全文获取类型
收费全文 | 3376篇 |
免费 | 285篇 |
国内免费 | 277篇 |
专业分类
测绘学 | 700篇 |
大气科学 | 133篇 |
地球物理 | 354篇 |
地质学 | 804篇 |
海洋学 | 131篇 |
天文学 | 200篇 |
综合类 | 264篇 |
自然地理 | 1352篇 |
出版年
2024年 | 28篇 |
2023年 | 55篇 |
2022年 | 184篇 |
2021年 | 188篇 |
2020年 | 151篇 |
2019年 | 196篇 |
2018年 | 125篇 |
2017年 | 193篇 |
2016年 | 181篇 |
2015年 | 148篇 |
2014年 | 148篇 |
2013年 | 240篇 |
2012年 | 177篇 |
2011年 | 150篇 |
2010年 | 131篇 |
2009年 | 150篇 |
2008年 | 146篇 |
2007年 | 152篇 |
2006年 | 136篇 |
2005年 | 123篇 |
2004年 | 148篇 |
2003年 | 120篇 |
2002年 | 104篇 |
2001年 | 89篇 |
2000年 | 90篇 |
1999年 | 61篇 |
1998年 | 57篇 |
1997年 | 46篇 |
1996年 | 32篇 |
1995年 | 32篇 |
1994年 | 33篇 |
1993年 | 21篇 |
1992年 | 15篇 |
1991年 | 22篇 |
1990年 | 10篇 |
1989年 | 16篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 5篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有3938条查询结果,搜索用时 15 毫秒
51.
银河系厚盘的本地空间密度 总被引:1,自引:0,他引:1
本文利用北银极天区的现代恒星计数资料,采用标准的银河系模型,通过恒星统计积分方程对参数值组合模型的亮度函数进行计数分析,证明了银河系厚盘恒星成份存在的必要性,并进一步得出太阳附近厚盘成份的空间密度的最佳期望值为薄盘密度的0.041. 相似文献
52.
53.
Hari Om Vats 《Journal of Astrophysics and Astronomy》2006,27(2-3):227-235
Examples of extreme events of solar wind and their effect on geomagnetic conditions are discussed here. It is found that there
are two regimes of high speed solar wind streams with a threshold of ∼ 850 km s-1. Geomagnetic activity enhancement rate (GAER) is defined as an average increase in Ap value per unit average increase in
the peak solar wind velocity (Vp) during the stream. GAER was found to be different in the two regimes of high speed streams
with +ve and-ve IMF. GAER is 0.73 and 0.53 for solar wind streams with +ve and -ve IMF respectively for the extremely high
speed streams (< 850 km s-1). This indicates that streams above the threshold speed with +ve IMF are 1.4 times more effective in enhancing geomagnetic
activity than those with -ve IMF. However, the high speed streams below the threshold with -ve IMF are 1.1 times more effective
in enhancing geomagnetic activity than those with +ve IMF. The violent solar activity period (October–November 2003) of cycle
23 presents a very special case during which many severe and strong effects were seen in the environment of the Earth and
other planets; however, the z-component of IMF (Bz) is mostly positive during this period. The most severe geomagnetic storm
of this cycle occurred when Bz was positive. 相似文献
54.
55.
Considering the actual seaway condition, stability and capsizing of nonlinear ship rolling system in stochastic beam seas is of significant importance for voyage safety. Safe zone are defined in the phase space plan of the unperturbed Hamilton system to qualitatively distinguish ship motions as capsize and noncapsize. Capsize events are defined by solutions passing out of the safe zone. The probability of such an occurrence is studied by virtue of the random Melnikov function and the concept of phase space flux. In this paper, besides conventional wave excitation, the effect of wind load is also taken into account. The introduction of wind load will lead to asymmetry, in other words, it transforms the symmetric heteroclinic orbits into asymmetric homoclinic orbits. For asymmetric dynamical system, the orbital analytic solutions and its power spectrum are not readily available, and the technique of discrete time Fourier transformation (DTFT) is used. In the end, as verification of theoretical critical significant wave height, capsizing probability contour diagram is generated by means of numerical simulation. The contour diagram shows that these analytical methods provide reliable and predictive results about the likelihood of a vessel capsizing in a given seaway condition. 相似文献
56.
57.
Understanding our star, the Sun, is of fundamental interest for life on Earth. In this paper, the status of knowledge in solar physics of roughly two decades ago is summarised, and the most recent developments in this very active field are shown, many of them achieved by means of space based missions. The Sun–Earth connection is described and, finally, an outlook on future solar research is given. 相似文献
58.
59.
John L. Tassoulas 《地震工程与结构动力学》2011,40(5):531-550
A half‐space finite element and a transmitting boundary are developed for a water‐saturated layered half‐space using a paraxial boundary condition. The exact dynamic stiffness of a half‐space in plane strain is derived and a second‐order paraxial approximation of the stiffness is obtained. A half‐space finite element and a transmitting boundary are then formulated. The development is verified by comparison of the dynamic stiffness of impermeable and permeable rigid strip foundations with other published results. The advantage of using the paraxial boundary condition in comparison with the rigid boundary condition is examined. It is shown that the paraxial boundary condition offers significant gain and the resulting half‐space finite element and transmitting boundary can represent the effects of a water‐saturated layered half‐space with good accuracy and efficiency. In addition, the numerical method described herein maintains the strengths and advantages of the finite element method and can be easily applied to demanding problems of soil–structure interaction in a water‐saturated layered half‐space. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
60.
采用欧洲空间局ERS-2的星载干涉雷达数据,选取1997年11月8日MW7.6级玛尼地震作为研究对象,采用了差分干涉方法,在通过对覆盖同一地区的SAR数据进行差分干涉处理,得到了玛尼地震的视线向同震形变场。经研究发现:该地震形变场呈长轴近北东东向不规则椭圆形分布,地表破裂带长度约为130km,发震断层走向约为78°,断裂为左行走滑特征。断层以南为隆起区,在发震断层附近最大视线向隆起位移量为113.6cm,断层以北为沉降区,最大视线向沉降位移量为170.4cm。基于Okada模型实现了具有复杂结构的4段断层段参数的InSAR形变场数据模拟,获得断层的最大走滑为6m,估计出玛尼地震的标量地震矩M0为2.69×10^20Nm,计算得到的矩震级MW为7.6。证明了研究方法的正确性和研究结论的可靠性。 相似文献