全文获取类型
| 收费全文 | 38504篇 |
| 免费 | 6424篇 |
| 国内免费 | 11167篇 |
专业分类
| 测绘学 | 1145篇 |
| 大气科学 | 1555篇 |
| 地球物理 | 5141篇 |
| 地质学 | 31328篇 |
| 海洋学 | 3398篇 |
| 天文学 | 7706篇 |
| 综合类 | 2246篇 |
| 自然地理 | 3576篇 |
出版年
| 2024年 | 146篇 |
| 2023年 | 518篇 |
| 2022年 | 1109篇 |
| 2021年 | 1248篇 |
| 2020年 | 1231篇 |
| 2019年 | 1606篇 |
| 2018年 | 1270篇 |
| 2017年 | 1430篇 |
| 2016年 | 1643篇 |
| 2015年 | 1660篇 |
| 2014年 | 2423篇 |
| 2013年 | 2180篇 |
| 2012年 | 2626篇 |
| 2011年 | 2694篇 |
| 2010年 | 2498篇 |
| 2009年 | 2957篇 |
| 2008年 | 2743篇 |
| 2007年 | 2866篇 |
| 2006年 | 2729篇 |
| 2005年 | 2440篇 |
| 2004年 | 2237篇 |
| 2003年 | 2101篇 |
| 2002年 | 1752篇 |
| 2001年 | 1735篇 |
| 2000年 | 1551篇 |
| 1999年 | 1485篇 |
| 1998年 | 1363篇 |
| 1997年 | 1005篇 |
| 1996年 | 909篇 |
| 1995年 | 763篇 |
| 1994年 | 670篇 |
| 1993年 | 616篇 |
| 1992年 | 484篇 |
| 1991年 | 352篇 |
| 1990年 | 260篇 |
| 1989年 | 205篇 |
| 1988年 | 150篇 |
| 1987年 | 115篇 |
| 1986年 | 94篇 |
| 1985年 | 57篇 |
| 1984年 | 27篇 |
| 1983年 | 21篇 |
| 1982年 | 18篇 |
| 1981年 | 13篇 |
| 1980年 | 17篇 |
| 1979年 | 15篇 |
| 1978年 | 12篇 |
| 1977年 | 18篇 |
| 1976年 | 5篇 |
| 1954年 | 14篇 |
排序方式： 共有10000条查询结果，搜索用时 15 毫秒
31.
32.
We study the self-consistent, linear response of a galactic disc to vertical perturbations, as induced, say, by a tidal interaction. We calculate the self-gravitational potential corresponding to a non-axisymmetric, self-consistent density response of the disc using the Green's function method. The response potential is shown to oppose the perturbation potential because the self-gravity of the disc resists the imposed potential, and this resistance is stronger in the inner parts of a galactic disc. For the m = 1 azimuthal wavenumber, the disc response opposes the imposed perturbation up to a radius that spans a range of 4–6 disc scalelengths, so that the disc shows a net warp only beyond this region. This physically explains the well known but so far unexplained observation that warps typically set in beyond this range of radii. We show that the inclusion of a dark matter halo in the calculation only marginally changes (by ∼10 per cent) the radius for the onset of warps. For perturbations with higher azimuthal wavenumbers, the net signature of the vertical perturbations can only be seen at larger radii – for example, beyond 7 exponential disc scalelengths for m = 10 . Also, for the high- m cases, the magnitude of the negative disc response due to the disc self-gravity is much smaller. This is shown to result in corrugations of the mid-plane density, which explains the puzzling scalloping with m = 10 detected in H i in the outermost regions ∼30 kpc in the Galaxy. 相似文献
33.
34.
We present results from a new simulation code that accounts for the evolution of the reservoirs of carbon dioxide on Mars, from its early years to the present. We establish a baseline model parameter set that produces results compatible with the present (i.e., Patm?6.5 mbar with permanent CO2 ice cap) for a wide range of initial inventories. We find that the initial inventory of CO2 broadly determines the evolutionary course of the reservoirs of CO2. The reservoirs include the atmosphere, ice cap, adsorbed CO2 in the regolith, and carbonate rocks. We track the evolution of the free inventory: the atmosphere, ice cap and regolith. Simulations begin at 4.53 Gyr before present with a rapid loss of free inventory to space in the early Noachian. Models that assume a relatively small initial inventory (?5 bar) have pronounced minima in the free inventory of CO2 toward the end of the Noachian. Under baseline parameters, initial inventories below ∼4.5 bar result in a catastrophic loss of the free inventory to space. The current free inventory would be then determined by the balance between outgassing, sputtering losses and chemical weathering following the end of the late bombardment. We call these “thin” models. They generically predict small current free inventories in line with expectations of a small present CO2 ice cap. For “thick” models, with initial inventories ?5 bar, a surplus of 300-700 mbar of free CO2 remains during the late-Noachian. The histories of free inventory in time for thick models tend to converge within the last 3.5 Gyr toward a present with an ice cap plus atmospheric inventory of about 100 mbar. For thick models, the convergence is largely due to the effects of chemical weathering, which draws down higher free inventories more rapidly than the low. Thus, thick models have ?450 mbar carbonate reservoirs, while thin models have ?200 mbar. Though both thick and thin scenarios can reproduce the current atmospheric pressure, the thick models imply a relatively large current CO2 ice cap and thin models, little or none. While the sublimation of a massive cap at a high obliquity would create a climate swing of greenhouse warming for thick models, under the thin model, mean temperatures and pressures would be essentially unaffected by increases in obliquity. 相似文献
35.
Zhan-Le Du Hua-Ning Wang Xiang-Tao He National Astronomical Observatories Chinese Academy of Sciences Beijing Department of Astronomy Beijing Normal University Beijing 《中国天文和天体物理学报》2006,6(4):489-494
The maximum amplitudes of solar activity cycles are found to be well anti-correlated (r = -0.72) with the newly defined solar cycle lengths three cycles before (at lag -3) in 13-month running mean sunspot numbers during the past 190 years. This result could be used for predicting the maximum sunspot numbers. The amplitudes of Cycles 24 and 25 are estimated to be 149.5±27.6 and 144.3±27.6, respectively. 相似文献
36.
37.
38.
39.
40.