Pollen-based reconstructions of Holocene vegetation and climatic change of Tibetan Plateau     

唐领余  李春海  于革  沈才明 《极地研究(英文版)》2003,14(2)

A synthesis of Holocene pollen records from the Tibetan Plateau shows the history of vegetation and climatic changes during the Holocene. Palynological evidences from 24 cores/sections have been compiled and show that the vegetation shifted from subalpine/alpine conifer forest to subalpine/alpine evergreen sclerophyllous forest in the southeastern part of the plateau; from alpine steppe to alpine desert in the central, western and northern part; and from alpine meadow to alpine steppe in the eastern and southern plateau regions during the Holocene. These records show that increases in precipitation began about 9 ka from the southeast, and a wide ranging level of increased humidity developed over the entire of the plateau around 8-7 ka, followed by aridity from 6 ka and a continuous drying over the plateau after 4-3 ka. The changes in Holocene climates of the plateau can be interpreted qualitatively as a response to orbital forcing and its secondary effects on the Indian Monsoon which expanded northwards  相似文献   

Isotope fractionation of neon during stepheating extraction?: a comment on ‘Re‐interpretation of the existence of a primitive plume under Australia based on neon isotope fractionation during step heating’ by Gautheron and Moreira (2003)     

Takuya Matsumoto  Masahiko Honda  Ian McDougall  Igor Yatsevich  Suzanne Y. O'Reilly 《地学学报》2004,16(1):23-26

Using a Rayleigh distillation fractionation model, we calculate that the maximum isotope fractionation potentially achievable is less than 5% during the early stages of gas release from a sample. Our calculation corrects the erroneous conclusions of Gautheron and Moreira (2003), who re‐interpreted the plume‐like neon isotopic compositions found in metasomatic apatite from a south‐eastern Australian xenolith (Matsumoto et al., 1997) to be the result of Rayleigh‐type isotope fractionation of originally MORB‐type neon during stepheating gas extraction. We stress that the modelling of neon isotopic fractionation by Gautheron and Moreira (2003) is incorrect, and that the finding of a plume‐like neon isotopic composition in the apatite by Matsumoto et al. (1997) remains a quite valid and robust conclusion.  相似文献   

A critical state model for sands dependent on stress and density     

Y.P. Yao  D.A. Sun  T. Luo 《国际地质力学数值与分析法杂志》2004,28(4):323-337

An elastoplastic model for sands is presented in this paper, which can describe stress–strain behaviour dependent on mean effective stress level and void ratio. The main features of the proposed model are: (a) a new state parameter, which is dependent on the initial void ratio and initial mean stress, is proposed and applied to the yield function in order to predict the plastic deformation for very loose sands; and (b) another new state parameter, which is used to determine the peak strength and describe the critical state behaviour of sands during shearing, is proposed in order to predict simply negative/positive dilatancy and the hardening/softening behaviour of medium or dense sands. In addition, the proposed model can also predict the stress–strain behaviour of sands under three-dimensional stress conditions by using a transformed stress tensor instead of ordinary stress tensor. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

The Siberian Solar Radio Telescope: the current state of the instrument,observations, and data   总被引：2，自引：0，他引：2  

Grechnev  V.V.  Lesovoi  S.V.  Smolkov  G. Ya.  Krissinel  B.B.  Zandanov  V.G.  Altyntsev  A.T.  Kardapolova  N.N.  Sergeev  R.Y.  Uralov  A.M.  Maksimov  V.P.  Lubyshev  B.I. 《Solar physics》2003,216(1-2):239-272

The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in 1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec. Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive prominences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument. We compare SSRT observations at 5.7 GHz, including computed `light curves', both morphologically and quantatively, with observations made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray observations, and dynamic radio spectra.  相似文献   

A Type IV Radio Burst Associated with Type III Burst,Pulsation and Spike Emissions     

Wang  M.  Xie  R.X.  Duan  C.C.  Yan  Y.H.  Sych  R.A.  Altyntsev  A.T. 《Solar physics》2003,212(2):407-424

