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641.
Sphenoidal tensile microfractures with particular patterns have been discovered in the Late Devonian quartzose sandstone in islands and peninsula of the Taihu Lake area. Microfractures present a '人'-shape collective pattern, occurring in a single row or multiple rows. The quartz crystals in the inner acute angle area of some '人'-shape microfractures have severely been non-crystallized, showing the feature of diaplectic glass. The studies show that these special microfractures were formed during the unloading process after the compression was up to the peak of an impact event. 相似文献
642.
谢志东 《极地研究(英文版)》2009,20(2):172-186
Shock metamorphism resulting from hypervelocity collisions between planetary bodies, is a fundamental processes in the solar system. The term "shock metamorphism" is used to describe all changes in rocks and minerals resulting from the passage of shock waves. Most meteorites have experienced collisions and have a record of shock metamorphism, which includes brecciation, deformation, phase transformation, local melting and crystallization. The key to reading this record is to use the shock features to estimate the pressure and duration of shock event. In this paper, the history of the study of shock metamorphism is reviewed; basic knowledge of shock physics is discussed; recent 10 years' studies of shock-induced melt veins are summarized; and finally a short note to the shock metamorphism in general is given. 相似文献
643.
Forecasting space weather more accurately from solar observations requires an understanding of the variations in physical
properties of interplanetary (IP) shocks as solar activity changes. We examined the characteristics (occurrence rate, physical
parameters, and types of shock driver) of IP shocks. During the period of 1995 – 2001, a total of 249 forward IP shocks were
observed. In calculating the shock parameters, we used the solar wind data from Wind at the solar minimum period (1995 – 1997) and from ACE since 1998 including the solar maximum period (1999 – 2001). Most
of IP shocks (68%) are concentrated in the solar maximum period. The values of physical quantities of IP shocks, such as the
shock speed, the sonic Mach number, and the ratio of plasma density compression, are larger at solar maximum than at solar
minimum. However, the ratio of IMF compression is larger at solar minimum. The IP shock drivers are classified into four groups:
magnetic clouds (MCs), ejecta, high speed streams (HSSs), and unidentified drivers. The MC is the most dominant and strong
shock driver and 150 out of total 249 IP shocks are driven by MCs. The MC is a principal and very effective shock driver not
only at solar maximum but also at solar minimum, in contrast to results from previous studies, where the HSS is considered
as the dominant IP shock driver. 相似文献
644.
Simone Bianchi Raffaella Schneider 《Monthly notices of the Royal Astronomical Society》2007,378(3):973-982
The presence of dust at high redshift requires efficient condensation of grains in supernova (SN) ejecta, in accordance with current theoretical models. Yet observations of the few well-studied supernovae (SNe) and supernova remnants (SNRs) imply condensation efficiencies which are about two orders of magnitude smaller. Motivated by this tension, we have (i) revisited the model of Todini & Ferrara for dust formation in the ejecta of core collapse SNe, and (ii) followed, for the first time, the evolution of newly condensed grains from the time of formation to their survival – through the passage of the reverse shock – in the SNR. We find that 0.1–0.6 M⊙ of dust form in the ejecta of 12–40 M⊙ stellar progenitors. Depending on the density of the surrounding interstellar medium, between 2 and 20 per cent of the initial dust mass survives the passage of the reverse shock, on time-scales of about 4–8 × 104 yr from the stellar explosion. Sputtering by the hot gas induces a shift of the dust size distribution towards smaller grains. The resulting dust extinction curve shows a good agreement with that derived by observations of a reddened QSO at z = 6.2 . Stochastic heating of small grains leads to a wide distribution of dust temperatures. This supports the idea that large amounts (∼0.1 M⊙ ) of cold dust ( T ∼ 40 K) can be present in SNRs, without being in conflict with the observed infrared emission. 相似文献
645.
646.
Santabrata Das 《Monthly notices of the Royal Astronomical Society》2007,376(4):1659-1670
We investigate the behaviour of dissipative accreting matter close to a black hole, as this provides important observational features of galactic and extragalactic black hole candidates. We find a complete set of global solutions in the presence of viscosity and synchrotron cooling. We show that advective accretion flow can have a standing shock wave and the dynamics of the shock is controlled by the dissipation parameters (both viscosity and cooling). We study the effective region of the parameter space for standing as well as oscillating shock. We find that the shock front always moves towards the black hole as the dissipation parameters are increased. However, viscosity and cooling have opposite effects in deciding the solution topologies. We obtain two critical cooling parameters that separate the nature of the accretion solution. 相似文献
647.
Urban shrinkage and growth: Measurement and determinants of economic resilience in the Pearl River Delta 总被引:2,自引:0,他引:2
Journal of Geographical Sciences - In the aftermath of the global financial crisis of 2008, China witnessed gradual shrinkage of cities in the Pearl River Delta (PRD). In this study, we introduce... 相似文献
648.
摆型波现象是深部开采中块系岩体固有的动力学特征之一。为研究深部块系岩体摆型波传播规律,采用理论分析与室内试验相结合的方法,研究了在冲击载荷作用下摆型波的传播特性。该试验采用12块花岗岩和橡胶夹层材料建立了模型,采用TST5915动态数据采集系统进行信号监测,分别获得了在均质夹层和部分夹层介质增厚情况下的加速度响应曲线,并运用摆型波动力模型理论进行分析。结果表明,摆型波在块体间传播过程中,频率大小不受能量大小的影响,而能量大小决定了波的衰减时间。当块体间介质黏性增加时,加速度幅值均有所下降,但对加速度衰减周期没有影响,而影响加速度衰减的主要因素是冲击载荷的能量大小。此次试验研究将为今后进一步研究摆型波动力传播特性提供一定的基础。 相似文献
649.
S. V. Bogovalov D. V. Khangulyan A. V. Koldoba G. V. Ustyugova F. A. Aharonian 《Monthly notices of the Royal Astronomical Society》2008,387(1):63-72
In this paper, we present a detailed hydrodynamical study of the properties of the flow produced by the collision of a pulsar wind with the surrounding in a binary system. This work is the first attempt to simulate interaction of the ultrarelativistic flow (pulsar wind) with the non-relativistic stellar wind. Obtained results show that the wind collision could result in the formation of an 'unclosed' (at spatial scales comparable to the binary system size) pulsar wind termination shock even when the stellar wind ram pressure exceeds significantly the pulsar wind kinetic pressure. Moreover, the post-shock flow propagates in a rather narrow region, with very high bulk Lorentz factor (γ∼ 100). This flow acceleration is related to adiabatic losses which are purely hydrodynamical effects. Interestingly, in this particular case, no magnetic field is required for formation of the ultrarelativistic bulk outflow. The obtained results provide a new interpretation for the orbital variability of radio, X-ray and gamma-ray signals detected from binary pulsar system PSR B1259−63/SS2883. 相似文献
650.
An intermediate shock-like event was observed by Voyager 2 on 9 January 1979. The discontinuity structure is well identified to be a 2→3 type intermediate shock by fitting the Rankine – Hugoniot
relations. The shock satisfies the following conditions: i) the plasma density increases from the upstream region to the downstream region, ii) The normal Alfvén Mach number (M
AN) is greater than unity in the preshock state and less than unity in the postshock state, iii) The fast-mode Mach numbers in the upstream and downstream regions are less than unity and both the slow-mode Mach numbers
are greater than unity, iv) The tangential component of the magnetic field changes sign across the shock front. 相似文献