全文获取类型
收费全文 | 4090篇 |
免费 | 61篇 |
国内免费 | 56篇 |
专业分类
测绘学 | 99篇 |
大气科学 | 283篇 |
地球物理 | 796篇 |
地质学 | 1642篇 |
海洋学 | 249篇 |
天文学 | 934篇 |
综合类 | 11篇 |
自然地理 | 193篇 |
出版年
2023年 | 18篇 |
2022年 | 31篇 |
2021年 | 31篇 |
2020年 | 27篇 |
2019年 | 41篇 |
2018年 | 131篇 |
2017年 | 130篇 |
2016年 | 174篇 |
2015年 | 82篇 |
2014年 | 177篇 |
2013年 | 222篇 |
2012年 | 156篇 |
2011年 | 186篇 |
2010年 | 189篇 |
2009年 | 231篇 |
2008年 | 177篇 |
2007年 | 171篇 |
2006年 | 201篇 |
2005年 | 131篇 |
2004年 | 129篇 |
2003年 | 108篇 |
2002年 | 83篇 |
2001年 | 75篇 |
2000年 | 75篇 |
1999年 | 79篇 |
1998年 | 77篇 |
1997年 | 87篇 |
1996年 | 75篇 |
1995年 | 46篇 |
1994年 | 32篇 |
1993年 | 57篇 |
1992年 | 34篇 |
1991年 | 43篇 |
1990年 | 27篇 |
1989年 | 36篇 |
1988年 | 37篇 |
1987年 | 43篇 |
1986年 | 52篇 |
1985年 | 41篇 |
1984年 | 46篇 |
1983年 | 46篇 |
1982年 | 46篇 |
1981年 | 35篇 |
1980年 | 32篇 |
1979年 | 35篇 |
1977年 | 20篇 |
1976年 | 22篇 |
1975年 | 23篇 |
1974年 | 20篇 |
1973年 | 23篇 |
排序方式: 共有4207条查询结果,搜索用时 15 毫秒
1.
2.
A spectacular change in the lower corona on the south-west limb has been found in solar images taken by the Yohkoh soft X-ray telescope. The event is characterized by a large topological change in magnetic field and a large intensity decrease observed after the X1. 1/1B flare on 9 November, 1991. A coronal mass ejection (CME) was observed by the Mark III K-coronameter (MK3) at the HAO/Mauna Loa Observatory. Both the MK3 (white-light) and soft X-ray observations showed that one leg of this CME was located above the flare site. An interplanetary shock associated with this event was observed by Pioneer Venus Orbiter, and, possibly, by IMP-8.Also Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, U.S.A. 相似文献
3.
4.
5.
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
6.
An empirical kinematic method developed by Hakamada and Akasofu (1982) is calibrated on the basis of a one-dimensional MHD solution. The calibrated results are used to simulate the stream-stream interaction and the background corotating structure in a simple situation and also during 22 November–6 December 1977. The solar wind disturbances caused by solar activities during this period are then introduced into the above background stream in simulating the heliospheric disturbance event which was observed by an aligned set of spacecraft at distances between 0.6 and 1.6 a.u. The observations and the simulated results are satisfactory, and a little more refinement in the simulation could reconstruct reasonably well the data by filling the data gaps in the solar wind speed, the density and the IMF magnitude. 相似文献
7.
We have developed a method for analytically solving the porous medium flow equation in many different geometries for horizontal (two‐dimensional), homogeneous and isotropic aquifers containing impermeable boundaries and any number of pumping or injection wells located at arbitrary positions within the system. Solutions and results are presented for rectangular and circular aquifers but the method presented here is easily extendible to many geometries. Results are also presented for systems where constant head boundary conditions can be emulated internal to the aquifer boundary. Recommendations for extensions of the present work are briefly discussed. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
8.
A new approach to constraining seawater δ34S and sulphate concentration using francolite‐bound sulphate reveals an abrupt increase in δ34S to +50‰ around the Early–Middle Cambrian boundary. Such high δ34S values are best explained by increased rates of pyrite burial due to ocean anoxia coupled with an increased sensitivity of the ocean sulphate reservoir to perturbations due to low sulphate concentrations of 500–700 μgL?1. We argue that the spread of anoxic waters at this time was partly the result of greenhouse warming related to the eruption of the Kalkarindji Large Igneous Province of northern Australia and that it triggered the collapse of early metazoan reef ecosystems during the latest Early Cambrian. Mass extinctions of the last 260 Myr have all coincided with enhanced volcanic activity, while several are also associated with positive shifts in seawater δ34S. Extending this correlation back in time further implicates volcanically induced climate change as a major determining factor in biosphere evolution. Terra Nova, 18, 257–263, 2006 相似文献
9.
In the coastal and estuarine waters of Goa, particulate organic carbon (POC) varied from 0.52 to 2.51 mg l?1 and from 0.28 to 5.24 mg l?1 and particulate phosphorus (PP) varied from 0.71 to 5.18 μg l?1 and from 0.78 to 20.34 μg l?1, respectively. The mean values of chlorophyll and primary productivity were 1.94 mg m?3 and 938.1 mg C m?2 day?1 in the coastal waters and 4.3 mg m?3 and 636.5 mg C m?1 day?1 in the estuarine waters, respectively. ratios were low in June and October even when POC values were quite high. The POC in surface waters was linearly correlated with the chlorophyll content. Also PP increased when chlorophyll and primary productivity remained high. The results suggest that the phytoplankton was sharply increasing and contributed to POC and PP content. The percentage of detritus calculated from the intercept values of chlorophyll on POC varied from 46 to 76% depending on season. Results indicate that the major portion of POC and PP during postmonsoon (October–January) is derived from phytoplankton production while the allochthonous matter predominate during monsoon (June–September). 相似文献
10.
Z. Balog † A. J. Delgado ‡ A. Moitinho G. Frész † G. Kaszás † J. Vinkó E. J. Alfaro 《Monthly notices of the Royal Astronomical Society》2001,323(4):872-886
CCD photometry in Johnson UBV and Strömgren uvby systems and medium-resolution spectroscopy of the galactic open cluster NGC 7128 are presented. Spectral types of the brightest 12 stars in the cluster field were determined based on equivalent widths of the H α and the He i 6678-Å line. The spectroscopic observations also revealed two obvious and one probable Be-type stars showing H α emission. The analysis of the photometric diagrams gave a colour excess of E ( B − V )=1.03±0.06 mag, a distance modulus DM =13.0±0.2 mag and an age above 10 Myr. Time-resolved photometric observations obtained on one night resulted in the detection of short time-scale light variations of seven new and three already known variable stars in the cluster field. 相似文献