Exploring the Bimodal Solar System via Sample Return from the Main Asteroid Belt: The Case for Revisiting Ceres
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Thomas H. Burbine↗
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Power Management and Distribution System Developed for Thermionic Power Converters
A spacecraft solar, bimodal system combines propulsion and power generation into a single integrated system. An Integrated Solar Upper Stage (ISUS) provides orbital transfer capabilities, power generation for payloads, and onboard propulsion to the spacecraft. A key benefit of a bimodal system is a greater payload-to-spacecraft mass ratio resulting in lower launch vehicle requirements. Scaling down to smaller launch vehicles increases space access by reducing overall mission cost. NASA has joined efforts with the Air Force Phillips Laboratory to develop enabling technologies for such a system. The NASA/Air Force bimodal concept uses solar concentrators to focus energy into an integrated power plant. This power plant consists of a graphite core that stores thermal energy within a cavity. An array of thermionic converters encircles the graphite cavity and provides electrical energy conversion functions. During the power generation phase of the bimodal system, the thermionic converters are exposed to the heated cavity and convert the thermal energy to electricity. Near-term efforts of the ISUS bimodal program are focused on a ground demonstration of key technologies in order to proceed to a full space flight test. Thermionic power generation is one key technology of the bimodal concept. Thermionic power converters impose unique operating requirements upon a power management and distribution (PMAD) system design. Single thermionic converters supply large currents at very low voltages. Operating voltages can vary over a range of up to 3 to 1 as a function of operating temperature. Most spacecraft loads require regulated 28-volts direct-current (Vdc) power. A combination of series-connected converters and powerprocessing boosters is required to deliver power to the spacecraft's payloads at this level.
Baez, Anastacio N.↗
Ultraviolet+Infrared Star Formation Rates: Hickson Compact Groups with Swift and SPitzer
We present Swift UVOT ultraviolet (UV; 1600-3000 A) data with complete three-band UV photometry for a sample of 41 galaxies in 11 nearby (<4500 km/s) Hickson Compact Groups (HCGs) of galaxies. We use UVOT uvw2-band (2000A) photometry to estimate the dust-unobscured component, SFR(sub uv), of the total star formation rate, SFR(sub TOTAL). We use Spitzer MIPS 24 micron photometry to estimate SFR(sub IR), the component of SFR(sub TOTAL) that suffers dust extinction in the UV and is re-emitted in the IR. By combining the two components, we obtain SFR(sub TOTAL) estimates for all HCG galaxies. We obtain total stellar mass, M(sub *) estimates by means of Two Micron All Sky Survey K(sub s)-band luminosities, and use them to calculate specific star formation rates, SSFR is identical with SFR(sub TOTAL)/ M (sub *). SSFR values show a clear and significant bimodality, with a gap between low (approximately <3.2 x 10(exp -11) / yr) and high-SSFR (approximately > 1.2 x lO)exp -10)/yr) systems. We compare this bimodality to the previously discovered bimodality in alpha-IRAC, the MIR activity index from a power-law fit to the Spitzer IRAC 4.5-8 micron data for these galaxies. We find that all galaxies with alpha-IRAC <= 0 (> 0) are in the high- (low-) SSFR locus, as expected if high levels of star-forming activity power MIR emission from polycyclic aromatic hydrocarbon molecules and a hot dust continuum. Consistent with this finding, all elliptical/SO galaxies are in the low-SSFR locus, while 22 out of 24 spirals / irregulars are in the high-SSFR locus, with two borderline cases. We further divide our sample into three subsamples (I, II, and III) according to decreasing H I richness of the parent galaxy group to which a galaxy belongs. Consistent with the SSFR and alpha-IRAC bimodality, 12 out of 15 type I (11 out of 12 type III) galaxies are in the high- (low-) SSFR locus, while type II galaxies span almost the full range of SSFR values. We use the Spitzer Infrared Nearby Galaxy Survey (SINGS) to construct a comparison subsample of galaxies that (1) match HCG galaxies in J-band total galaxy luminosity and (2) are not strongly interacting and largely isolated. This selection eliminates mostly low-luminosity dwarfs and galaxies with some degree of peculiarity, providing a substantially improved, quiescent control sample. Unlike HCG galaxies, galaxies in the comparison SINGS subsample are continuously distributed both in SSFR and alpha-IRAC, although they show ranges in SFR(sub TOTAL) values, morphologies and stellar masses similar to those for HCG systems. We test the SSFR bimodality against a number of uncertainties, and find that these can only lead to its further enhancement. Excluding galaxies belonging to HCGs with three giant galaxies (triplets) leaves both the SSFR and the alpha-IRAC bimodality completely unaffected. We interpret these results as further evidence that an environment characterized by high galaxy number densities and low galaxy velocity dispersions, such as the one found in compact groups, plays a key role in accelerating galaxy evolution by enhancing star formation processes in galaxies and favoring a fast transition to quiescence.