A type IV radio burst accompanied by several normal- and reverse-drifting type III bursts, multiple long-term quasi-periodic pulsations and spikes was observed with the radio spectrometers (1.0–2.0 and 2.6–3.8 GHz) at National Astronomical Observatories of China (NAOC) on 23 September 1998. In combination with the images of Siberian Solar Radio Telescope (SSRT) of Russia, the complex and multiple magnetic structures inferred from the radio bursts reveal the existence of both large-scale and small-scale magnetic structures. This event suggests that the geometries of coronal magnetic fields contain multiple discrete electron acceleration/injection sites at different heights, and extended open and closed magnetic field lines. It can be shown that the energetic electrons gain access to open, diverging and closed field lines thus producing different types of radio bursts. From the characteristics of position, polarization, dispersion and displacement of the sources, the model of the type IV event is supported, which involves synchrotron emission from the electrons confined by the rapid scattering through the interaction of hydromagnetic wave with particles.  相似文献   

New evidence for a linear colour–magnitude relation and a single Schechter function for red galaxies in a nearby cluster of galaxies down to M*+ 8     

S. Andreon  J.-C. Cuillandre  E. Puddu  Y. Mellier 《Monthly notices of the Royal Astronomical Society》2006,372(1):60-68

  相似文献   

The HNCO ring in the Sgr B2 region     

Y.C. Minh  W.M. Irvine   《New Astronomy》2006,11(8):594-599

The large-scale structure associated with the 2′N HNCO peak in Sgr B2 [Minh, Y.C., Haikala, L., Hjalmarson, Å., Irvine, W.M., 1998. ApJ 498, 261 (Paper I)] has been investigated. A ring-like morphology of the HNCO emission has been found; this structure may be colliding with the Principal Cloud of Sgr B2. This “HNCO Ring” appears to be centered at (l,b) = (0.7°,−0.07°), with a radius of 5 pc and a total mass of 1.0 × 10⁵ to 1.6 × 10⁶ M. The expansion velocity of the Ring is estimated to be 30–40 km s⁻¹, which gives an expansion time scale of 1.5 × 10⁵ year. The morphology suggests that collision between the Ring and the Principal Cloud may be triggering the massive star formation in the Sgr B2 cloud sequentially, with the latest star formation taking place at the 2′N position. The chemistry related to HNCO is not certain yet, but if it forms mainly via reaction with the evaporated OCN⁻ from icy grain mantles, the observed enhancement of the HNCO abundance can be understood as resulting from shocks associated with the collision between the Principal Cloud and the expanding HNCO Ring.  相似文献   

A multi‐level parallelized substructuring‐frontal solution for coupled thermo/hydro/mechanical problems in unsaturated soil     

H.R. Thomas  H.T. Yang  Y. He  P.J. Cleall 《国际地质力学数值与分析法杂志》2003,27(11):951-965

This paper proposes a multi‐level parallelized substructuring–frontal combined algorithm for the analysis of the problem of thermo/hydraulic/mechanical behaviour of unsaturated soil. Temperature, displacement, pore water pressure and pore air pressure are treated as the primary variables in a non‐linear analysis. Details are given firstly of the substructuring–frontal combined approach. The incorporation of the algorithm in a multi‐level parallel strategy is then discussed. The parallel processing can thus be carried out at different substructural levels. The method thus developed impacts, in a positive way, on both computer storage requirement and execution time. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

On the most general accurate solutions for Buchdahl's fluid spheres     

Y.K. Gupta  M.K. Jasim 《Astrophysics and Space Science》2003,283(3):337-346

The most general accurate solutions for the Buchdhal fluid sphere were obtained and matched with the Schwarzchild's exterior solution at the pressure free interface. Various parameters of the solutions were so adjusted that the energy density, pressure and temperature were positive and decreasing away from the centre, and the velocity of sound was less than unity throughout the spheroid model. Using this procedure, the maximum mass of the fluid sphere with a surface density of2 × 10¹⁴gm cm^-3 was determined to be 3.82MΘ and 4.57 MΘ for strong and weak energy conditions respectively. PACS number: 0402, 0402J, 0440D, 95301 This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Contribution to the search for binaries among Am stars – V. Orbital elements of eight short-period spectroscopic binaries     

J.-M. Carquillat  N. Ginestet  J.-L. Prieur  Y. Debernardi 《Monthly notices of the Royal Astronomical Society》2003,346(2):555-564

We present the results of a radial-velocity study of eight Am stars (HD 341, 55822, 61250, 67317, 93991, 162950, 224890 and 225137) observed at Observatoire de Haute-Provence with the CORAVEL instrument. We find that these systems are single-line spectroscopic binaries whose orbital elements are determined for the first time.  相似文献