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971.
A solution to the fixed-time minimum-fuel two-impulse rendezvous problem for the general non-coplanar elliptical orbits is
provided. The optimal transfer orbit is obtained using the constrained multiple-revolution Lambert solution. Constraints consist
of lower bound for perigee altitude and upper bound for apogee altitude. The optimal time-free two-impulse transfer problem
between two fixed endpoints implies finding the roots of an eighth order polynomial, which is done using a numerical iterative
technique. The set of feasible solutions is determined by using the constraints conditions to solve for the short-path and
long-path orbits semimajor axis ranges. Then, by comparing the optimal time-free solution with the feasible solutions, the
optimal semimajor axis for the two fixed-endpoints transfer is identified. Based on the proposed solution procedure for the
optimal two fixed-endpoints transfer, a contour of the minimum cost for different initial and final coasting parameters is
obtained. Finally, a numerical optimization algorithm (e.g., evolutionary algorithm) can be used to solve this global minimization
problem. A numerical example is provided to show how to apply the proposed technique. 相似文献
972.
Katherine C. Filippino Peter W. Bernhardt Margaret R. Mulholland 《Estuaries and Coasts》2009,32(3):410-424
To determine the effects of the Chesapeake Bay outflow plume on the coastal ocean, nutrient concentrations and climatology
were evaluated in conjunction with nitrogen (N) and carbon (C) uptake rates during a 3-year field study. Sixteen cruises included
all seasons and captured high- and low-flow freshwater input scenarios. Event-scale disturbances in freshwater flow and wind
speed and direction strongly influenced the location and type of plume present and thus the biological uptake of N and C.
As expected, volumetric primary productivity rates did not always correlate with chlorophyll a concentrations, suggesting that high freshwater flow does not translate into high productivity in the coastal zone; rather,
high productivity was observed during periods where recycling processes may have dominated. Results suggest that timing of
meteorological events, with respect to upwelling or downwelling favorable conditions, plays a crucial role in determining
the impact of the estuarine plume on the coastal ocean. 相似文献
973.
Veronika Gälman Johan Rydberg Andrei Shchukarev Staffan Sjöberg Antonio Martínez-Cortizas Richard Bindler Ingemar Renberg 《Journal of Paleolimnology》2009,42(1):141-153
Easily discernible sediment varves (annual laminations) may be formed in temperate zone lakes, and reflect seasonal changes in the composition of the accumulating material derived from the lake and its catchment (minerogenic and organic material). The appearance of varves may also be influenced by chemical processes. We assessed the role of iron (Fe) and sulfur (S) in the appearance of varves in sediments from Lake Nylandssjön in northern Sweden. We surveyed Fe in the lake water and established whether there is internal transport of Fe within the sediment. We used a unique collection of seven stored freeze cores of varved sediment from the lake, collected from 1979 to 2004. This suite of cores made it possible to follow long-term changes in Fe and S in the sediment caused by processes that occur in the lake bottom when the sediment is ageing. We compared Fe and S concentrations using X-ray fluorescence spectroscopy (XRF) in specific years in the different cores. No diagenetic front was found in the sediment and the data do not suggest that there is substantial vertical transport of Fe and S in the sediment. We also modeled Fe and S based on thermodynamic, limnological, and sediment data from the lake. The model was limited to the five components H+, e?, Fe3+, SO4 2?, H2CO3 and included the formation of solid phases such as Fe(OH)3 (amorphous), FeOOH (aged, microcrystalline), FeS and FeCO3. Modeling showed that there are pe (redox) ranges within which either FeS or Fe(OH)3/FeOOH is the only solid phase present and there are pe ranges within which the two solid phases co-exist, which supports the hypothesis that blackish and grey-brownish layers that occur in the varves were formed at the time of deposition. This creates new possibilities for deciphering high-temporal-resolution environmental information from varves. 相似文献
974.