Tzanavaris, P.↗
Comparison study of fluorine/hydrazine engine concepts
The weight and performance were examined of 600 lbF thrust liquid-liquid and bimodal engines as chamber pressure, mixture ratio, contraction ratio, combustion chamber and nozzle lengths, area ratio, and operating duration were varied. Results show the bimodal engine to have a lower weight, shorter length, and slightly higher performance over the entire range of operating conditions. Advanced engine designs were also considered. It was determined that the weight of a bimodal engine with a ducted thrust chamber using hydrazine decomposition products for a combination of regenerative, inter-regen, and film cooling would be nearly independent of operating duration. The length of the combustion chamber of the liquid-liquid engine can be reduced by the incorporation of a splash-plate element transverse-platelet injector to obtain improved fuel atomization and vaporization. The use of this injector would result in the performance and weight of the liquid-liquid engine being virtually the same as that of the baseline bimodal engine.
Schindler, R. C.↗
The Influence of SLD Drop Size Distributions on Ice Accretion in the NASA Icing Research Tunnel
An ice shape database has been created to document ice accretions on a 21-inch chord NACA0012 model and a 72-inch chord NACA 23012 airfoil model resulting from an exposure to a Supercooled Large Drop (SLD) icing cloud with a bimodal drop size distribution. The ice shapes created were documented with photographs, laser scanned surface measurements over a section of the model span, and measurement of the ice mass over the same section of each accretion. The icing conditions used in the test matrix were based upon previously used conditions on the same models but with an alternate approach to evaluation of drop distribution effects. Ice shapes resulting from the bimodal distribution as well as from equivalent monomodal drop size distributions were obtained and compared. Results indicate that the ice shapes resulting from the monomodal and bimodal drop size distributions had similar shapes, but the bimodal distributions had greater mass and volume measurements and icing limits that extended further back on the chord of the model.
Potapczuk, Mark G.↗
A kinetic-theory approach to turbulent chemically reacting flows
The paper examines the mathematical and physical foundations for the kinetic theory of reactive turbulent flows, discussing the differences and relation between the kinetic and averaged equations, and comparing some solutions of the kinetic equations obtained by the Green's function method with those obtained by the approximate bimodal method. The kinetic method described consists essentially in constructing the probability density functions of the chemical species on the basis of solutions of the Langevin stochastic equation for the influence of eddies on the behavior of fluid elements. When the kinetic equations are solved for the structure of the diffusion flame established in a shear layer by the bimodal method, discontinuities in gradients of the mean concentrations at the two flame edges appear. This is a consequence of the bimodal approximation of all distribution functions by two dissimilar half-Maxwellian functions, which is a very crude approximation. These discontinuities do not appear when the solutions are constructed by the Green's function method described here.
