全文获取类型
收费全文 | 66061篇 |
免费 | 11503篇 |
国内免费 | 13478篇 |
专业分类
测绘学 | 5135篇 |
大气科学 | 6788篇 |
地球物理 | 19751篇 |
地质学 | 34373篇 |
海洋学 | 7793篇 |
天文学 | 2302篇 |
综合类 | 4703篇 |
自然地理 | 10197篇 |
出版年
2024年 | 327篇 |
2023年 | 942篇 |
2022年 | 1908篇 |
2021年 | 2296篇 |
2020年 | 2343篇 |
2019年 | 2630篇 |
2018年 | 2225篇 |
2017年 | 2458篇 |
2016年 | 2539篇 |
2015年 | 2864篇 |
2014年 | 3528篇 |
2013年 | 3405篇 |
2012年 | 3855篇 |
2011年 | 4057篇 |
2010年 | 3526篇 |
2009年 | 4328篇 |
2008年 | 4007篇 |
2007年 | 4476篇 |
2006年 | 4368篇 |
2005年 | 3894篇 |
2004年 | 3678篇 |
2003年 | 3493篇 |
2002年 | 3096篇 |
2001年 | 2717篇 |
2000年 | 2445篇 |
1999年 | 2268篇 |
1998年 | 2001篇 |
1997年 | 1829篇 |
1996年 | 1702篇 |
1995年 | 1445篇 |
1994年 | 1422篇 |
1993年 | 1184篇 |
1992年 | 960篇 |
1991年 | 678篇 |
1990年 | 555篇 |
1989年 | 494篇 |
1988年 | 340篇 |
1987年 | 223篇 |
1986年 | 143篇 |
1985年 | 94篇 |
1984年 | 39篇 |
1983年 | 32篇 |
1982年 | 32篇 |
1981年 | 22篇 |
1980年 | 26篇 |
1979年 | 42篇 |
1978年 | 39篇 |
1977年 | 26篇 |
1975年 | 4篇 |
1954年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
C. Gontikakis C. Efthymiopoulos A. Anastasiadis 《Monthly notices of the Royal Astronomical Society》2006,368(1):293-304
We consider the possibility of particles being injected at the interior of a reconnecting current sheet (RCS), and study their orbits by dynamical systems methods. As an example we consider orbits in a 3D Harris type RCS. We find that, despite the presence of a strong electric field, a 'mirror' trapping effect persists, to a certain extent, for orbits with appropriate initial conditions within the sheet. The mirror effect is stronger for electrons than for protons. In summary, three types of orbits are distinguished: (i) chaotic orbits leading to escape by stochastic acceleration, (ii) regular orbits leading to escape along the field lines of the reconnecting magnetic component, and (iii) mirror-type regular orbits that are trapped in the sheet, making mirror oscillations. Dynamically, the latter orbits lie on a set of invariant KAM tori that occupy a considerable amount of the phase space of the motion of the particles. We also observe the phenomenon of 'stickiness', namely chaotic orbits that remain trapped in the sheet for a considerable time. A trapping domain, related to the boundary of mirror motions in velocity space, is calculated analytically. Analytical formulae are derived for the kinetic energy gain in regular or chaotic escaping orbits. The analytical results are compared with numerical simulations. 相似文献
993.
994.
宋国玄 《中国天文和天体物理学报》1989,(1)
巨分子云的碰撞造成了大质量恒星在碰撞分子云中的形成,这些大质量恒星的形成产生了膨胀的HII区域,从而使巨分于云碎裂成小质量的分子云。这是本文提出的巨分子云碎裂机制。因此巨分子云的寿命也主要由区分子云间的碰撞几率所决定。我们的分析表明,巨分子云的寿命有赖于巨分子云所在的旋涡星系中的不同位置。寿命的最大可能存在区间为8.18×10~7yr与2.45×10~8yr。利用我们提出的机制可以在分子云研究的数值计算与数值模拟中得到应用。 相似文献
995.
