首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   965篇
  免费   37篇
  国内免费   23篇
测绘学   38篇
大气科学   49篇
地球物理   187篇
地质学   612篇
海洋学   32篇
天文学   64篇
综合类   6篇
自然地理   37篇
  2024年   2篇
  2023年   5篇
  2022年   34篇
  2021年   37篇
  2020年   34篇
  2019年   27篇
  2018年   79篇
  2017年   86篇
  2016年   63篇
  2015年   44篇
  2014年   67篇
  2013年   83篇
  2012年   41篇
  2011年   52篇
  2010年   38篇
  2009年   41篇
  2008年   33篇
  2007年   21篇
  2006年   24篇
  2005年   24篇
  2004年   18篇
  2003年   16篇
  2002年   16篇
  2001年   9篇
  2000年   11篇
  1999年   11篇
  1998年   3篇
  1997年   4篇
  1996年   5篇
  1995年   6篇
  1994年   7篇
  1993年   5篇
  1992年   6篇
  1991年   3篇
  1990年   5篇
  1989年   4篇
  1988年   3篇
  1987年   2篇
  1986年   7篇
  1985年   8篇
  1984年   5篇
  1983年   3篇
  1982年   8篇
  1981年   3篇
  1978年   4篇
  1976年   3篇
  1975年   2篇
  1973年   3篇
  1970年   2篇
  1969年   2篇
排序方式: 共有1025条查询结果,搜索用时 718 毫秒
121.
Abstract

Further ratification of the Kyoto Protocol by non-Annex 1 countries such as the State of Qatar will not affect the entrance into force of the Treaty; however, ratification remains an important decision due to other considerations, primarily the economic costs and benefits associated with ratification. As a member of the Organization of Petroleum Exporting Countries (OPEC), Qatar's economic position is closely allied with revenue generated from its oil and natural gas resources. Qatar expects adverse impacts from implementing the Kyoto Protocol, though the estimated magnitude varies enormously with different models. Also, the impacts depend significantly on how the implementation is done; for example, the kind of policies that other countries use. Qatar is able to counter adverse impacts by exploiting its greater share of natural gas and developing energy-intensive industries that produce nonenergy goods such as steel, petrochemicals, and chemicals. Furthermore, it is clear that clauses in the Protocol can be developed to protect OPEC interests, and if Qatar does not ratify the Protocol, it will be excluded from the subsequent rule-making processes. On balance, there are benefits to ratifying the Treaty, and there is also a strong need for extensive further research.  相似文献   
122.
Taking into account of ion temperature effect, existence conditions of arbitrary amplitude solitary Kinetic Alfvén Waves (KAWs) in a plasma with q-nonextensive electrons are investigated by the conventional Sagdeev pseudo potential method. It is found that only solitons with density hump can exist, the amplitude of which depends sensitively on the parameter q, ion temperature ( \(\sigma= \frac{T_{i}}{T_{e}}\) ) and plasma β. There is an upper limit of solitary wave amplitude which decreases with increase of q, σ and β. The amplitude of solitary KAWs is found to increase with increase in ion temperature. The results obtained in the framework of Maxwellian distribution are reproduced when q→1.  相似文献   
123.
Two petrographic settings of carbonaceous components, mainly filling open fractures and occasionally enclosed in shock‐melt veins, were found in the recently fallen Tissint Martian meteorite. The presence in shock‐melt veins and the deuterium enrichments (δD up to +1183‰) of these components clearly indicate a pristine Martian origin. The carbonaceous components are kerogen‐like, based on micro‐Raman spectra and multielemental ratios, and were probably deposited from fluids in shock‐induced fractures in the parent rock of Tissint. After precipitation of the organic matter, the rock experienced another severe shock event, producing the melt veins that encapsulated a part of the organic matter. The C isotopic compositions of the organic matter (δ13C = ?12.8 to ?33.1‰) are significantly lighter than Martian atmospheric CO2 and carbonate, providing a tantalizing hint for a possible biotic process. Alternatively, the organic matter could be derived from carbonaceous chondrites, as insoluble organic matter from the latter has similar chemical and isotopic compositions. The presence of organic‐rich fluids that infiltrated rocks near the surface of Mars has significant implications for the study of Martian paleoenvironment and perhaps to search for possible ancient biological activities on Mars.  相似文献   
124.