Chung, P. M.↗
Particle track densities in double drive tube 60009/10
Measurements are reported of particle track densities in grains from a suite of eleven samples from the Apollo 16 double drive tube 60009/10. All of the samples studied in 60009 show a clearly bimodal distribution of track densities in plagioclase. This bimodal distribution is interpreted to indicate a mixing of a relatively immature soil with a relatively mature soil. This mixing was not followed by any appreciable reworking which would have altered the distribution. The three lower samples in 60010 also show a bimodal distribution. However, the three uppermost samples show a unimodal distribution. The percentage of high track density plagioclase grains shows a good correlation with the other maturity indices of agglutinate content and the ferromagnetic resonance intensity.
Blanford, G. E.↗
A continuous record of tectonic evolution from 3.5 Ga to 2.6 Ga in Swaziland and northern Natal
The approx. 3.5 Ga-old bimodal suite underlying an extensive area in southwestern Swaziland comprises the oldest-dated sialic rocks in the Kaapvaal structural province. The suite consists of leucocratic, layered tonalitic-trondhjemitic gneisses and amphibolites characterized by the effects of repeated high strains. This suite is considered to represent a sialic basement on which metavolcanic and metasedimentary rocks, now preserved as scattered greenstone remnants, accumulated. Direct evidence to confirm this temporal relationship is lacking, but structural data from the Dwalile, Assegaai and Commondale areas indicate that (1) the bimodal gneisses experienced a complex structural history prior to the first recognizable deformation in the supracrustal rocks (i.e., D1 in the supracrustals is equivalent to Dn + 1 in the gneisses) and (2) scattered remnants of the Dwalile rocks infolded with the bimodal suite structurally overlie the gneisses and are preserved in synformal keels. Significant proportions of metaquartzites and metapelites are present in the Assegaai greenstone sequence, the presence of which implies the existence of felsic crust in the source area from which these sediments were derived, a conclusion that is consistent with the structural data.
Hunter, D. R.↗
On the distribution of sunspot cycle periods
A comparison is made between the observed distribution of sunspot cycle periods and distributions based on uniform, normal, and bimodal distributions. The bimodal distribution, composed of short-period and long-period cycles, is found to best describe the observed distribution. Compared to the normal distribution for the most reliably determined cycles (cycles 8-20), the bimodal distribution has a residual (sum of squares of differences) that is about 86 percent smaller. Means for short-period and long-period cycles are estimated to be 122 + or - 4 months and 140 + or - 5 months, respectively.
Wilson, Robert M.↗
Secondary atomization in the combustion of electrostatic sprays
The combustion of electrosprays in a laminar counterflow diffusion flame has been experimentally studied by measuring droplet size and velocity distributions and gas-phase temperature. Detailed examination of the evolution of droplet size distribution as droplets approach the flame shows that, if substantial evaporation occurs before droplets 'interact' with the flame, the size distribution becomes bimodal. A secondary, sharp peak, in fact, develops in correspondence of diameters about one order of magnitude smaller than the mean. No evaporation mechanism can account for the development of such bimodality, that can be explained only in terms of a disintegration of droplets into finer fragments of size much smaller than the parent ones. This fission is of electric nature and it occurs when the repulsion of electric charges overcomes the surface tension cohesive force ultimately leading to a disintegration into finer fragments at or about the so-called Rayleigh limit. We here report on the first observation in combustion environments of such 'explosions'. If, on the other hand, droplets enter the very high temperature region before exploding, there appears to be no evidence of bimodality in their size distribution. In this case, in fact, flame chemi-ions may neutralize the charge on the droplets and thus prevent disruption.
