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At least 631 records · Page 35
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NASA NTRS1999

Thin Film Ba(x)Sr(1-x)TiO3 Ku- and K-Band Phase Shifters Grown on MgO Substrates

We report measurements of gold circuits fabricated on four Ba(x)Sr(1-x)TiO3 films doped with 1% Mn grown on MgO substrates by laser ablation. Low frequency measurements of epsilon(sub r) and tan(delta) on interdigital capacitors are compared with high frequency measurements of phase shift and insertion loss on coupled microstrip phase shifters done on the same films. The variation in temperature of both high and low frequency device parameters is compared. Annealed and unannealed films are compared. Room temperature figures of merit of phase shift per insertion loss of up to 58.4 C/dB at 18 GHz and 400 V dc bias were measured.

VanKeuls, F. W.↗
NASA NTRS1995

Chromospheric variations in main-sequence stars

The fluxes in passbands 0.1 nm wide and centered on the Ca II H and K emission cores have been monitored in 111 stars of spectral type F2-M2 on or near the main sequence in a continuation of an observing program started by O. C. Wilson. Most of the measurements began in 1966, with observations scheduled monthly until 1980, when observations were schedueld sevral times per week. The records, with a long-term precision of about 1.5%, display fluctuations that can be idntified with variations on timescales similar to the 11 yr cycle of solar activity as well as axial rotation, and the growth and decay of emitting regions. We present the records of chromospheric emission and general conclusions about variations in surface magnetic activity on timescales greater than 1 yr but less than a few decades. The results for stars of spectral type G0-K5 V indicate a pattern of change in rotation and chromospheric activity on an evolutionary timescale, in which (1) young stars exhibit high average levels of activity, rapid rotation rates, no Maunder minimum phase and rarely display a smooth, cyclic variation; (2) stars of intermediate age (approximately 1-2 Gyr for 1 solar mass) have moderate levels of activity and rotation rates, and occasional smooth cycles; and (3) stars as old as the Sun and older have slower rotation rates, lower activity levels and smooth cycles with occasional Maunder minimum-phases.

Baliunas, S. L.↗
NASA NTRS1979

Inverse multiple scattering problems. III - Inadequacy of certain limb darkening and phase curves for retrieving atmospheric information and limitations of approximate scattering models

The paper considers three complementary inverse multiple scattering problems relating to a uniquely defined atmospheric scattering model. Consideration is given to the appropriateness, for data inversion purposes, of intensities observed in diffuse reflection under a variety of experimental conditions; the uniqueness of the inverse solution is investigated. It is found that light curves representing monotonic variations, such as limb darkening curves and phase curves for a planetary (e.g., Venus) disk center are unsuitable for inferring atmospheric and scattering parameters.

Fymat, A. L.↗
NASA NTRS2004

X-ray Monitoring of eta Carinae: Variations on a Theme

We present monitoring observations by the Rossi X-ray Timing Explorer of the 2-10 keV X-ray emission from the supermassive star eta Carinae from 1996 through late 2003. These data cover more than one of the stellar variability cycles in temporal detail and include especially detailed monitoring through two X-ray minima. We compare the current X-ray minimum which began on June 29, 2003 to the previous X-ray minimum which began on December 15, 1997, and refine the X-ray period to 2024 days. We examine the variations in the X-ray spectrum with phase and with time, and also refine our understanding of the X-ray peaks which have a quasi-period of 84 days, with significant variation. Cycle-to-cycle differences are seen in the level of X-ray intensity and in the detailed variations of the X-ray flux on the rise to maximum just prior to the X-ray minimum. Despite these differences the similarities between the decline to minimum, the duration of the minimum, and correlated variations of the X-ray flux and other measures throughout the electromagnetic spectrum leave little doubt that that the X-ray variation is strictly periodic and produced by orbital motion as the wind from eta Carinae collides with the wind of an otherwise unseen companion.

Corcoran, M. F.↗
NASA NTRS1989

Time-periodic variations in stratospheric ozone from satellite observations

The time-periodic variations in stratospheric zonal mean ozone number density are examined with emphasis on annual and semi-annual oscillations. The data base for this analysis is the ozone observation from the SAGE II satellite instrument. A multiple linear regression method is adopted for the analysis. The results show that the amplitudes and phases of the time-periodic ozone variations are functions of altitude and latitude.