Equilibrium models of differentially rotating nascent neutron stars are constructed, which represent the result of the accretion-induced collapse of rapidly rotating white dwarfs. The models are built in a two-step procedure: (1) a rapidly rotating pre-collapse white dwarf model is constructed; (2) a stationary axisymmetric neutron star having the same total mass and angular momentum distribution as the white dwarf is constructed. The resulting collapsed objects consist of a high-density central core of size roughly 20 km, surrounded by a massive accretion torus extending over 1000 km from the rotation axis. The ratio of the rotational kinetic energy to the gravitational potential energy of these neutron stars ranges from 0.13 to 0.26, suggesting that some of these objects may have a non-axisymmetric dynamical instability that could emit a significant amount of gravitational radiation. 相似文献
975.
The mean value concept in mono-linear regression of multi-variables and its application to trace studies in geosciences 总被引:3,自引:1,他引:3
A J T JULL 《中国科学D辑(英文版)》2007,50(12):1828-1834
The "mean value concept" in a mono-linear regression of multi-variables is clarified.Its applications to reconstruction of the past 90-year salinity of the sea surface water in Xisha Islands and to tracing the past 80 ka paleo-geomagnetic events from the Luochuan loess 10Be record are introduced in detail,which show the significance and the potential of the "mean value concept" in geoscience research. 相似文献
976.
Automatic Interpretation of Magnetic Data Using Euler Deconvolution with Nonlinear Background 总被引:1,自引:0,他引:1
Pawan Dewangan T. Ramprasad M. V. Ramana M. Desa B. Shailaja 《Pure and Applied Geophysics》2007,164(11):2359-2372
The voluminous gravity and magnetic data sets demand automatic interpretation techniques like Naudy, Euler and Werner deconvolution.
Of these techniques, the Euler deconvolution has become a popular choice because the method assumes no particular geological
model. However, the conventional approach to solving Euler equation requires tentative values of the structural index preventing
it from being fully automatic and assumes a constant background that can be easily violated if the singular points are close
to each other. We propose a possible solution to these problems by simultaneously estimating the source location, depth and
structural index assuming nonlinear background. The Euler equation is solved in a nonlinear fashion using the optimization
technique like conjugate gradient. This technique is applied to a published synthetic data set where the magnetic anomalies
were modeled for a complex assemblage of simple magnetic bodies. The results for close by singular points are superior to
those obtained by assuming linear background. We also applied the technique to a magnetic data set collected along the western
continental margin of India. The results are in agreement with the regional magnetic interpretation and the bathymetric expressions. 相似文献
977.
James Flocks Michael D. Miner David C. Twichell Dawn L. Lavoie Jack Kindinger 《Geo-Marine Letters》2009,29(6):359-378
The barrier-island systems of the Mississippi River Delta plain are currently undergoing some of the highest rates of shoreline
retreat in North America (~20 m/year). Effective management of this coastal area requires an understanding of the processes
involved in shoreline erosion and measures that can be enacted to reduce loss. The dominant stratigraphy of the delta plain
is fluvial mud (silts and clays), delivered in suspension via a series of shallow-water delta lobes that prograded across
the shelf throughout the Holocene. Abandonment of a delta lobe through avulsion leads to rapid land subsidence through compaction
within the muddy framework. As the deltaic headland subsides below sea level, the marine environment transgresses the bays
and wetlands, reworking the available sands into transgressive barrier shorelines. This natural process is further complicated
by numerous factors: (1) global sea-level rise; (2) reduced sediment load within the Mississippi River; (3) diversion of the
sediment load away from the barrier shorelines to the deep shelf; (4) storm-induced erosion; and (5) human alteration of the
littoral process through the construction of hardened shorelines, canals, and other activities. This suite of factors has
led to the deterioration of the barrier-island systems that protect interior wetlands and human infrastructure from normal
wave activity and periodic storm impact. Interior wetland loss results in an increased tidal prism and inlet cross-sectional
areas, and expanding ebb-tidal deltas, which removes sand from the littoral processes through diversion and sequestration.
Shoreface erosion of the deltaic headlands does not provide sufficient sand to balance the loss, resulting in thinning and
dislocation of the islands. Abatement measures include replenishing lost sediment with similar material, excavated from discrete
sandy deposits within the muddy delta plain. These sand bodies were deposited by the same cyclical processes that formed the
barrier islands, and understanding these processes is necessary to characterize their location, extent, and resource potential.