Gomez, Alessandro↗
Charge-induced secondary atomization in diffusion flames of electrostatic sprays
The combustion of electrostatic sprays of heptane in laminar counterflow diffusion flames was experimentally studied by measuring droplet size and velocity distributions, as well as the gas-phase temperature. A detailed examination of the evolution of droplet size distribution as droplets approach the flame shows that, if substantial evaporation occurs before droplets interact with the flame, an initially monodisperse size distribution becomes bimodal. A secondary sharp peak in the size histogram develops in correspondence of diameters about one order of magnitude smaller than the mean. No evaporation mechanism can account for the development of such bimodality, that can be explained only in terms of a disintegration of droplets into finer fragments of size much smaller than that of the parent. Other evidence in support of this interpretation is offered by the measurements of droplet size-velocity correlation and velocity component distributions, showing that, as a consequence of the ejection process, the droplets responsible for the secondary peak have velocities uncorrelated with the mean flow. The fission is induced by the electric charge. When a droplet evaporates, in fact, the electric charge density on the droplet surface increases while the droplet shrinks, until the so-called Rayleigh limit is reached at which point the repulsion of electric charges overcomes the surface tension cohesive force, ultimately leading to a disintegraton into finer fragments. We report on the first observation of such fissions in combustion environments. If, on the other hand, insufficient evaporation has occurred before droplets enter the high temperature region, there appears to be no significant evidence of bimodality in their size distribution. In this case, in fact, the concentration of flame chemi-ions or, in the case of positively charged droplets, electrons may be sufficient for them to neutralize the charge on the droplets and to prevent disruption.
Gomez, Alessandro↗
Generic Signatures of the Time Profiles of BATSE Cosmic Gamma-Ray Bursts
A new method is proposed, which allows the study of generic signatures of cosmic gamma-ray burst time histories. We average the 64 ms resolution time profiles of 275 bright bursts detected by BATSE. The profile of each burst is normalized by the maximum number of counts at the peak of the primary pulse, and individual pulses and interpulse valleys are selected from the normalized profiles by identical selection criteria. New generic temporal parameters are introduced, which characterize the duration and equivalent width of each pulse and the duration of each valley. The histograms of the total equivalent pulse width and summed pulse duration are bimodal. Bimodality is also seen in the histogram of the mean duration of individual pulses. Bursts from the short and long peaks of these distributions correspond to the two modes of the Third BATSE Burst Catalog T(sub 50) and T(sub 90) distributions. Therefore, these new burst parameters demonstrate that the observed bimodal temporal behavior results from properties of the pulsed emission of gamma-ray bursts. The long mode of the T(sub 90) histogram includes bursts with from one to approximately 20 pulses; the logarithmic mean pulse duration is 1.17 +/- 0.09 S; for the long events with more than one pulse, the logarithmic mean valley duration is 1.28 +/- 0.15 S. Bursts of the short mode of T(sub 90) are mainly single-pulse events, and the logarithmic mean pulse duration is much smaller, 0.20 +/- 0.01 s. For multipulse bursts of the T(sub 90) long mode, marginal correlations were found between the parameters of the pulses and valleys and the number of pulses. The basic signatures of the evolution of pulses and valleys along the time course of bursts are examined. Conclusions are drawn concerning the physics of gamma-ray emission by taking into account these signatures.
Mitrofanov, Igor G.↗
Spectral Measurements of PMCs from SBUV/2 Instruments
The SBUV/2 (Solar Backscattered Ultraviolet, model 2) instrument is designed to monitor ozone stratospheric profile and total column ozone using measurements of the Earth's backscattered ultraviolet albedo. We have previously demonstrated that the normal radiance measurements from SBUV/2 instruments, which sample 12 discrete wavelengths between 252 and 340 nm during each scan, can be used to identify polar mesospheric clouds (PMCs). Some SBUV/2 instruments also periodically view the earth in continuous scan mode, covering the wavelength range 160-400 nm with 0.15 nm sampling. Analysis of these data show PMC occurrence rates similar to the normal discrete scan results, although the observation technique reduces the number of daily measurements by a factor of six. PMC observed by SBUV/2 instruments show a monotonic variation in the residual spectral albedo over the wavelength range 250 300 nm, with maximum enhancements of 10 15% at 250 nm. This result is consistent with microphysical model predictions from Jensen [1989. A numerical model of polar mesospheric cloud formation and evolution, Ph. D. Thesis, University of Colorado]. We find no evidence for a systematic localized increase in PMC residual albedo for wavelengths near 260 nm, in contrast to the recently reported results from the MSX UVISI instrument [Carbary J.F., et al., 2004. Evidence for bimodal particle distribution from the spectra of polar mesospheric clouds. Geophysics Research. Letters 31, L13108]. This result is observed for three different SBUV/2 instruments in both Northern and Southern Hemisphere data over a 13-year span. Our Mie scattering calculations show that the location and magnitude of the 260 nm hump feature is dependent upon the specific scattering angles appropriate to the MSX measurements. Although it explains the MSX spectrum, the bimodal size distribution proposed by Carbary et al. (2004), cannot explain the lack of scattering angle dependence of the SBUV/2 spectral shapes. The spectral signature of the SBUV/2 continuous scan PMC data is thus inconsistent with the bimodal particle size distribution suggested by Carbary et al. (2004).