This paper reports on the occurrence of layered Pan African dioritetonalite-granodiorite (DTG) rocks. The layering is marked by alternation of melanocratic (M) layers (diorites and tonalites) and leucocratic (L) layers (tonalites and granodiorites). M-samples have cumulus biotite+hornblende+relict pyroxene+plagioclase+K-feldspars+magnetite+apatite, and have transitional calc-alkaline and metaluminous affinities. They were derived from subduction-related magma enriched in Rb, Ba, K and LREE and depleted in Sr and Nb. L-samples have cumulus plagioclase+hornblende. They are enriched in Sr and depleted in Rb, Ba, K, Nb and LREE. They have calc-alkaline and peraluminous affinites.
The formation of the rhythmic layers of DTG composition can be attributed to periodical replenishment of pulses of basic magma into a more evolved acidic magma chamber under open system conditions. Field relations, mineralogy and element concentration among the M- and L-layers indicate that at the subduction zone, the ascending magma was contaminated with lower crustal materials (marginal basin metasediments) which led to LILE-enrichment, Nb-depletion and transition from calc-alkaline to alkaline and from metaluminous to peraluminous affinities as well.  相似文献   
125.
Employing the Haselgrove ray tracing equations and a diffusive equilibrium model of the ionosphere, the propagation characteristics of hook whistlers recorded at low-latitude ground station Varanasi (geomag. lat., 16°6′.N) are discussed. It is shown that the two traces of the hook whistlers are caused by the VLF waves radiated from the return stroke of a lightning discharge which after penetrating the ionosphere at two different entry points, propagated to the opposite hemisphere in the whistler mode and were received at 16 geomagnetic latitude. Further the crossing of ray paths for the same frequency leads to the explanation of the hook whistler. The lower and higher cut-off frequencies are explained in terms of their deviating away from the bunch of the recorded whistler waves and crossing of ray paths for the same frequency.  相似文献   
126.
The magnetohydrodynamic stability of a streaming liquid cylinder subject to surface tension and pervaded by a magnetic vacuum field has been elaborated for all axisymmetric and non-axisymmetric disturbances. The dispersion relation is obtained and studied analytically and numerically. the streaming has always a destabilizing effect. The axial magnetic fields inside and outside the jet have always stabilizing effects for all perturbations. The transverse magnetic field has a destabilizing effect. However, if the axial field intensity is so high and paramount over that the transverse field, the destabilizing character of the model is suppressed. The latter is satisfied if the Alfvén wave velocity is greater than the equilibrium liquid velocity.  相似文献   
127.
Lie transforms and the Hamiltonization of non-Hamiltonian systems   总被引:1,自引:0,他引:1  
To develop the perturbation solution of the non-Hamiltonian system of differential equationsy=g(y, t; ), it is sufficient to obtain the perturbation solution of a Hamiltonian system represented by the HamiltonianK=Y·g(y, t; ) which is linear in the adjoint vectorY. This Hamiltonization allows the direct use of the perturbation methods already established for Hamiltonian systems. To demonstrate this fact, a Hamiltonian algorithm developed by this author and based on the Lie-Deprit transform is applied to the Hamiltonized system and is shown to be equivalent to the application of the non-Hamiltonian form of this same algorithm to the original non-Hamiltonian system.  相似文献   
128.
129.
Optimal and sustainable extraction of groundwater in coastal aquifers   总被引:1,自引:0,他引:1  
Four examples are investigated for the optimal and sustainable extraction of groundwater from a coastal aquifer under the threat of seawater intrusion. The objectives and constraints of these management scenarios include maximizing the total volume of water pumped, maximizing the profit of selling water, minimizing the operational and water treatment costs, minimizing the salt concentration of the pumped water, and controlling the drawdown limits. The physical model is based on the density-dependent advective-dispersive solute transport model. Genetic algorithm is used as the optimization tool. The models are tested on a hypothetical confined aquifer with four pumping wells located at various depths. These solutions establish the feasibility of simulating various management scenarios under complex three-dimensional flow and transport processes in coastal aquifers for the optimal and sustainable use of groundwater.  相似文献   
130.
PhotoelectricB andV observations of the W UMa-type system OO Aql have been obtained for 11 nights during the period from 22 June–30 July, 1987. A total of 978 observations were obtained in eachB andV filter. All these observations were transformed to theB andV colours of theUBV standard system. Five light curves for primary and secondary eclipses were obtained, its times of minima were determined and a new linear ephemeris was given. The period changes of the system were discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号