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831.
Wook-Hyun Nahm Gyoo Ho Lee Dong-Yoon Yang Ju-Yong Kim Kenji Kashiwaya Masayoshi Yamamoto Aya Sakaguchi 《Journal of Paleolimnology》2010,43(3):489-498
This paper describes mean grain-size data from the 137Cs- and 210Pb-dated sediment core BS-3 (33-cm long) recovered from Jinheung Pond, located in the southwestern part of the Korean Peninsula.
Grain-size analysis of the Jinheung Pond sediments shows a clear signal for changes in annual precipitation over the past
60 years. Instrumental records of annual precipitation (AP) and the annual summation of the precipitation of >50 mm per day
(AP50), which reflects the energy available for sediment transport, correlate well with the mean grain-size distributions
measured in the core. The most plausible mechanism for this response in mean grain size is variations in the annual amount
and intensity of precipitation. Heavy precipitation enhances soil erosion over the catchment area and increases the transport
capacity of streams and rivers. Thus, coarser mean grain size should reflect higher precipitation, and smaller mean grain
size should reflect lower rainfall. In the data from core BS-3, however, grain-size peaks attributed to increased annual precipitation
are not prominent. This is because a dam prevents removal of fine particles from the pond via the outflow. Therefore, the
mean grain-size value represents somewhat larger sediments together with fine clays. The results of this study show that sediments
of dammed lakes and ponds are well suited for high-resolution environmental investigations, especially for records of changes
in precipitation over time. 相似文献
832.
Dirk Enters Hermann Behling Christoph Mayr Lydie Dupont Bernd Zolitschka 《Journal of Paleolimnology》2010,44(1):265-277
Environmental changes of the last 9,300 years were reconstructed by geochemical and pollen analyses of a 14-m-long, laminated
sediment core from Lago Aleixo, south-eastern Brazil. Fossil pollen assemblages indicate open savannah vegetation (campo cerrado)
and gallery forests until approximately 6,900 cal. BP. During that time, siderite laminae were deposited under anoxic conditions
at the lake bottom. Then, increased rainfall and a shorter annual dry period allowed gallery forests and semi-deciduous forests
to expand, leading to more closed cerrado vegetation. High-intensity rainfall events during this period are recorded as peaks
in K and Ti concentrations. The sediment facies during this period consists of alternating layers of diatoms and minerogenic
matter. C/N ratios imply that algae and perhaps soils, too, were the main contributors to sediment organic matter. Biogenic
silica and δ13Corg variations indicate increasing primary productivity, which was related to higher nutrient flux from intensified leaching
of soils, as shown by rising K/Al ratios. Around 800 cal. BP, a closed, semi-deciduous forest developed under present-day
climate conditions. Slope stabilization diminished erosion processes in the catchment and caused reduced input of minerogenic
matter into the basin. Human impact is evident in the topmost homogeneous sediments, as removal of the stabilizing forest
cover amplified soil erosion. The continuous trend to more humid conditions during the Holocene probably reflects increased
influence of the Amazon Basin as a moisture source. We conclude that the Lago Aleixo sediment archive was a sensitive recorder
of environmental dynamics in tropical South America, which were mainly controlled by changes in precipitation patterns. 相似文献
833.
The evolution of the 27-day recurrence in the series of two solar indices (Wolf number WN and 10.7 cm radio flux F) and two geomagnetic indices (Dst and ζ, variance of the geomagnetic field recorded at a magnetic observatory) have been studied over the 1957 – 2007 time
span. Spectral energies contained in two period domains (25 – 27.3 and 27.3 – 31 days), designated as E
1 and E
2, have been computed. Whereas the evolution of E
1 is the same for the four indices, that of E
2 is essentially different for WN and F on the one hand, Dst and ζ on the other hand. Some general conclusions on the dynamics of the solar outer layers are inferred from these results.
First the solar activity, as measured by WN, and when averaged over a few years, evolves in the same way whatever the latitude.
Second, two families of coronal holes (CHs) are identified; the rapidly and the slowly rotating CHs evolve quite differently. 相似文献
834.
Nikolay A. Sokolov 《Astrophysics and Space Science》2010,325(1):37-40
We presented a phenomenological mode that attributes the precession of perihelion of planets to the relativistic correction.
This modifies Newton’s equation by adding an inversely cube term with distance. The total energy of the new system is found
to be the same as the Newtonian one. Moreover, we have deduced the deflection of light formula from Rutherford scattering.