DeLand, Matthew T.↗
Revisiting Scaling Relations for Giant Radio Halos in Galaxy Clusters
Many galaxy clusters host megaparsec-scale radio halos, generated by ultrarelativistic electrons in the magnetized intracluster medium. Correlations between the synchrotron power of radio halos and the thermal properties of the hosting clusters were established in the last decade, including the connection between the presence of a halo and cluster mergers. The X-ray luminosity and redshift-limited Extended GMRT Radio Halo Survey provides a rich and unique dataset for statistical studies of the halos. We uniformly analyze the radio and X-ray data for the GMRT cluster sample, and use the new Planck Sunyaev-Zel'dovich (SZ) catalog to revisit the correlations between the power of radio halos and the thermal properties of galaxy clusters. We find that the radio power at 1.4 GHz scales with the cluster X-ray (0.1-2.4 keV) luminosity computed within R(sub 500) as P(sub 1.4) approx. L(2.1+/-0.2) - 500). Our bigger and more homogenous sample confirms that the X-ray luminous (L(sub 500) > 5 × 10(exp 44) erg/s)) clusters branch into two populations-radio halos lie on the correlation, while clusters without radio halos have their radio upper limits well below that correlation. This bimodality remains if we excise cool cores from the X-ray luminosities. We also find that P(sub 1.4) scales with the cluster integrated SZ signal within R(sub 500), measured by Planck, as P(sub 1.4) approx. Y(2.05+/-0.28) - 500), in line with previous findings. However, contrary to previous studies that were limited by incompleteness and small sample size, we find that "SZ-luminous" Y(sub 500) > 6×10(exp −5) Mpc(exp 2) clusters show a bimodal behavior for the presence of radio halos, similar to that in the radio-X-ray diagram. Bimodality of both correlations can be traced to clusters dynamics, with radio halos found exclusively in merging clusters. These results confirm the key role of mergers for the origin of giant radio halos, suggesting that they trigger the relativistic particle acceleration.
radiation mechanisms↗
Improvement of Stratospheric Aerosol Extinction Retrieval from OMPS/LP Using a New Aerosol Model
The Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP) has been flying on the Suomi National Polar-orbiting Partnership (S-NPP) satellite since October 2011. It is designed to produce ozone and aerosol vertical profiles at 2-kilometer vertical resolution over the entire sunlit globe. Aerosol extinction profiles are computed with Mie theory using radiances measured at 675 nanometers. The operational Version 1.0 (V1.0) aerosol extinction retrieval algorithm assumes a bimodal lognormal aerosol size distribution (ASD) whose parameters were derived by combining an in situ measurement of aerosol microphysics with the Stratospheric Aerosol and Gas Experiment (SAGE II) aerosol extinction climatology. Internal analysis indicates that this bimodal lognormal ASD does not sufficiently explain the spectral dependence of LP-measured radiances. In this paper we describe the derivation of an improved aerosol size distribution, designated Version 1.5 (V1.5), for the LP retrieval algorithm. The new ASD uses a gamma function distribution that is derived from Community Aerosol and Radiation Model for Atmospheres (CARMA)-calculated results. A cumulative distribution fit derived from the gamma function ASD gives better agreement with CARMA results at small particle radii than bimodal or unimodal functions. The new ASD also explains the spectral dependence of LP-measured radiances better than the V1.0 ASD. We find that the impact of our choice of ASD on the retrieved extinctions varies strongly with the underlying reflectivity of the scene. Initial comparisons with collocated extinction profiles retrieved at 676 nanometers from the SAGE III instrument on the International Space Station (ISS) show a significant improvement in agreement for the LP V1.5 retrievals. Zonal mean extinction profiles agree to within 10 percent between 19 and 29 kilometers, and regression fits of collocated samples show improved correlation and reduced scatter compared to the V1.0 product. This improved agreement will motivate development of more sophisticated ASDs from CARMA results that incorporate latitude, altitude and seasonal variations in aerosol properties.