Wang, Pi-Huan↗
NASA NTRS1984

Observations of shadow bands at the total solar eclipse of 16 February 1980

Photoelectric observations of short term light variations (shadow bands) at the 16 Feb. 1980 total solar eclipse have been made using a set of spatially separated PIN diodes. Light variations in a bandpass of 1-500 Hz were detected during the half-minutes preceding and following the total phase. Fourier analysis of the noise spectrum of the variations reveals a sharp drop-off for frequencies above 50 Hz and an overall spectrum quite similar to previously reported power spectra of stellar scintillation. This is consistent with an atmospheric origin for the shadow bands. Cross-correlations between the detector outputs are low, suggesting a short persistence time for the turbulent elements causing the patterns.

Marschall, L. A.↗
NASA NTRS2000

Downward Link of Solar Activity Variations Through Wave Driven Equatorial Oscillations (QBO and SAO)

Signatures of the 11-year solar activity/irradiance cycle are observed in the Quasi Biennial Oscillation (QBO) of the lower stratosphere. At these altitudes, the QBO is understood to be the result of "downward control" exerted by the wave mean flow interactions that drive the phenomenon. It is reasonable then to speculate that the QBO is a natural conduit to lower altitudes of solar activity variations in radiance (SAV). To test this hypothesis, we conducted experiments with a 2D version of our Numerical Spectral Model that incorporates Hines' Doppler Spread Parameterization for small-scale gravity waves (GW). To account for the SAV, we change the solar heating rate on a logarithmic scale from 0.1% at the surface to 1% at 50 kin to 10% at 100 km. With the same GW flux, we then conduct numerical experiments to evaluate the magnitude of the solar activity irradiance effect (SAE) on the zonal circulation at low latitudes. The numerical results obtained show that, under certain conditions, the SAE is significant in the zonal circulation and does extend to lower altitudes where the SAV is small. The differences in the wind velocities can be as large as 5 m/s at 20 kin. We carried out two numerical experiments with integrations over more than 20 years: 1) With the QBO period "tuned" to be 30 months, of academic interest but instructive, the seasonal cycle in the solar forcing [through the Semi-annual Oscillation (SAO)] acts as a strong pacemaker to produce a firm lock on the period and phase of the QBO. The SAE then shows up primarily as a distinct but relatively weak amplitude modulation. 2) With the QBO period between 30 and 34 (or less than 30, presumably) months, the seasonal phase lock is weak compared with (1). The SAV in the seasonal cycle then causes variations in the QBO period and phase, and this amplifies the SAE to produce relatively large variations in the wind field. We conclude that, under realistic conditions as in (2), the solar seasonal forcing, with activity variations in radiance, causes the phase and period of the QBO to change to produce a relatively large solar activity effect in the zonal circulation at low latitudes and low altitudes.

Mengel, J. G.↗
NASA NTRS2021

A Broadband X-Ray View of the Precessing Accretion Disk and Pre-eclipse Dip in the Pulsar Her X-1 with NuSTAR and XMM-Newton

We present a broad-band X-ray study of the variation with the super orbital cycle of the pulse profiles and spectral shape in the low-mass X-ray binary Her X-1. This source shows a 35-day superorbital modulation in X-ray flux that is most likely caused by occultation by a warped, precessing accretion disk. Our data set consists of four joint XMM-Newton and NuSTAR observations of Her X-1 which span a complete super orbital cycle. However, We focus our analysis on the first and fourth observa-tion, which occur during the bright “main-on” phase, that have sufficient signal to noise to resolve pulsations. In order to broaden our coverage of the superorbital cycle, we supplemented our weakly pulsed observations with an archival XMM-Newton observation during the “short-on” phase of the superorbital cycle. We find that the energy-resolved pulse profiles show variations with superorbital phase and that pulse profiles from observations at similar superorbital phases show the same shape and relative phase, which we expect from a precessing disk. We determine that the broad-band spectrum is well fit by an absorbed power law with a soft black body component. We demonstrate that a simple precessing accretion disk model is sufficient to reproduce the observed pulse profiles. The results of this model support the idea that the similarities in the observed pulse profiles are due to reprocessing by a precessing disk that has returned to its original precession phase. We also present a brief analysis of the energy resolved light curves of a pre-eclipse dip, which shows soft X-ray absorption and hard X-ray variability during the dip.