The relativistic term can be accounted for quantum correction of the gravitational potential and/or self energy of objects. 相似文献
835.
To investigate the dependence of the Holmberg effect of paired galaxies on three-dimensional separations, we have constructed
four subsamples characterized by three-dimensional separations s ≤ 50 kpc, 50 kpc < s ≤ 100 kpc , 100kpc < s ≤150kpc , and
150 kpc < s ≤ 200 kpc , respectively. In this study, linear correlation coefficients and surplus standard deviations of color
indices, luminosity, and size between two components of paired galaxies are calculated. It is found that there is a weak tendency
for correlation coefficients of color indices to increase with decreasing three-dimensional separations, but such a tendency
does not exist for ones of luminosity and size. We also note that there is no tendency for surplus standard deviations to
decrease with decreasing three-dimensional separations. This suggests that it is still very difficult to clearly affirm the
dependence of the Holmberg Effect of paired galaxies on three-dimensional separations. 相似文献
836.
Jayanta Roy Yashwant Gupta Ue-Li Pen Jeffrey B. Peterson Sanjay Kudale Jitendra Kodilkar 《Experimental Astronomy》2010,28(1):25-60
The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation
of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components,
is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes
with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline
uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility
in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection.
Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the
GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using
this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate
sized array like the GMRT. 相似文献
837.
In this paper we present a new method of restoration of the true three-dimensional (3D) trajectories of prominence knots using
ground-based observations taken with a single telescope that is equipped with a Multi-Channel Subtractive Double Pass imaging
spectrograph. Our method allows us to evaluate the true 3D trajectories of the prominence knots without any assumptions concerning
the shape of the trajectories or the dynamics of the motion. The reconstructed trajectories of several knots observed in three
prominences are presented. 相似文献
838.
Nonlinear electron-acoustic solitary waves (EASWs) are studied using Sagdeev’s pseudo-potential technique in a collisionless
unmagnetized plasma consisting of a cold electron fluid, nonthermal hot electrons and stationary ions. It is shown that the
presence of fast nonthermal electrons may modify the parametric region where electron-acoustic solitons may exist. Our investigation
is of wide relevance to astronomers and space scientists working on interstellar space plasmas. 相似文献
839.
Murli Manohar Verma 《Astrophysics and Space Science》2010,330(1):101-105
We propose a time-varying cosmological constant with a fixed equation of state, which evolves mainly through its interaction
with the background during most of the long history of the universe. However, such an interaction does not exist in the very
early- and the late-time universe and this produces the acceleration during these eras when it becomes very nearly a constant.
It is found that after the initial inflationary phase, the cosmological constant, which we call the lambda parameter, rolls
down from a large constant value to another but very small constant value and further dominates the present epoch showing
up in the form of dark energy, driving the acceleration. 相似文献
840.
The use of invariant manifolds for transfers between unstable periodic orbits of different energies 总被引:1,自引:0,他引:1
Kathryn E. Davis Rodney L. Anderson Daniel J. Scheeres George H. Born 《Celestial Mechanics and Dynamical Astronomy》2010,107(4):471-485
Techniques from dynamical systems theory have been applied to the construction of transfers between unstable periodic orbits
that have different energies. Invariant manifolds, trajectories that asymptotically depart or approach unstable periodic orbits,
are used to connect the initial and final orbits. The transfer asymptotically departs the initial orbit on a trajectory contained
within the initial orbit’s unstable manifold and later asymptotically approaches the final orbit on a trajectory contained
within the stable manifold of the final orbit. The manifold trajectories are connected by the execution of impulsive maneuvers.
Two-body parameters dictate the selection of the individual manifold trajectories used to construct efficient transfers. A
bounding sphere centered on the secondary, with a radius less than the sphere of influence of the secondary, is used to study
the manifold trajectories. A two-body parameter, κ, is computed within the bounding sphere, where the gravitational effects of the secondary dominate. The parameter κ is defined as the sum of two quantities: the difference in the normalized angular momentum vectors and eccentricity vectors
between a point on the unstable manifold and a point on the stable manifold. It is numerically demonstrated that as the κ parameter decreases, the total cost to complete the transfer decreases. Preliminary results indicate that this method of
constructing transfers produces a significant cost savings over methods that do not employ the use of invariant manifolds. 相似文献