Ozone Mapping Profiler Suite Limb Profiler (OMPS/L↗
Alternative Vegetation States in Tropical Forests and Savannas: The Search for Consistent Signals in Diverse Remote Sensing Data
Globally, the spatial distribution of vegetation is governed primarily by climatological factors (rainfall and temperature, seasonality, and inter-annual variability). The local distribution of vegetation, however, depends on local edaphic conditions (soils and topography) and disturbances (fire, herbivory, and anthropogenic activities). Abrupt spatial or temporal changes in vegetation distribution can occur if there are positive (i.e., amplifying) feedbacks favoring certain vegetation states under otherwise similar climatic and edaphic conditions. Previous studies in the tropical savannas of Africa and other continents using the MODerate Resolution Imaging Spectroradiometer (MODIS) vegetation continuous fields (VCF) satellite data product have focused on discontinuities in the distribution of tree cover at different rainfall levels, with bimodal distributions (e.g., concentrations of high and low tree cover) interpreted as alternative vegetation states. Such observed bimodalities over large spatial extents may not be evidence for alternate states, as they may include regions that have different edaphic conditions and disturbance histories. In this study, we conduct a systematic multi-scale analysis of diverse MODIS data streams to quantify the presence and spatial consistency of alternative vegetation states in Sub-Saharan Africa. The analysis is based on the premise that major discontinuities in vegetation structure should also manifest as consistent spatial patterns in a range of remote sensing data streams, including, for example, albedo and land surface temperature (LST). Our results confirm previous observations of bimodal and multimodal distributions of estimated tree cover in the MODIS VCF. However, strong disagreements in the location of multimodality between VCF and other data streams were observed at 1 km scale. Results suggest that the observed distribution of VCF over vast spatial extents are multimodal, not because of local-scale feedbacks and emergent bifurcations (the definition of alternative states), but likely because of other factors including regional scale differences in woody dynamics associated with edaphic, disturbance, and/or anthropogenic processes. These results suggest the need for more in-depth consideration of bifurcation mechanisms and thus the likely spatial and temporal scales at which alternative states driven by different positive feedback processes should manifest.
Savanna↗
Oil-film-parameter investigation
A counter rotating fixture was designed and built to measure starved EHD film thicknesses. Some good geometry glass races were obtained. A theoretical analysis of starvation was made. Measurements of retainer stability during bimode torque disturbances were made. The most common form of bimode was related to stability of retainer cock.
Kingsbury, E. P.↗
Tensile properties to 650 C and deformation structures in a precipitation strengthened titanium-aluminum alloy
Appreciable strength levels were retained to 650 C in a Ti-10Al-1Si alloy aged in the (alpha + alpha sub 2) phase field to yield optimum room temperature strength and ductility. The aging treatment precipitated a uniform distribution of alpha sub 2-particles such that, at room temperature, dislocations bypassed instead of shearing the particles at low strains. Specimens fractured at room temperature exhibited fine uniform dimples even for those aging conditions that imparted no macroscopic ductility. The main crack appeared to propagate through the planar slip bands that had cut through the alpha sub 2-particles. A two-step aging process produced a higher volume fraction of bimodally distributed alpha sub 2-particles that led to higher strength levels at elevated temperatures. Both for the single size and the bimodal alpha sub 2-particle distributions, elevated-temperature deformation structures consisted mainly of planar slip bands that sheared through the alpha sub 2-particles.
Mendiratta, M. G.↗