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At least 1,333 records · Page 74
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NASA NTRS1998

A Comparison of Wolf's Reconstructed Record of Annual Sunspot Number with Schwabe's Observed Record of Clusters of Spots for the Interval of 1826-1868

Samuel Heinrich Schwabe, the discoverer of the sunspot cycle, observed the Sun routinely from Desau, Germany during the interval of 1826-1869, averaging about 290 observing days per year. His yearly counts of 'clusters of spots' (or, more correctly, the yearly number of newly appearing sunspot groups) provided a simple means for describing the overt features of the sunspot cycle (i.e., the timing and relative strengths of cycle minimum and maximum). In 1848, Rudolf Wolf, a Swiss astronomer, having become aware of Schwabe's discovery, introduced his now familiar 'relative sunspot number' and established an international cadre of observers for monitoring the future behavior of the sunspot cycle and for reconstructing its past behavior (backwards in time to 1818, based on daily sunspot number estimates). While Wolf's reconstruction is complete (without gaps) only from 1849 (hence, the beginning of the modern era), the immediately preceding interval of 1818-1848 is incomplete, being based on an average of 260 observing days per year. In this investigation, Wolf's reconstructed record of annual sunspot number is compared against Schwabe's actual observing record of yearly counts of clusters of spots. The comparison suggests that Wolf may have misplaced (by about 1-2 yr) and underestimated (by about 16 units of sunspot number) the maximum amplitude for cycle 7. If true, then, cycle 7's ascent and descent durations should measure about 5 years each instead of 7 and 3 years, respectively, the extremes of the distributions, and its maximum amplitude should measure about 96 instead of 70. This study also indicates that cycle 9's maximum amplitude is more reliably determined than cycle 8's and that both appear to be of comparable size (about 130 units of sunspot number) rather than being significantly different. Therefore, caution is urged against the indiscriminate use of the pre-modern era sunspot numbers in long-term studies of the sunspot cycle, since such use may lead to specious results.

Wilson, Robert M.↗
NASA NTRS1966

Rocket Engine Clustering and Vehicle Integration as Influenced by Base Thermal Environments

Clustered rocket engines create severe thermal environments in the base of rocket vehicle stages. Boosters burning hydrocarbon fuels experience severe radiant heating early in flight; as the plumes interact at higher altitudes, convective heating becomes significant. For hydrogen-fueled upper stages radiation is not important, but convective heating is severe during the entire stage operation. Predicted and measured heating rates are discussed. The base region thermal environments of stages with clustered engines present a variety of engine/vehicle interaction problems. Components and structures in the base region, including the rocket engines, cannot survive radiant and convective heating from engine exhausts without such remedies as protective insulation, shielding, air-scooping, and proper disposal of the fuel-rich turbine exhaust gases. Different thermal protection concepts evolve for booster and upper stages due to the differences in ground test and flight environments. Solutions to the engine/vehicle interaction and design integration problems are described.

Hopson, George D.↗
NASA NTRS1966

Some Observations on Exhaust Recirculation from Clustered Rocket Nozzles

A short-duration rocket testing technique which exactly duplicates the combustion flows of multi-engine rocket boosters at high altitudes is described. The application of this technique to the investigation of the reverse flow characteristics of a 4-engine rocket cluster model is discussed. The use of a comparatively new diagnostic tool, the electron beam density probe, in mapping the recirculating flow field of a rocket cluster is described and experimental density results presented. Finally, the use of a reflection plane for the examination of reverse flow phenomena, whereby a plane of symmetry is replaced by a solid surface on which measurements may be made,is evaluated and experimental data presented.

Hendershot, K. C.↗
NASA NTRS1999

The Low-Mass X-Ray Binary X1832-330 in the Globular Cluster NGC 6652: A Serendipitous ASCA Observation

The Low Mass X-ray Binary (LMXB) X1832-330 in NGC 6652 is one of about 10 bright X-ray sources to have been discovered in Globular Clusters. We report on a serendipitous ASCA observation of this Globular Cluster LMXB, during which a Type I burst was detected and the persistent, non-burst emission of the source was at its brightest level recorded to date. No orbital modulation was detected, which argues against a high inclination for the X1832-330 system. The spectrum of the persistent emission can be fit with a power law plus a partial covering absorber, although other models are not ruled out. Our time-resolved spectral analysis through the burst shows, for the first time, clear evidence for spectral cooling from kT = 2.4 +/- 0.6 keV to kT = 1.0 +/- 0.1 keV during the decay. The measured peak flux during the burst is approximately 10% of the Eddington luminosity for a 1.4 Solar Mass neutron star. These are characteristic of a Type I burst, in the context of the relatively low quiescent luminosity of X1832-330.

