Chemical alteration of tephra in the depositional environment: theoretical stability modelling     

A. M. Pollard  S. P. E. Blockley  K. R. Ward 《第四纪科学杂志》2003,18(5):385-394

The study of the chemical stability of vitreous material in aqueous media is well‐established. There has to date been little consideration of the implications of variations in the chemical durability of tephra in Quaternary tephrochronology. Chemical alteration can take the form of cationic leaching from the matrix, or complete destruction of the silica network, either of which could constrain the ability to chemically identify distal tephra. Here we apply established models of vitreous durability to the published chemical analyses of a large number of Icelandic tephras in order to predict their relative durabilities under equivalent conditions. This suggests that some important tephras have relatively poor chemical stability, and that rhyolitic tephras are, in general, more stable than basaltic. We conclude that tephras should be expected to show predictable differential chemical stability in the post‐depositional environment. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Parameters of the intense X-ray and gamma-ray radiation from the solar flare of May 20, 2002, as observed from the Coronas-F spacecraft     

A. V. Bogomolov  A. P. Ignat’ev  K. Kudela  S. N. Kuznetsov  Yu. I. Logachev  O. V. Morozov  I. N. Myagkova  S. N. Oparin  A. A. Pertsov  S. I. Svertilov  B. Yu. Yushkov 《Astronomy Letters》2003,29(3):199-204

We consider temporal, spectral, and polarization parameters of the hard X-ray and gamma-ray radiation observed during the solar flare of May 20, 2002, in the course of experiments with the SONG and SPR-N instruments onboard the Coronas-F spacecraft. This flare is one of the most intense gamma-ray events among all of the bursts of solar hard electromagnetic radiation detected since the beginning of the Coronas-F operation (since July 31, 2001) and one of the few gamma-ray events observed during solar cycle 23. A simultaneous analysis of the Coronas-F and GOES data on solar thermal X-ray radiation suggests that, apart from heating due to currents of matter in the the flare region, impulsive heating due to the injection of energetic electrons took place during the near-limb flare S21E65 of May 20, 2002. These electrons produced intense hard X-ray and gamma-ray radiation. The spectrum of this radiation extends up to energies ≥7 MeV. Intense gamma-ray lines are virtually unobservable against the background of the nonthermal continuum. The polarization of the hard X-ray (20–100 keV) radiation was estimated to be ≤15–20%. No significant increase in the flux of energetic protons from the flare under consideration was found. At the same time, according to ACE data, the fluxes of energetic electrons in interplanetary space increased shortly (～25 min) after the flare.  相似文献   

ANALYSIS OF DRIVING FORCES ON THE CHANGE OF HISTORICAL FARMLAND USE IN SHANDONG PROVINCE DURING THE QING DYNASTY （1644-1911）     

ZHANGXue-qin GEQuan-sheng ZHENGJing-yun 《中国地理科学(英文版)》2003,13(3):207-215

How land-cover has been changed by human use over the last 300 years is one of the five overarching questions guiding the Land-use/Cover Change (LUCC) Science/Research Plan. China has variety of historical docu-ments providing unique data superiority. So the characteristics of farmland area in Shandong Province during the Qing Dynasty (1644-1911) are summarized firstly: 1) the rising trend of farmland area was striking; 2) farmland area had re-markable fluctuation; 3) farmland area per capita decreased dramatically; 4) wasteland reclamation index increased rapidly. Then, the driving forces of farmland area change are analyzed. It is concluded that natural and human factors are jointly influential. Among the driving forces, human dimensions are the main factors of farmland area change,which direct the general tendency of the changes mentioned above. And the natural factors influence the stability of farmland area as well. Variation of the natural factors would act as the major contributory factor to farmland area change during years or periods of abrupt climatic changes, or during the intensive occurring periods of natural hazards.Besides, the passive aspects of human factors, such as war chaos also influenced the fluctuation of the farmland area.This research indicates that it is feasible to study the land-use/cover change by Chinese historical literatures, which has huge potential to provide a comprehensive picture of the growing dominance of human land-use and land-cover pat-terns that can be used in many global change research oroiects.  相似文献   

SIMULATION AND PREDICTION OF DEBRIS FLOW USING ARTIFICIAL NEURAL NETWORK     

WANGXie-kang HUANGEr CUIPeng 《中国地理科学(英文版)》2003,13(3):262-266

Debris flow is one of the most destructive phenomena of natural hazards. Recently, major natural haz-ard, claiming human lives and assets, is due to debris flow in the world. Several practical methods for forecasting de-bris flow have been proposed, however, the accuracy of these methods is not high enough for practical use because of the stochastic and non-linear characteristics of debris flow. Artificial neural network has proven to be feasible and use-fill in developing models for nonlinear systems. On the other hand, predicting the future behavior based on a time se-ries of collected historical data is also an important tool in many scientific applications. In this study we present a three-layer feed-forward neural network model to forecast surge of debris flow according to the time series data collect-ed in the Jiangjia Ravine, situated in north part of Yunnan Province of China. The simulation and prediction of debris flow using the proposed approach shows this model is feasible, however, further studies are needed.  相似文献   

