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321.
Ralph E. Pudritz 《Astrophysics and Space Science》2004,292(1-4):471-478
This article summarizes recent theoretical, numerical and observational progress on the nature of jets and outflows in regions of star formation. The emphasis is placed on the role of hydromagnetic phenonmena in understanding these new results. The confluence of sophisticated 3D numerical MHD simulations and the recent observations of rotating jets makes it possible, for the first time, to rigorously test jet models. 相似文献
322.
Abstract— The carbonate mineralogy of several complex carbonate‐rich regions in Allan Hills (ALH) 84001 has been examined. These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform “slab” carbonates, distinct “post‐slab” magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite‐rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite‐siderite layers on the exterior surfaces of the carbonate. Post‐slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts. 相似文献
323.
Peter McGregor Michael Dopita Ralph Sutherland Tracy Beck Thaisa Storchi-Bergmann 《Astrophysics and Space Science》2007,311(1-3):223-230
We present first results from the Near-infrared Integral Field Spectrograph (NIFS) located at Gemini North. For the active
galaxies Cygnus A and Perseus A we observe rotationally-supported accretion disks and adduce the existence of massive central
black holes and estimate their masses. In Cygnus A we also see remarkable high-excitation ionization cones dominated by photoionization
from the central engine. In the T-Tauri stars HV Tau C and DG Tau we see highly-collimated bipolar outflows in the [Fe II]
λ 1.644 micron line, surrounded by a slower molecular bipolar outflow seen in the H2 lines, in accordance with the model advocated by Pyo, T.-S., et al., Astrophys. J. 570, 724 (2002). 相似文献
324.
Laura Duncanson Wenli Huang Kristofer Johnson Anu Swatantran Ronald E. McRoberts Ralph Dubayah 《Carbon balance and management》2017,12(1):18
Background
Carbon accounting in forests remains a large area of uncertainty in the global carbon cycle. Forest aboveground biomass is therefore an attribute of great interest for the forest management community, but the accuracy of aboveground biomass maps depends on the accuracy of the underlying field estimates used to calibrate models. These field estimates depend on the application of allometric models, which often have unknown and unreported uncertainties outside of the size class or environment in which they were developed.Results
Here, we test three popular allometric approaches to field biomass estimation, and explore the implications of allometric model selection for county-level biomass mapping in Sonoma County, California. We test three allometric models: Jenkins et al. (For Sci 49(1): 12–35, 2003), Chojnacky et al. (Forestry 87(1): 129–151, 2014) and the US Forest Service’s Component Ratio Method (CRM). We found that Jenkins and Chojnacky models perform comparably, but that at both a field plot level and a total county level there was a ~ 20% difference between these estimates and the CRM estimates. Further, we show that discrepancies are greater in high biomass areas with high canopy covers and relatively moderate heights (25–45 m). The CRM models, although on average ~ 20% lower than Jenkins and Chojnacky, produce higher estimates in the tallest forests samples (> 60 m), while Jenkins generally produces higher estimates of biomass in forests < 50 m tall. Discrepancies do not continually increase with increasing forest height, suggesting that inclusion of height in allometric models is not primarily driving discrepancies. Models developed using all three allometric models underestimate high biomass and overestimate low biomass, as expected with random forest biomass modeling. However, these deviations were generally larger using the Jenkins and Chojnacky allometries, suggesting that the CRM approach may be more appropriate for biomass mapping with lidar.Conclusions
These results confirm that allometric model selection considerably impacts biomass maps and estimates, and that allometric model errors remain poorly understood. Our findings that allometric model discrepancies are not explained by lidar heights suggests that allometric model form does not drive these discrepancies. A better understanding of the sources of allometric model errors, particularly in high biomass systems, is essential for improved forest biomass mapping.325.
J. Alan RoebuckJr. G. Brooks Avery J. David Felix Robert J. Kieber Ralph N. Mead Stephen A. Skrabal 《Aquatic Geochemistry》2016,22(3):177-195
This study reports the first ethanol and acetaldehyde measurements in sediment porewaters collected at multiple freshwater locations. Ethanol concentrations ranged from 11 to 2535 nM and acetaldehyde concentrations ranged from 6 to 320 nM. A significant positive correlation (p < 0.001) was observed between ethanol concentrations and the percent organic carbon content of sediments (TOC). Porewater depth profiles at two sites within the same lake indicated potential diffusion of ethanol into sediments from the overlying water at a lower TOC site and upwards diffusion from sediment into the water column at a higher TOC site. Diffusion of water column ethanol into sediments was observed at individual sites from October to January, whereas the opposite was true from June to August indicating the seasonal variability of ethanol flux from sediments. Changes in ethanol concentrations during a long-term sediment incubation experiment showed an inverse relation with acetaldehyde concentrations. The lack of a quantitative conversion was likely due to other sources and sinks that control their abundance. Our study provided new information on the biogeochemistry of ethanol in freshwater sediments and shed light on the potential role of ethanol in the global carbon cycle. 相似文献
326.
