全文获取类型
收费全文 | 32972篇 |
免费 | 628篇 |
国内免费 | 856篇 |
专业分类
测绘学 | 978篇 |
大气科学 | 2777篇 |
地球物理 | 5796篇 |
地质学 | 15634篇 |
海洋学 | 2246篇 |
天文学 | 5290篇 |
综合类 | 422篇 |
自然地理 | 1313篇 |
出版年
2023年 | 81篇 |
2022年 | 152篇 |
2021年 | 166篇 |
2020年 | 141篇 |
2019年 | 181篇 |
2018年 | 3547篇 |
2017年 | 3322篇 |
2016年 | 2083篇 |
2015年 | 394篇 |
2014年 | 407篇 |
2013年 | 471篇 |
2012年 | 1558篇 |
2011年 | 3325篇 |
2010年 | 3126篇 |
2009年 | 3274篇 |
2008年 | 2569篇 |
2007年 | 3241篇 |
2006年 | 450篇 |
2005年 | 824篇 |
2004年 | 670篇 |
2003年 | 746篇 |
2002年 | 465篇 |
2001年 | 265篇 |
2000年 | 276篇 |
1999年 | 277篇 |
1998年 | 251篇 |
1997年 | 264篇 |
1996年 | 231篇 |
1995年 | 151篇 |
1994年 | 173篇 |
1993年 | 162篇 |
1992年 | 112篇 |
1991年 | 84篇 |
1990年 | 85篇 |
1989年 | 65篇 |
1988年 | 74篇 |
1987年 | 67篇 |
1986年 | 74篇 |
1985年 | 54篇 |
1984年 | 62篇 |
1983年 | 54篇 |
1982年 | 65篇 |
1981年 | 54篇 |
1980年 | 59篇 |
1979年 | 41篇 |
1977年 | 21篇 |
1976年 | 27篇 |
1975年 | 24篇 |
1974年 | 21篇 |
1973年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
We consider the particular solutions of the evolutionary system of equations in elements that correspond to planar and spatial circular orbits of the singly averaged Hill problem. We analyze the stability of planar and spatial circular orbits to inclination and eccentricity, respectively. We construct the instability regions of both particular solutions in the plane of parameters of the problem. 相似文献
12.
13.
A formula for the thickness of a shear band formed in saturated soils under a simple shear or a combined stress state has been proposed. It is shown that the shear band thickness is dependent on the pore pressure properties of the material and the dilatancy rate, but is independent of the details of the combined stress state. This is in accordance with some separate experimental observations. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
14.
15.
16.
Stress wave attenuation across fractured rock masses is a great concern of underground structure safety. When the wave amplitude is large, fractures experience nonlinear deformation during the wave propagation. This paper presents a study on normal transmission of P‐wave across parallel fractures with nonlinear deformational behaviour (static Barton–Bandis model). The results show that the magnitude of transmission coefficient is a function of incident wave amplitude, nondimensional fracture spacing and number of fractures. Two important indices of nondimensional fracture spacing are identified, and they divide the area of nondimensional fracture spacing into three parts (individual fracture area, transition area and small spacing area). In the different areas, the magnitude of transmission coefficient has different trends with nondimensional fracture spacing and number of fractures. In addition, the study reveals that under some circumstances, the magnitude of transmission coefficient increases with increasing number of fractures, and is larger than 1. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
17.
Xie Rui-xiang Wang Min Shi Shuo-biao Xu Chun Li Wei-hua Yan Yi-hua 《Chinese Astronomy and Astrophysics》2003,27(4):426-434
Using the decimetric (700–1500 MHz) radio spectrometer and the synchronous observational system with high temporal resolution at four frequencies (1420, 2130, 2840 and 4260 MHz) of Yunnan Observatory, two rare events were observed on 2001 June 24 and 1990 July 30. The former was a small radio burst exhibiting pulsations with short periods (about 29, 40 and 100 ms) in the impulsive phase. The latter was a large radio burst, which at 2840 MHz produced radio pulsations with period of about 30 ms. This paper focuses on pulsations with very short periods in the range of 29–40 ms. The mechanism of generation of such pulsations may be modulation of radio radiation by the periodic trains of whistler packets originating in unstable regions of the corona. Alternatively, these pulsations can be attributed to wave-wave non-linear interactions of electrostatic upper hybrid waves driven by beams of precipitating electrons in flaring loops. 相似文献
18.
In late October and early November 2003, a series of space weather hazard events erupted in solar-terrestrial space. Aiming
at two intense storm (shock) events on 28 and 29 October, this paper presents a Two-Step method, which combines synoptic analysis
of space weather–`observing’ and quantitative prediction – ‘palpating’, and uses it to test predictions. In the first step,
‘observing’, on the basis of observations of the source surface magnetic field, interplanetary scintillation (IPS) and ACE
spacecraft, we find that the propagation of the shock waves is asymmetric and northward relative to the normal direction of
their solar sources due to the large-scale configuration of the coronal magnetic fields, and the Earth is located near the
direction of the fastest speed and greatest energy of the shocks. Being two fast ejection shock events, the fast explosion
of extremely high temperature and strong magnetic field, and background solar wind velocity as high as 600 and 1000 km s−1, are also helpful to their rapid propagation. According to the synoptic analysis, the shock travel times can be estimated
as 21 and 20 h, which are close to the observational results of 19.97 and 19.63 h, respectively. In the second step, ‘palpating’,
we adopt a new membership function of the fast shock events for the ISF method. The predicted results here show that for the
onset time of the geomagnetic disturbance, the relative errors between the observational and the predicted results are 1.8
and 6.7%, which are consistent with the estimated results of the first step; and for the magnetic disturbance magnitude, the
relative errors between the observational and the predicted results are 4.1 and 3.1%, respectively. Furthermore, the comparison
among the predicted results of our Two-Step method with those of five other prevailing methods shows that the Two-Step method
is advantageous in predicting such strong shock event. It can predict not only shock arrival time, but also the magnitude
of magnetic disturbance. The results of the present paper tell us that understanding the physical features of shock propagation
thoroughly is of great importance in improving the prediction efficiency. 相似文献
19.
A. Habib J. Vernin Z. Benkhaldoun H. Lanteri 《Monthly notices of the Royal Astronomical Society》2006,368(3):1456-1462
In this paper, we present a new method to estimate, for each turbulent layer labelled i , the horizontal wind speed v ( h i ) , the standard deviation of the horizontal wind speed fluctuations σ v ( hi ) and the integrated value of C 2 n over the thickness Δ hi of the turbulent layer C 2 n ( hi )Δ hi , where hi is the altitude of the turbulent layer. These parameters are extracted from single star scintillation spatiotemporal cross-correlation functions of atmospheric speckles obtained within the generalized mode. This method is based on the simulated annealing algorithm to find the optimal solution required to solve the problem. Astrophysics parameters for adaptive optics are also calculated using C 2 n ( hi ) and v ( hi ) values. The results of other techniques support this new method. 相似文献
20.