Mukai, Koji↗
NASA NTRS1999

Evolution of Local Microstructures: Spatial Instabilities of Coarsening Clusters

This work examines the diffusional growth of discrete phase particles dispersed within a matrix. Engineering materials are microstructurally heterogeneous, and the details of the microstructure determine how well that material performs in a given application. Critical to the development of designing multiphase microstructures with long-term stability is the process of Ostwald ripening. Ripening, or phase coarsening, is a diffusion-limited process which arises in polydisperse multiphase materials. Growth and dissolution occur because fluxes of solute, driven by chemical potential gradients at the interfaces of the dispersed phase material, depend on particle size. The kinetics of these processes are "competitive," dictating that larger particles grow at the expense of smaller ones, overall leading to an increase of the average particle size. The classical treatment of phase coarsening was done by Todes, Lifshitz, and Slyozov, (TLS) in the limit of zero volume fraction, V(sub v), of the dispersed phase. Since the publication of TLS theory there have been numerous investigations, many of which sought to describe the kinetic scaling behavior over a 0 range of volume fractions. Some studies in the literature report that the relative increase in coarsening rate at low (but not zero) volume fractions compared to that predicted by TLS is proportional to v(sub v)(exp 1/2), whereas others suggcest V(sub v)(exp 1/3). This issue has been resolved recently by simulation studies at low volume fractions in three dimensions by members of the Rensselaer/MSFC team. Our studies of ripening behavior using large-scale numerical simulations suggest that although there are different circumstances which can lead to either scaling law, the most important length scale at low volume fractions is the diffusional analog of the Debye screening length. The numerical simulations we employed exploit the use of a recently developed "snapshot" technique, and identifies the nature of the coarsening dynamics at various volume fractions. Preliminary results of numerical and experimental investigations, focused on the growth of finite particle clusters, provide important insight into the nature of the transition between the two scaling regimes. The companion microgravity experiment centers on the growth within finite particle clusters, and follows the temporal dynamics driving microstructural evolution, using holography.

Frazier, Donald O.↗
NASA NTRS1999

HST Observations of New Horizontal Branch Structures in the Globular Cluster w Centauri

The globular cluster omega Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HB objects are "extreme" HB or hot post-HB stars. A wide gap in the color distribution of the hot HB stars may correspond to gaps found earlier in several other clusters, which argues for a common mass loss mechanism. The diagram contains a significant population of hot sub-HB stars, which we interpret as the "blue-hook" objects predicted by D'Cruz (1996a). These are produced by late He-flashes in stars which have undergone unusually large giant branch mass loss. Omega Cen has a well-known spread of metal abundance, and the diagram is consistent with a giant branch mass loss efficiency which increases with metallicity. There is no evidence for a dynamical origin of the hot HB stars.

Stecher, Theodore↗
NASA NTRS1999

X-Ray Measured Masses for Clusters of Galaxies

Results of the analysis of Chandra observations fo the gas density and temperature profiles for several clusters f galaxies will be used to determine the amount and distribution of the total mass in clusters. These results will be compared with past results from ASCA. with theoretically predicted mass profiles and with mass profiles determined from gravitational lensing.

Forman, William↗
NASA NTRS1999

Chandra Observations of Clusters of Galaxies

The high spatial resolution of the Chandra X-ray telescope, along with the broad energy coverage will allow a new view of the X-ray emission from clusters of galaxies. Results from the first Chandra X-ray images, including any structure in clusters on fine scales, gas temperature distributions and heavy element abundances will be presented.

Jones, Christine↗
NASA NTRS1999

Clustering of the Diffuse Infrared Light from the COBE DIRBE Maps: Power Spectrum Analysis and Excess Isotropic Component of Fluctuations - 3