The Cretaceous aquifers underlying the central part of the North Carolina Coastal Plain are used extensively as the preferred
source of water for municipalities, industries, and agriculture. Regional decline in head in the middle parts of the system
(the Upper Cape Fear and Black Creek aquifers) began in the late 1950s to early 1960s. The trends in head during the 1960s,
together with data on land subsidence, were analyzed to demonstrate the value of using early data, in conjunction with conceptual
models, before collecting additional data. The sources of withdrawals from the Cretaceous aquifers identified in the conceptual
model are: (1) movement of water (recharge) across confining beds, (2) reduction in natural discharge, (3) release of water
from aquifer storage, (4) drainage of water from fine-grained lenses in aquifers and confining beds, and (5) release of water
resulting from advance of the salt-water front. Estimates of the amounts derived from each of these sources resulted in a
reduction of the estimated safe yield of the Cretaceous aquifers in the 2,486-mi2 (6,439-km2) area of the central Coastal Plain from 75 mgd (million gallons per day) (284,000 m3/day) to less than 5 mgd (19,000 m3/day).
Electronic Publication 相似文献
327.
Ralph S. Sutherland 《Astrophysics and Space Science》2010,327(2):173-206
A new hydrodynamics code aimed at astrophysical applications has been developed. The new code and algorithms are presented
along with a comprehensive suite of test problems in one, two, and three dimensions. The new code is shown to be robust and
accurate, equalling or improving upon a set of comparison codes. Fyris Alpha will be made freely available to the scientific community. 相似文献
328.
Charles A. Wood Ralph Lorenz Rosaly Lopes Ellen Stofan The Cassini RADAR Team 《Icarus》2010,206(1):334-344
Five certain impact craters and 44 additional nearly certain and probable ones have been identified on the 22% of Titan’s surface imaged by Cassini’s high-resolution radar through December 2007. The certain craters have morphologies similar to impact craters on rocky planets, as well as two with radar bright, jagged rims. The less certain craters often appear to be eroded versions of the certain ones. Titan’s craters are modified by a variety of processes including fluvial erosion, mass wasting, burial by dunes and submergence in seas, but there is no compelling evidence of isostatic adjustments as on other icy moons, nor draping by thick atmospheric deposits. The paucity of craters implies that Titan’s surface is quite young, but the modeled age depends on which published crater production rate is assumed. Using the model of Artemieva and Lunine (2005) suggests that craters with diameters smaller than about 35 km are younger than 200 million years old, and larger craters are older. Craters are not distributed uniformly; Xanadu has a crater density 2-9 times greater than the rest of Titan, and the density on equatorial dune areas is much lower than average. There is a small excess of craters on the leading hemisphere, and craters are deficient in the north polar region compared to the rest of the world. The youthful age of Titan overall, and the various erosional states of its likely impact craters, demonstrate that dynamic processes have destroyed most of the early history of the moon, and that multiple processes continue to strongly modify its surface. The existence of 24 possible impact craters with diameters less than 20 km appears consistent with the Ivanov, Basilevsky and Neukum (1997) model of the effectiveness of Titan’s atmosphere in destroying most but not all small projectiles. 相似文献
329.
Radarclinometry is a powerful technique for estimating heights of landforms in synthetic aperture radar (SAR) images of planetary surfaces. In particular, it has been used to estimate heights of dunes in the sand seas of Saturn’s moon Titan (Lorenz, R.D., and 39 colleagues [2006]. Science 312, 724-727). In this work, we verify the technique by comparing dune heights derived from radarclinometry to known topography of dune fields in the Namib sand sea of western Africa. We compared results from three different image grid spacings, and found that 350 m/pixel (the same spacing at which the Cassini RADAR data was processed) is sufficient to determine dune height for dunes of similar morphometry to those of the Namib sand sea. At this grid spacing, height estimates derived from radarclinometry are largely representative of, though may underestimate by as much as 30%, or overestimate by as much as 40%, true dune height. Applying the technique to three regions on Titan, we estimate dune heights of 45-180 m, and dune spacings of 2.3-3.3 km. Obtaining accurate heights of Titan’s dunes will help to constrain the total organic inventory on Titan. 相似文献
330.
Ralph Lorenz 《Icarus》2011,215(1):381-390