全文获取类型
收费全文 | 4187篇 |
免费 | 439篇 |
国内免费 | 684篇 |
专业分类
测绘学 | 59篇 |
大气科学 | 359篇 |
地球物理 | 1266篇 |
地质学 | 663篇 |
海洋学 | 1398篇 |
天文学 | 707篇 |
综合类 | 123篇 |
自然地理 | 735篇 |
出版年
2024年 | 16篇 |
2023年 | 50篇 |
2022年 | 88篇 |
2021年 | 77篇 |
2020年 | 102篇 |
2019年 | 146篇 |
2018年 | 103篇 |
2017年 | 111篇 |
2016年 | 110篇 |
2015年 | 125篇 |
2014年 | 149篇 |
2013年 | 245篇 |
2012年 | 111篇 |
2011年 | 152篇 |
2010年 | 150篇 |
2009年 | 257篇 |
2008年 | 295篇 |
2007年 | 357篇 |
2006年 | 280篇 |
2005年 | 214篇 |
2004年 | 215篇 |
2003年 | 234篇 |
2002年 | 205篇 |
2001年 | 152篇 |
2000年 | 171篇 |
1999年 | 186篇 |
1998年 | 169篇 |
1997年 | 88篇 |
1996年 | 114篇 |
1995年 | 88篇 |
1994年 | 94篇 |
1993年 | 74篇 |
1992年 | 74篇 |
1991年 | 57篇 |
1990年 | 50篇 |
1989年 | 44篇 |
1988年 | 30篇 |
1987年 | 27篇 |
1986年 | 19篇 |
1985年 | 17篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 19篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1954年 | 4篇 |
排序方式: 共有5310条查询结果,搜索用时 15 毫秒
11.
12.
Sensitivity kernels for finite-frequency surface waves 总被引:1,自引:1,他引:1
13.
The characteristics of the response of equatorial Pacific upper ocean current to westerly wind bursts(WWB)were analyzed in the frequency domain by using wind and ADCP data collected by the Shiyan3 during TOGA-COARE IOP,1992-1993.The preliminary results showed that the response consistedof an eastward surface jet at shallower than 60m depth,a westward counter current centering near100m and a shear layer between them,with the variations of all three being nonlinear and nearlysynchronous.The oceanic responses in the frequency domain were characterized by occurrences of a remotely forced mixed Rossby-gravity wave with period of 8-10 days in the surface jet andcountercurrent at shallower than 110 m depth,and two locally forced waves with periods of 24 daysand 4-5 days limited in shallower than 70m depth.These fluctuations of the responses depended much more on zonal wind than meridional wind.The results also revealed that the oceanic response toWWB resulted from momentum transport and energy propagation assoc 相似文献
14.
15.
Soumen Mondal Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2006,371(3):1418-1426
Standing, propagating or oscillating shock waves are common in accretion and winds around compact objects. We study the topology of all possible solutions using the pseudo-Kerr geometry. We present the parameter space spanned by the specific energy and angular momentum and compare it with that obtained from the full general relativity to show that the potential can work satisfactorily in fluid dynamics also, provided the polytropic index is suitably modified. We then divide the parameter space depending on the nature of the solution topology. We specifically study the nature of the standing Rankine–Hugoniot shocks. We also show that as the Kerr parameter is increased, the shock location generally moves closer to the black hole. In future, these solutions can be used as guidelines to test numerical simulations around compact objects. 相似文献
16.
Santabrata Das Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2008,389(1):371-378
We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy , the specific angular momentum (λ) and the accretion rate of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes. 相似文献
17.
18.
In an attempt to elucidate the mechanics of deep-water wave breaking, a variety of breaking waves, including spilling and plunging waves, of different length scales and geometries was studied. The waves were generated through wave-wave interactions using wave packets with constant-steepness components, constant-amplitude components, and also components following the Pierson-Moskowitz distribution. Wave steepening prior to breaking were found to cause an increase in the high frequency spectral slope of the wave spectrum. The slopes were correlated to the type of breaking and the intensity of the breaking. The energy loss through breaking varied with the spectral characteristics of the wave packet. On the other hand, it was also noted that, irrespective of the wave packet, the losses were from the higher frequency end of the first harmonics. 相似文献
19.
M. D. Sharma 《Geophysical Journal International》2008,174(3):971-977
Out of the four waves in an anisotropic poroelastic medium, two are termed as quasi-transverse waves. The prefix 'quasi' refers to their polarizations being nearly, but not exactly, perpendicular to direction of propagation. In this composite medium, unlike perfectly elastic medium, the propagation of a longitudinal wave along a phase direction may not be accompanied by transverse waves. The existence of a transverse wave in anisotropic poroelastic media is ensured by the two equations restricting the choice of elastic coefficients of porous aggregate as well as fluid–solid coupling. Necessary and sufficient conditions for the existence of transverse waves along the coordinate axes and in the coordinate planes for general anisotropy are discussed. The discussion is extended to the case of orthotropic materials and existence for few specific phase directions is also explored. The conditions for the transverse waves decided on the basis of their apparent polarizations, that is, particle motion being perpendicular to ray direction, are also discussed. For a particular numerical model, the existence of these apparent transverse waves is solved numerically for phase directions in coordinate planes. For general directions of phase propagation, the existence of these transverse waves is checked graphically for the chosen numerical model. 相似文献
20.
M. E. Dieckmann B. Eliasson M. Parviainen P. K. Shukla A. Ynnerman 《Monthly notices of the Royal Astronomical Society》2006,367(3):865-872
Initially, inhomogeneous plasma jets, ejected by active galactic nuclei and associated with gamma-ray bursts, are thermalized by the formation of internal shocks. Jet subpopulations can hereby collide at Lorentz factors of a few. As the resulting relativistic shock expands into the upstream plasma, a significant fraction of the upstream ions is reflected. These ions, together with downstream ions that leak through the shock, form relativistic beams of ions that outrun the shock. The thermalization of these beams via the two-stream instability is thought to contribute significantly to plasma heating and particle acceleration by the shock. Here, the capability of a two-stream instability to generate relativistic field-aligned and cross-field electron flow, is examined for a magnetized plasma by means of a particle-in-cell (PIC) simulation. The electrons interact with the developing quasi-electrostatic waves and oblique magnetic fields. The simulation results bring forward evidence that such waves, by their non-linear interactions with the plasma, produce a highly relativistic field-aligned electron flow and electron energies, which could contribute to the radio synchrotron emissions from astrophysical jets, to ultrarelativistic leptonic subpopulations propagating with the jet and to the halo particles surrounding the accretion disc of the black hole. 相似文献