The cosmic infrared background (CIB) radiation is the cosmic repository for energy release throughout the history of the universe. The spatial fluctuations of the CIB resulting from galaxy clustering are expected to be at least a few percent on scales of a degree, depending on the luminosity and clustering history of the early universe. Using the all-sky data from the COBE DIRBE instrument at wavelengths 1.25 - 100 microns we attempt to measure the CIB fluctuations. In the near-IR, foreground emission is dominated by small scale structure due to stars in the Galaxy. There we find a strong correlation between the amplitude of the fluctuations and Galactic latitude after removing bright foreground stars. Using data outside the Galactic plane (absolute value of b > 20 deg) and away from the center (90 deg < l < 270 deg) we extrapolate the amplitude of the fluctuations to cosec absolute value of b = 0. We find a positive intercept of delta.F(sub rms) = 15.5(sup +3.7, sub -7.0), 5.9(sup +1.6, sub -3.7), 2.4(sup +0.5, sub -0.9), 2.0(sup +0.25, sub -0.5) nW/sq m.sr at 1.25, 2.2, 3.5 and 4.9 microns respectively, where the errors are the range of 92% confidence limits. For color subtracted maps between band 1 and 2 we find the isotropic part of the fluctuations at 7.6(sup +1.2, sub -2.4) nW/sq m.sr. Based on detailed numerical and analytic models, this residual is not likely to originate from the Galaxy, our clipping algorithm, or instrumental noise. We demonstrate that the residuals from the fit used in the extrapolation are distributed isotropically and suggest that this extra variance may result from structure in the CIB. We also obtain a positive intercept from a linear combination of maps at 1.25 and 2.2 microns. For 2 deg < theta < 15 deg, a power-spectrum analysis yields limits of (theta/5 deg) x delta.F(sub rms)(theta) < 6, 2.5, 0.8, 0.5 nW/sq m.sr at 1.25, 2.2, 3.5 and 4.9 microns respectively. From 10 - 100 microns, the dominant foregrounds are emission by dust in the Solar system and the Galaxy. There the upper limits on the CIB fluctuations are below 1 nW/sq m.sr and are lowest (< equal 0.5 nW/sq m.sr) at 25 microns.

Kashlinsky, A.↗
NASA NTRS1999

Chandra Observations of the Young Cluster NGC 2516

The Chandra observatory was Launched July 23 1999. The first two months of the mission have been spent performing on-orbit calibration. This phase of the mission has included about 60 Ks of observing time on the young cluster NGC 2516. The observations have been made as part of the measurements of the boresight and the plate-scale of the various instruments. These observations will augment planned (deeper) GTO observation of this cluster. Here we present the results of these early observations. As of September 7, in a single ACIS 5'x5' field (one backside chip) examined 26 sources were detected in a single 10 Ks exposure. This represents less than one sixth of the total exposure time and less than one sixth of the total area to be observed during the calibration phase. we will compare the characteristics of those sources detected with Chandra with those detected by ROSAT. Since these observations are being made using all detectors, we will also compare observations among the various detectors.

Wolk, Scott J.↗
NASA NTRS1999

Heterogeneous Clustering: Operational and User Impacts

Heterogeneous clustering can improve overall utilization of multiple hosts and can provide better turnaround to users by balancing workloads across hosts. Building a cluster requires both operational changes and revisions in user scripts.

Salm, Saita Wood↗
NASA NTRS2000

Characterization of Acousto-Electric Cluster and Array Levitation and its Application to Evaporation

An acousto-electric levitator has been developed to study the behavior of liquid drop and solid particle clusters and arrays. Unlike an ordinary acoustic levitator that uses only a standing acoustic wave to levitate a single drop or particle, this device uses an extra electric static field and the acoustic field simultaneously to generate and levitate charged drops in two-dimensional arrays in air without any contact to a solid surface. This cluster and array generation (CAG) instrument enables us to steadily position drops and arrays to study the behavior of multiple drop and particle systems such as spray and aerosol systems relevant to the energy, environmental, and material sciences.

Robert E. Apfel↗
NASA NTRS1994

Cluster Computation of Flight Reynolds Number Flows

The performance of a workstation cluster used for the solution of the Reynolds-averaged Navier-Stokes equations is compared with a conventional vector supercomputer architecture. The application simulation of the steady flowfield about a transonic transport was computed using an implicit diagonal scheme in an overset mesh framework. Static load balancing was used, while coarse grain decomposition was achieved by solution of a grid zone per processor. Price/performance ratios are estimated for several scenarios in which such clusters may be utilized.