McKinley C Brumback↗
NASA NTRS1977

Large Area Crop Inventory Experiment (LACIE). Phase 2 evaluation report

Documentation of the activities of the Large Area Crop Inventory Experiment during the 1976 Northern Hemisphere crop year is presented. A brief overview of the experiment is included as well as phase two area, yield, and production estimates for the United States Great Plains, Canada, and the Union of Soviet Socialist Republics spring winter wheat regions. The accuracies of these estimates are compared with independent government estimates. Accuracy assessment of the United States Great Plains yardstick region based on a through blind sight analysis is given, and reasons for variations in estimating performance are discussed. Other phase two technical activities including operations, exploratory analysis, reporting, methods of assessment, phase three and advanced system design, technical issues, and developmental activities are also included.

Source record↗
NASA NTRS1960

Tracking and Orbit-Determination Program of the Jet Propulsion Laboratory

The lunar-probe tracking program at the Jet Propulsion Laboratory has two prime objectives: (1) provide real-time predictions of the direction of the probe from various observation stations; (2) establish a reliable trajectory corresponding to the actual flight path of the probe. The tracking program, although developed for use with lunar probes, can be used for interplanetary probes if certain modifications are made. The program, as developed for the IBM 704 digital computer, has two distinct phases. First, the equations of motion and the variational equations are integrated to each observation time where the elements of the equation A (sub u) equals b [linearization of the maximum likelihood equations] are computed. The second phase is concerned with the solution of a specified subset of A (sub u) equals b. Flexibility and ease of operation have been major objectives in writing the 704 program. The number of data points and tracking stations that may be used is limited only by computing time and core storage. Input formats and operating instructions are presented for utilizing the various computational options available in the program.

Carr, Russell E.↗
NASA NTRS1985

Propagation of the phase of solar modulation

The phase of the 11 year galactic cosmic ray variation, due to a varying rate of emission of long lived propagating regions of enhanced scattering, travels faster than the scattering regions themselves. The radial speed of the 11 year phase in the quasi-steady, force field approximation is exactly twice the speed of the individual, episodic decreases. A time dependent, numerical solution for 1 GeV protons at 1 and 30 Au gives a phase speed which is 1.85 times the propagation speed of the individual decreases.

Forman, M. A.↗
NASA NTRS1998

A Report from the Thermal Science Research Center (TSRC)

A vertical flow loop was designed and assembled to determine the local (circumferential and axial) and mean wall temperature distributions for single-phase and two-phase (subcooled and saturated) downward flow in both uniformly-heated and single-side heated vertical channels. Freon-11 was used as the working fluid in order to directly relate and compare the results with a previous experimental campaign which employed this same working fluid. For a given steady-state experiment, the following parameters were held constant: (1) exit pressure, (2) inlet temperature, and (3) mass velocity. For a given configuration of the 2.2 m long cylindrical channel test section, which had a 1.2 m long heated section, the applied heat rate was varied from zero through successive quasi-steady states to a level which corresponded to localized film boiling in the test section. The measurements showed that the boiling curve changes significantly at higher mass velocities with respect to both the circumferential and axial directions. The slope of the boiling curve changes in a non-monotonic fashion with respect to the circumferential directions. The slope of the boiling curve changes in a non-monotonic fashion with respect to the circumferential direction. The measurements point to the existence of a dry-out phenomenon occurring at multiple levels of the applied heat for the single-side heated channel. In comparing the heat transfer for horizontal channel flow with a vertically downward flow, the results show that significantly lower heat transfer occurs in the horizontal flow. However, this trend reverses as both the Reynolds number and the applied heat rate increase. Both the Liu-Winterton and Shah correlations were compared with the experimental data. The Shah correlation predicted the uniformly heated tube data better. When a thermal hydraulic diameter approach was used for the single-side heated case, the data at upstream locations for Z/L less than 0.5 was bounded above by the Liu-Winterton correlation and below by the Shah correlation. At Z/L = 0.5, the Shah correlation bounded the data; and for Z/L greater than 0.5, both correlations overpredicted the data with the Shah correlation being closest to the data. The present results indicate that additional correlational development is needed. In addressing some of the advanced space thermal management objectives concerning accommodating high heat fluxes in non-uniformly heated systems, a large battery of experiments 88 have been completed where local two-dimensional wall temperature variations were measured for both single-phase and two-phase flow in a single-side heated circular tube. As noted above, the results show significant axial and circumferential variations. Accurately accounting for such variations can result in optimized future advanced space, enhanced (high heat flux) thermal management systems.