In this paper we demonstrate the dominant fluvial and marine-transgressive depositional processes that occur on the inner
shelf, and identify the preservation and resource potential of fluvio-deltaic deposits for coastal management in Louisiana. 相似文献
978.
We analysed the hard X-ray and microwave flux spectra of the solar flare (BATSE No. 1791) on 2 November 1991, which started at 16:11:03 UT and ended at 16:56:10 UT. This flare is particularly interesting because of its deep cyclic intensity modulation. Data are available simultaneously from the 16-channel BATSE/LAD hard X-ray and 45-frequency OVRO microwave database. We quantitatively compare the time variations in profiles of the hard X-ray spectral photon index, the 50 keV X-ray flux, and microwave spectral indices (at both high and low frequency ends). As expected, the X-ray photon spectral index decreases as the hard X-ray flux increases. This pattern appears in all the sub-peaks. This is consistent with previous observations that hard X-ray emission hardens at the emission peak. However, the behaviour of the high-frequency microwave index is unexpected. We observe an anti-correlation between the high-frequency microwave index and the hard X-ray photon index during the course of the flare. Finally, we study the arrival time of microwave flux peaks as a function of frequency and find that the microwave peak at a higher frequency comes earlier than that at a lower frequency. A maximum delay of 72 s is found among the main peaks at different frequencies. Shorter delays are found for the other five sub-peaks. 相似文献
979.
This paper addresses temporal variability in bottom hypoxia in broad shallow areas of Mobile Bay, Alabama. Time-series data
collected in the summer of 2004 from one station (mean depth of 4 m) exhibit bottom dissolved oxygen (DO) variations associated
with various time scales of hours to days. Despite a large velocity shear, stratification was strong enough to suppress vertical
mixing most of the time. Bottom DO was closely related to the vertical salinity gradient (ΔS). Hypoxia seldom occurred when
ΔS (over 2.5 m) was <2 psu and occurred almost all the time when ΔS was >8 psu in the absence of extreme events like hurricanes.
Oxygen balance between vertical mixing and total oxygen demand was considered for bottom water from which oxygen demand and
diffusive oxygen flux were estimated. The estimated decay rates at 20°C ranging between 0.175–0.322 d−1 and the corresponding oxygen consumption as large as 7.4 g O2 m−2 d−1 fall at the upper limit of previously reported ranges. The diffusive oxygen flux and the corresponding vertical diffusivity
estimated for well mixed conditions range between 8.6–9.5 g O2 m−2 d−1 and 2.6–2.9 m2 d−1, respectively. Mobile Bay hypoxia is likely to be associated with a large oxygen demand, supported by both water column and
sediment oxygen demands, so that oxygen supply from surface water during destratification events would be quickly exhausted
to return to hypoxic conditions within a few hours to days after destratification events are terminated. 相似文献
980.
Manish P. Kale Shirish A. Ravan P. S. Roy Sarnam Singh 《Journal of the Indian Society of Remote Sensing》2009,37(3):457-471
Satellite remote sensing is a proven tool for mapping landuse patterns and estimating vegetation biomass/carbon. Present study
aims at estimating the potential of forests of Radhanagari WLS (Western Ghats, India) to sequester the atmospheric carbon-di-oxide,
using ground based observations coupled with satellite remote sensing. The study area was stratified for dominant forest types
based on the structure and composition of vegetation and elevation variations. Permanent sample plots were laid down in these
homogeneous vegetation strata (HVS) to make different observations during time 1 and time 2. Carbon sequestration by plantations
was also studied and compared with natural forests. Species and area-specific biomass equations were used for estimating carbon
pool and sequestration. Among natural forests ‘mixed moist deciduous’ forests exhibited highest sequestration rate (8%), whereas,
plantation as obvious had a comparatively higher sequestration rate than natural forests (20.27%). Total carbon sequestration
by forests of the Radhanagari WLS between 2004 and 2006 is 78742.09 tons. Eligible land for reforestation activity under clean
development mechanism (CDM) of Kyoto Protocol was identified using satellite remote sensing using 1989 and 2005 datasets and
it was observed that the potential land that can be used for reforestation activity is 10080 ha. 相似文献