Atwood, Christopher A.↗
NASA NTRS2001

Infrared Communications for Small Spacecraft: From a Wireless Bus to Cluster Concepts

Nanosatellites operating singly or in clusters are anticipated for future space science missions. To implement this new communications paradigm, we are approaching cluster communications by first developing an infrared (IR) intra-craft wireless bus capability, following initially the MIL-STD-1553B protocol. Benefits of an IR wireless bus are low mass, size, power, and cost, simplicity of implementation, ease of use, minimum EMI, and efficient and reliable data transfer. Our goals are to maximize the reliable link margin in order to afford greater flexibility in receiver placement, which will ease technology insertion. We have developed a concept demonstration using a high-speed visible-band silicon PIN photodiode and a high-efficiency visible LED operating at a data rate up to 4 Mb/sec. In designing an internal IR wireless bus, we have characterized various candidate materials, emitters, and geometries, assuming a single reflection. Thus, we have measured the bidirectional reflectance distribution function (BRDF) for five different materials characteristic of typical spacecraft structures, which range from nearly Lambertian to highly specular. We have fit our data to empirical BRDF functions and modeled the detected irradiance anywhere in the plane of incidence for a divergent (LED) emitter. We have also determined the angular limits on the link geometry to remain within the required bit error rate by determining the received signal-to-noise ratio (SNR) for minimum values of irradiance received at the detector.

Webb, Suzanne C.↗
NASA NTRS2001

From the Cluster Temperature Function to the Mass Function at Low Z

The project consists of three X-ray Multimirror Mission (XMM) observations of the hot, nearby galaxy cluster Triangulum Australis (one pointing to the cluster center and two offsets). The major goal of the project is to obtain an accurate radial gas temperature profile out to large radii and derive the total gravitating mass within the radius of over density 500. A secondary goal is to obtain a detailed two-dimensional gas temperature map of the central region. We have recently received the central pointing (of the two CDROMs received, one contains the central pointing and the other, a zero-exposure dummy observation). The analysis of the data at hand is underway with the aim of deriving the two-dimensional temperature map. At present, we have to limit quantitative analysis to the EPIC-MOS data because of the absence of the necessary calibration products for the Extended Full Frame mode used for EPIC-pn in these observations. We anticipate the arrival of the data from the remaining two offset pointings in the near future, after which we will be able to undertake the total mass derivation. Money from this year's installment of the grant was partially spent on an upgrade of the computer used for the above data analysis, and on travel to a conference that extensively discussed the XMM calibration issues.

Markevitch, Maxim↗
NASA NTRS2001

Enhancement of Tropical Land Cover Mapping with Wavelet-Based Fusion and Unsupervised Clustering of SAR and Landsat Image Data

The characterization and the mapping of land cover/land use of forest areas, such as the Central African rainforest, is a very complex task. This complexity is mainly due to the extent of such areas and, as a consequence, to the lack of full and continuous cloud-free coverage of those large regions by one single remote sensing instrument, In order to provide improved vegetation maps of Central Africa and to develop forest monitoring techniques for applications at the local and regional scales, we propose to utilize multi-sensor remote sensing observations coupled with in-situ data. Fusion and clustering of multi-sensor data are the first steps towards the development of such a forest monitoring system. In this paper, we will describe some preliminary experiments involving the fusion of SAR and Landsat image data of the Lope Reserve in Gabon. Similarly to previous fusion studies, our fusion method is wavelet-based. The fusion provides a new image data set which contains more detailed texture features and preserves the large homogeneous regions that are observed by the Thematic Mapper sensor. The fusion step is followed by unsupervised clustering and provides a vegetation map of the area.

LeMoigne, Jacqueline↗
NASA NTRS2002

Physics of Galaxy Clusters and How it Affects Cosmological Tests

We have worked on the analysis of the Chandra observations of the nearby and distant clusters of galaxies, and on the expansion of the sample of distant X-ray clusters based on the archival ROSAT PSPC data. Some of the scientific results are discussed.

Vikhlinin, Alexey↗
NASA NTRS2002

Progress on the Cluster Mission

Prof M. G. Kivelson and Dr. K. K. Khurana (UCLA (University of California, Los Angeles)) are co-investigators on the Cluster Magnetometer Consortium (CMC) that provided the fluxgate magnetometers and associated mission support for the Cluster Mission. The CMC designated UCLA as the site with primary responsibility for the inter-calibration of data from the four spacecraft and the production of fully corrected data critical to achieving the mission objectives. UCLA will also participate in the analysis and interpretation of the data. The UCLA group here reports its excellent progress in developing fully intra-calibrated data for large portions of the mission and an excellent start in developing inter-calibrated data for selected time intervals, especially extended intervals in August, 2001 on which a workshop held at ESTEC in March, 2002 focused. In addition, some scientific investigations were initiated and results were reported at meetings.

Kivelson, Margaret↗