Assuming that an unknown mechanism (e.g., gas turbulence) removes most of the subnebula gas disk in a timescale shorter than that for satellite formation, we develop a model for the formation of regular (and possibly at least some of the irregular) satellites around giant planets in a gas-poor environment. In this model, which follows along the lines of the work of Safronov et al. [1986. Satellites. Univ. of Arizona Press, Tucson, pp. 89-116], heliocentric planetesimals collide within the planet's Hill sphere and generate a circumplanetary disk of prograde and retrograde satellitesimals extending as far out as ∼RH/2. At first, the net angular momentum of this proto-satellite swarm is small, and collisions among satellitesimals leads to loss of mass from the outer disk, and delivers mass to the inner disk (where regular satellites form) in a timescale ?105 years. This mass loss may be offset by continued collisional capture of sufficiently small <1 km interlopers resulting from the disruption of planetesimals in the feeding zone of the giant planet. As the planet's feeding zone is cleared in a timescale ?105 years, enough angular momentum may be delivered to the proto-satellite swarm to account for the angular momentum of the regular satellites of Jupiter and Saturn. This feeding timescale is also roughly consistent with the independent constraint that the Galilean satellites formed in a timescale of 105-106 years, which may be long enough to accommodate Callisto's partially differentiated state [Anderson et al., 1998. Science 280, 1573; Anderson et al., 2001. Icarus 153, 157-161]. In turn, this formation timescale can be used to provide plausible constraints on the surface density of solids in the satellitesimal disk (excluding satellite embryos for satellitesimals of size ∼1 km), which yields a total disk mass smaller than the mass of the regular satellites, and means that the satellites must form in several ∼10 collisional cycles. However, much more work will need to be conducted concerning the collisional evolution both of the circumplanetary satellitesimals and of the heliocentric planetesimals following giant planet formation before one can assess the significance of this agreement. Furthermore, for enough mass to be delivered to form the regular satellites in the required timescale one may need to rely on (unproven) mechanisms to replenish the feeding zone of the giant planet. We compare this model to the solids-enhanced minimum mass (SEMM) model of Mosqueira and Estrada [2003a. Icarus 163, 198-231; 2003b. Icarus 163, 232-255], and discuss its main consequences for Cassini observations of the saturnian satellite system. 相似文献
996.
ICE, the electric field experiment on DEMETER 总被引:9,自引:0,他引:9
997.
Qualitative and quantitative composition of meiobenthos was studied in Amursky Bay (Peter the Great Bay, the East Sea). Ten
taxonomic groups were found, where nematodes were dominant. Density of meiobenthos in ground sediments of the Bay were not
uniform, and the average density was measured at 126.4±62.3 ind. m-2. 56 species of nematodes were detected, and dominant species wereSabatieria palmaris, Paracanthonchus macrodon, Sphaerolaimus limosus, S. gracilis andOncholaimium ramosum. Five taxocenes of nematodes were allocated based on the results of cluster analysis and species domination according to density.
Low diversity in species composition of nematodes was noted in the northwestern part of the Bay, which is a zone of desalination,
and in the eastern part, which is exposed to household drains. 相似文献
998.
Numerical prediction of performance of submerged breakwaters 总被引:1,自引:0,他引:1
The results of a numerical model study on the transmission characteristics of a submerged breakwater are presented. Study aimed to determine the effect of depth of submergence, crest width, initial wave conditions and material properties on the transmission characteristics of the submerged breakwater. The results highlight the optimum crest width of the breakwater and optimum clear spacing between two breakwaters. A submerged permeable breakwater with ds/d=0.5, p=0.3 and f=1.0, reduces the transmission coefficient by about 10% than the impermeable breakwater. The results indicates an optimum width ratio of B/d=0.75 for achieving minimum transmission. By restricting the effective width ratio of the series of breakwaters to 0.75, studies were conducted to determine the effect of clear spacing between breakwaters on transmission coefficient, suggesting an optimum clear spacing of w/b=2.00 to obtain Kt below 0.6. 相似文献
999.
1000.