Boyd, Ronald D.↗
NASA NTRS2005

Thermal Imaging of Earth for Accurate Pointing of Deep-Space Antennas

A report discusses a proposal to use thermal (long-wavelength infrared) images of the Earth, as seen from spacecraft at interplanetary distances, for pointing antennas and telescopes toward the Earth for Ka-band and optical communications. The purpose is to overcome two limitations of using visible images: (1) at large Earth phase angles, the light from the Earth is too faint; and (2) performance is degraded by large albedo variations associated with weather changes. In particular, it is proposed to use images in the wavelength band of 8 to 13 m, wherein the appearance of the Earth is substantially independent of the Earth phase angle and emissivity variations are small. The report addresses tracking requirements for optical and Ka-band communications, selection of the wavelength band, available signal level versus phase angle, background noise, and signal-to-noise ratio. Tracking errors are estimated for several conceptual systems employing currently available infrared image sensors. It is found that at Mars range, it should be possible to locate the centroid of the Earth image within a noise equivalent angle (a random angular error) between 10 and 150 nanoradians at a bias error of no more than 80 nanoradians

Ortiz, Gerardo↗
NASA NTRS1973

Distribution of atomic oxygen in the upper atmosphere deduced from Ogo 6 airglow observations.

The atomic oxygen distribution as a function of altitude between 80 and 120 km and as a function of latitude has been deduced from Ogo 6 557.7-nm airglow photometer data obtained between August 1969 and April 1970. The results indicate that the density ranges from 15 to 50 billion per cu cm at 120 km; that there is a semiannual variation by a factor of 3 in the global average density near 100 km in phase with the satellite drag semiannual effect; and that large latitudinal variations occur with maximums between 40 and 60 deg in the winter hemisphere and sometimes deep minimums in the tropics. The implication of these results for meridional and vertical transport patterns is discussed.

Donahue, T. M.↗
NASA NTRS1980

Simulation studies of a model of high-density metallic hydrogen

Upper bounds for the ground-state energies of liquid and solid phases of metallic hydrogen and metallic deuterium have been calculated with variational methods and Monte Carlo techniques. At four densities (0.8, 1.2, 1.36, and 1.488) crystalline phases are clearly preferred in the sense that the energy difference, when compared to the liquid, is in excess of the errors inherent in the numerical procedures. At a fifth density (1.6), the energy differences between solid and liquid phases are smaller than these errors.

Mon, K. K.↗
NASA NTRS1985

Solar tri-diurnal variation of cosmic rays in a wide range of rigidity

Solar tri-diurnal variations of cosmic rays have been analyzed in a wide range of rigidity, using data from neutron monitors, and the surface and underground muon telescopes for the period 1978-1983. The rigidity spectrum of the anisotropy in space is assumed to be of power-exponential type as (P/gamma P sub o) to the gamma exp (gamma-P/P sub o). By means of the best-fit method between the observed and the expected variations, it is obtained that the spectrum has a peak at P (=gamma P sub o) approx = 90 GV, where gamma=approx 3.0 and P sub o approx. 30 GV. The phase in space of the tri-diurnal variation is also obtained as 7.0 hr (15 hr and 23 hr LT), which is quite different from that of approx. 1 hr. arising from the axisymmetric distribution of cosmic rays with respect to the IMF.

Mori, S.↗
NASA NTRS1986

Phase propagation of the solar modulation of galactic cosmic rays

The phase of the 11-yr galactic cosmic ray variation, due to a varying rate of emission of long-lived, propagating regions of enhanced scattering, travels faster than the scattering regions themselves. The radial speed of the 11-yr phase in the quasi-steady force field approximation is exactly twice the speed of the individual episodic decreases. A time-dependent numerical solution for 1-GeV protons at 1 and 30 AU gives a phase speed which is 1.85 times the propagation speed of the individual decreases.

Forman, M. A.↗
NASA NTRS1994

Observations of the diurnal tide from space

This study presents a climatology of mesospheric and lower-thermospheric diurnal tidal winds obtained with the High Resolution Doppler Imager (HRDI) on the Upper Atmosphere Research Satellite (UARS). The observations reveal that although tidal structures are present at all times, like the prevailing zonal winds, they exhibit significant semiannual as well as other shorter-term variations in amplitude. Results are presented for a period extending over more than one year from November 1991 to July 1993. The 1,1 diurnal tidal amplitude of the meridional component, characterized by the value at an altitude of 90 km and a latitude of 22 deg, ranges from a minimum at solstice of less than 20 m per sec to an equinox maximum of over 70 m per sec. The vertical wavelength and phase of the tide show only slight variations throughout the year, with a suggestion of semiannual variations in both.

Hays, Paul B.↗