The D-CIXS X-ray mapping spectrometer on SMART-1     

M Grande  R BrowningN Waltham  D ParkerS.K Dunkin  B KentB Kellett  C.H PerryB Swinyard  A PerryJ Feraday  C HoweG McBride  K PhillipsJ Huovelin  P MuhliP.J Hakala  O VilhuJ Laukkanen  N ThomasD Hughes  H AlleyneM Grady  R LundinS Barabash  D BakerP.E Clark  C.D MurrayJ Guest  I CasanovaL.C d'Uston  S MauriceB Foing  D.J Heather  V FernandesK Muinonen  S.S RussellA Christou  C OwenP Charles  H KoskinenM Kato  K SipilaS Nenonen  M HolmstromN Bhandari  R ElphicD Lawrence 《Planetary and Space Science》2003,51(6):427-433

The D-CIXS Compact X-ray Spectrometer will provide high quality spectroscopic mapping of the Moon, the primary science target of the ESA SMART-1 mission. D-CIXS consists of a high throughput spectrometer, which will perform spatially localised X-ray fluorescence spectroscopy. It will also carry a solar monitor, to provide the direct calibration needed to produce a global map of absolute lunar elemental abundances, the first time this has been done. Thus it will achieve ground breaking science within a resource envelope far smaller than previously thought possible for this type of instrument, by exploiting two new technologies, swept charge devices and micro-structure collimators. The new technology does not require cold running, with its associated overheads to the spacecraft. At the same time it will demonstrate a radically novel approach to building a type of instrument essential for the BepiColombo mission and potential future planetary science targets.  相似文献   

Mezen Bay—a key area for understanding Weichselian glaciations in northern Russia     

Kurt H. Kjr  Igor N. Demidov  Eiliv Larsen  Andrew Murray  Jan K. Nielsen 《第四纪科学杂志》2003,18(1):73-93

Sediment successions in coastal cliffs around Mezen Bay, southeastern White Sea, record an unusually detailed history of former glaciations, interstadial marine and fluvial events from the Weichselian. A regional glaciation model for the Weichselian is based on new data from the Mezen Bay area and previously published data from adjacent areas. Following the Mikulinian (Eemian) interglacial a shelf‐centred glaciation in the Kara Sea is reflected in proglacial conditions at 100–90 ka. A local ice‐cap over the Timan ridge existed between 75 and 65 ka. Renewed glaciation in the Kara Sea spread southwestwards around 60 ka only, interrupted by a marine inundation, before it advanced to its maximum position at about 55–50 ka. After a prolonged ice‐free period, the Scandinavian ice‐sheet invaded the area from the west and terminated east of Mezen Bay about 17 ka. The previously published evidence of a large ice‐dammed lake in the central Arkhangelsk region, Lake Komi, finds no support in this study. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

X-ray spectral evolution of low-mass X-ray binary GX 349+2     

V. K. Agrawal  P. Sreekumar 《Monthly notices of the Royal Astronomical Society》2003,346(3):933-939

We present the results of a systematic investigation of spectral evolution in the Z source GX 349+2, using data obtained during 1998 with the Proportional Counter Array (PCA) on-board the RXTE satellite. The source traced a extended normal branch (NB) and flaring branch (FB) in the colour–colour diagram (CD) and the hardness-intensity diagram (HID) during these observations. The spectra at different positions of the Z-track were best fitted by a model consisting of a disc blackbody and a Comptonized spectrum. A broad (Gaussian) iron line at ∼6.7 keV is also required to improve the fit. The spectral parameters showed a systematic and significant variation with the position along the Z-track. The evolution in spectral parameters is discussed in view of the increasing mass accretion rate scenario, proposed to explain the motion of Z sources in the CD and the HID.  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   

Detailed comparison of the structures and kinematics of simulated and observed barred galaxies     

J. K. O'Neill  John Dubinski 《Monthly notices of the Royal Astronomical Society》2003,346(1):251-264

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Characters of the Pco2 and CO2 Flux in the East China Sea in Autumn     

纪雷  崔鹤  辛淑平  王军  张龙军  陆贤昆 《中国海洋湖沼学报》2003,21(2):180-186

Pco2 of air and seawater samples from the East China Sea(ECS) were measured in situ in autumn, 1994,Ocean currents,terrestrial fluviation,biological activities,etc.,Pco2 char-acters in air and seawater were investigated,CO2 flux and its character in the East China Sea are discussed on the basis of the Pco2 profiles of air and seawater,It was clear that the nearshore was the source of CO2;and tht the oulter sea area was the sink of CO2; and that the shelf area of the EXS is a net sink for atmospheric CO2 in autumn.  相似文献