全文获取类型
收费全文 | 54168篇 |
免费 | 8656篇 |
国内免费 | 12030篇 |
专业分类
测绘学 | 5247篇 |
大气科学 | 7412篇 |
地球物理 | 10423篇 |
地质学 | 25134篇 |
海洋学 | 7918篇 |
天文学 | 6990篇 |
综合类 | 3646篇 |
自然地理 | 8084篇 |
出版年
2024年 | 227篇 |
2023年 | 615篇 |
2022年 | 1671篇 |
2021年 | 1910篇 |
2020年 | 1994篇 |
2019年 | 2539篇 |
2018年 | 1806篇 |
2017年 | 2129篇 |
2016年 | 2143篇 |
2015年 | 2462篇 |
2014年 | 2906篇 |
2013年 | 3146篇 |
2012年 | 3052篇 |
2011年 | 3284篇 |
2010年 | 2810篇 |
2009年 | 3680篇 |
2008年 | 3709篇 |
2007年 | 4069篇 |
2006年 | 3972篇 |
2005年 | 3537篇 |
2004年 | 3112篇 |
2003年 | 2901篇 |
2002年 | 2503篇 |
2001年 | 2093篇 |
2000年 | 2133篇 |
1999年 | 1937篇 |
1998年 | 1691篇 |
1997年 | 1192篇 |
1996年 | 1050篇 |
1995年 | 869篇 |
1994年 | 808篇 |
1993年 | 677篇 |
1992年 | 517篇 |
1991年 | 408篇 |
1990年 | 299篇 |
1989年 | 267篇 |
1988年 | 192篇 |
1987年 | 134篇 |
1986年 | 94篇 |
1985年 | 68篇 |
1984年 | 44篇 |
1983年 | 26篇 |
1982年 | 29篇 |
1981年 | 20篇 |
1980年 | 32篇 |
1979年 | 11篇 |
1978年 | 23篇 |
1977年 | 12篇 |
1976年 | 4篇 |
1954年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
We examine possible locations for the primordial disk of the Edgeworth-Kuiper Belt (EKB), using several subsets of the known objects as markers of the total mass distribution. Using a secular perturbation theory, we find that the primordial plane of the EKB could have remained thin enough to escape detection only if it is clustered very closely about the invariable plane of the Solar System. 相似文献
92.
93.
李毅心 《成都信息工程学院学报》2006,21(2):206-209
积件技术是教学软件发展的方向,探讨了用Power Builder前台结合SQL Server关系数据库后台进行多媒体教学积件管理的应用模式,并结合具体教学示例,进行了分析说明。 相似文献
94.
95.
We consider the effects of projection, internal absorption, and gas-or stellar-velocity dispersion on the measured rotation curves of galaxies with edge-on disks. Axisymmetric disk models clearly show that the rotational velocity in the inner galaxy is highly underestimated. As a result, an extended portion that imitates nearly rigid rotation appears. At galactocentric distances where the absorption is low (i.e., it does not exceed 0.3–0.5m kpc?1), the line profiles can have two peaks, and a rotation curve with minimum distortions can be obtained by estimating the position of the peak that corresponds to a higher rotational velocity. However, the high-velocity peak disappears in high-absorption regions and the actual shape of the rotation curve cannot be reproduced from line-of-sight velocity estimates. In general, the optical rotation curves for edge-on galaxies are of little use in reconstructing the mass distribution in the inner regions, particularly for galaxies with a steep velocity gradient in the central region. In this case, estimating the rotation velocities for outer (transparent) disk regions yields correct results. 相似文献
96.
97.
Piero F. Spinnato Michael Fellhauer Simon F. Portegies Zwart 《Monthly notices of the Royal Astronomical Society》2003,344(1):22-32
We study the efficiency at which a black hole or dense star cluster spirals in to the Galactic Centre. This process takes place on a dynamical friction time-scale, which depends on the value of the Coulomb logarithm (ln Λ). We determine the accurate value of this parameter using the direct N -body method, a tree algorithm and a particle-mesh technique with up to two million plus one particles. The three different techniques are in excellent agreement. Our measurement for the Coulomb logarithm appears to be independent of the number of particles. We conclude that ln Λ= 6.6 ± 0.6 for a massive point particle in the inner few parsec of the Galactic bulge. For an extended object, such as a dense star cluster, ln Λ is smaller, with a value of the logarithm argument Λ inversely proportional to the object size. 相似文献
98.
99.
Groundbased radio observations indicate that Jupiter's ammonia is globally depleted from 0.6 bars to at least 4-6 bars relative to the deep abundance of ∼3 times solar, a fact that has so far defied explanation. The observations also indicate that (i) the depletion is greater in belts than zones, and (ii) the greatest depletion occurs within Jupiter's local 5-μm hot spots, which have recently been detected at radio wavelengths. Here, we first show that both the global depletion and its belt-zone variation can be explained by a simple model for the interaction of moist convection with Jupiter's cloud-layer circulation. If the global depletion is dynamical in origin, then important endmember models for the belt-zone circulation can be ruled out. Next, we show that the radio observations of Jupiter's 5-μm hot spots imply that the equatorial wave inferred to cause hot spots induces vertical parcel oscillation of a factor of ∼2 in pressure near the 2-bar level, which places important constraints on hot-spot dynamics. Finally, using spatially resolved radio maps, we demonstrate that low-latitude features exceeding ∼4000 km diameter, such as the equatorial plumes and large vortices, are also depleted in ammonia from 0.6 bars to at least 2 bars relative to the deep abundance of 3 times solar. If any low-latitude features exist that contain 3-times-solar ammonia up to the 0.6-bar ammonia condensation level, they must have diameters less than ∼4000 km. 相似文献
100.
A short summary of recent progress in measuring and understanding turbulence during magnetic reconnection in laboratory plasmas
is given. Magnetic reconnection is considered as a primary process to dissipate magnetic energy in laboratory and astrophysical
plasmas. A central question concerns why the observed reconnection rates are much faster than predictions made by classical
theories, such as the Sweet–Parker model based on MHD with classical Spitzer resistivity. Often, the local resistivity is
conjectured to be enhanced by turbulence to accelerate reconnection rates either in the context of the Sweet–Parker model
or by facilitating setup of the Pestchek model. Measurements at a dedicated laboratory experiment, called MRX or Magnetic
Reconnection Experiment, have indicated existence of strong electromagnetic turbulence in current sheets undergoing fast reconnection.
The origin of the turbulence has been identified as right-hand polarized whistler waves, propagating obliquely to the reconnecting
field, with a phase velocity comparable to the relative drift velocity. These waves are consistent with an obliquely propagating
electromagnetic lower-hybrid drift instability driven by drift speeds large compared to the Alfven speed in high-beta plasmas.
Interestingly, this instability may explain electromagnetic turbulence also observed in collisionless shocks, which are common
in energetic astrophysical phenomena. 相似文献