given to the consequences of modifying the equation of state by thermal cooling in the central regions of the cluster. It is suggested that, if the equation of state can be understood locally, an attempt may be made at (1) predicting the X-ray luminosities of the clusters at distant cosmological epochs and (2) measuring the evolution of the cluster's potential, through observation of the evolution of X-ray luminosity and surface brightness.

Cowie, L. L.↗
NASA NTRS1981

Anisotropy of the cosmic microwave background radiation

Theoretical predictions of the angular anisotropy in the cosmic microwave background radiation on both small and large angular scales are presented, and the effect of massive neutrinos on both the background radiation anisotropy and on the galaxy correlation function over very large scales is reviewed. Current observations show that the quadrupole anisotropy provides the greatest constraint on theory, and the values for the gravitational potential fluctuations indicate that small amplitude but sufficiently large-scale density fluctuations, both at the present epoch and on the surface of last scattering, can produce significant large angular scale variations in the radiation temperature. Most importantly, it is proposed that the quadrupole moment is most simply and elegantly interpreted in terms of the density fluctuations on very large scales whose presence is inferred from the requirement that an initial fluctuation spectrum is required in order for structure to develop.

Silk, J.↗
NASA NTRS1981

Comparison between variable and fixed dwell-time PN acquisition algorithms

Pseudo noise (PN) spread spectrum systems require a very accurate alignment between the PN code epochs at the transmitter and receiver. This synchronism is typically established through a two-step algorithm, including a coarse synchronization procedure and a fine synchronization procedure. A standard approach for the coarse synchronization is a sequential search over all code phases. The measurement of the power in the filtered signal is used to either accept or reject the code phase under test as the phase of the received PN code. This acquisition strategy, called a single dwell-time system, has been analyzed by Holmes and Chen (1977). A synopsis of the field of sequential analysis as it applies to the PN acquisition problem is provided. From this, the implementation of the variable dwell time algorithm as a sequential probability ratio test is developed. The performance of this algorithm is compared to the optimum detection algorithm and to the fixed dwell-time system.

Braun, W. R.↗
NASA NTRS1982

UV time-dependent emission in SY Muscae

IUE ultraviolet spectra of SY Mus for September 20, 1980 and June 11, 1981 indicate a substantial enhancement of UV emission over a nine-month period. The general UV flux level appears to have increased by about one order of magnitude between the two observing epochs. The strong UV continuum evident throughout the entire IUE spectral range 1200-3200 A on June 11, 1981 is closely approximated by a star with an effective temperature of 40,000 K. On September 20, 1980, however, the continuum distribution manifested a more complex structure, which is possibly explained by a combination of thermal emission from an early type main sequence star and nebular recombination emission.

Michalitsianos, A. G.↗