全文获取类型
收费全文 | 33084篇 |
免费 | 789篇 |
国内免费 | 384篇 |
专业分类
测绘学 | 781篇 |
大气科学 | 2594篇 |
地球物理 | 7280篇 |
地质学 | 11649篇 |
海洋学 | 2759篇 |
天文学 | 6483篇 |
综合类 | 74篇 |
自然地理 | 2637篇 |
出版年
2021年 | 225篇 |
2020年 | 286篇 |
2019年 | 273篇 |
2018年 | 481篇 |
2017年 | 451篇 |
2016年 | 643篇 |
2015年 | 488篇 |
2014年 | 662篇 |
2013年 | 1627篇 |
2012年 | 754篇 |
2011年 | 1149篇 |
2010年 | 989篇 |
2009年 | 1327篇 |
2008年 | 1198篇 |
2007年 | 1176篇 |
2006年 | 1145篇 |
2005年 | 991篇 |
2004年 | 1003篇 |
2003年 | 986篇 |
2002年 | 939篇 |
2001年 | 774篇 |
2000年 | 805篇 |
1999年 | 717篇 |
1998年 | 688篇 |
1997年 | 686篇 |
1996年 | 589篇 |
1995年 | 583篇 |
1994年 | 553篇 |
1993年 | 494篇 |
1992年 | 467篇 |
1991年 | 392篇 |
1990年 | 468篇 |
1989年 | 371篇 |
1988年 | 416篇 |
1987年 | 462篇 |
1986年 | 397篇 |
1985年 | 591篇 |
1984年 | 637篇 |
1983年 | 640篇 |
1982年 | 525篇 |
1981年 | 498篇 |
1980年 | 488篇 |
1979年 | 447篇 |
1978年 | 459篇 |
1977年 | 409篇 |
1976年 | 434篇 |
1975年 | 403篇 |
1974年 | 429篇 |
1973年 | 430篇 |
1972年 | 265篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
We present a new Very Large Array (VLA) image of Saturn, made from data taken in October 1998 at a wavelength of λ3.6 cm. The moderate ring opening angle (B≈15°) allows us to explore direct transmission of microwave photons through the A and C rings. We find a strong asymmetry of photons transmitted through the A ring, but not in the C ring, a new diagnostic of wake structure in the ring particles. We also find a weak asymmetry between east and west for the far side of the ansae. To facilitate quantitative comparison between dynamic models of the A ring and radio observations, we extend our Monte Carlo radiative transfer code (described in Dunn et al., 2002, Icarus 160, 132-160) to include idealized wakes. We show the idealized model can reproduce the properties of dynamic simulations in directly transmitted light. We examine the model behavior in directly transmitted and scattered light over a range of physical and geometric wake parameters. Finally, we present a wake model with a plausible set of physical parameters that quantitatively reproduces the observed intensity and asymmetry of the A ring both across the planet and in the ansae. 相似文献
112.
Chaoborid and chironomid (Diptera) fossils were examined in sediment cores form nine Ontario Precambrian Shield lakes that were fishless in 1979. An abundance of Chaoborus americamus (intolerant of predation by fish) throughout cores from four lakes indicated that they were fishless historically. Occurrence of Chaoborus punctipennis and C. flavicans and absence of C. americanus in cores, except near the surface in two cases, indicated that four other lakes were inhabited by fish historically. One of these was probably periodically fishless as suggested by fluctuations among these Chaoborus species. The ninth lake was not suitable for Chaoborus; only one specimen of C. trivittatus was found in this core. Chironomid fossil associations were substantially different in historically fishless and inhabited lakes. Fishless lakes had chironomids typical of eutrophic conditions indicating that seasonal oxygen depletion likely was inimical to fish. Chironomid associations of oligotrophic waters occurred in lakes historically inhabited by fish. Both good water quality and accessibility were required for long-term habitation by fish. Partial recovery of alkalinity and rise in pH were accompanied by substantial re-invasion by fish in two lakes which had been inhabited historically by fish. Diptera associations provide valuable information on the historical suitability of lakes for fish and the presence/absence of fish in regions where acidification of surface waters has occurred. 相似文献
113.
114.
Carey W.M. Evans R.B. Davis J.A. Botseas G. 《Oceanic Engineering, IEEE Journal of》1990,15(4):324-334
The structure of beam noise measured at the output of a vertical array in a range dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in the midbasin was calculated. The response of the array to the superposition of the noise sources was found by beamforming. An efficient and direct approach that superimposes the noise sources on the PE field as the field is marched toward the array was developed. Downslope calculations of the midbasin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter 相似文献
115.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
116.
117.
5个新发现的X选BL Lac天体 总被引:1,自引:0,他引:1
利用ROSATVLA方法筛选,从ROSATX射线源中选出了一批新的BLLac天体和类星体的候选体.1996年12月8日至17日,利用北京天文台2.16m望远镜和新从美国引进的OMR摄谱仪,对这批新候选体进行了光谱认证.经SUN工作站处理,又发现了5个新的X选BLLac天体.此外,还对去年作者在OMR引进之前发现的BLLac天体进行了检验,结果发现,去年发现的7个BLLac天体中,2322+343的CaIIH&K“Breakstrength”为26.9%,稍大于判据25%,因此,2322+343到底是BLLac天体还是射电星系还有待于进一步测光及偏振观测的验证. 相似文献
118.
Andrew J. L. Harris Anna L. Butterworth Richard W. Carlton Ian Downey Peter Miller Pedro Navarro David A. Rothery 《Bulletin of Volcanology》1997,59(1):49-64
Satellite data offer a means of supplementing ground-based monitoring during volcanic eruptions, especially at times or locations
where ground-based monitoring is difficult. Being directly and freely available several times a day, data from the advanced
very high resolution radiometer (AVHRR) offers great potential for near real-time monitoring of all volcanoes across large
(3000×3000 km) areas. Herein we describe techniques to detect and locate activity; estimate lava area, thermal flux, effusion
rates and cumulative volume; and distinguish types of activity. Application is demonstrated using data for active lavas at
Krafla, Etna, Fogo, Cerro Negro and Erebus; a pyroclastic flow at Lascar; and open vent systems at Etna and Stromboli. Automated
near real-time analysis of AVHRR data could be achieved at existing, or cheap to install, receiving stations, offering a supplement
to conventional monitoring methods.
Received: 21 January 1997 / Accepted: 3 April 1997 相似文献
119.
Richard P. BINZEL Ettore PEROZZI Andrew S. RIVKIN Alessandro ROSSI Alan W. HARRIS Schelte J. BUS Giovanni B. VALSECCHI Stephen M. SLIVAN 《Meteoritics & planetary science》2004,39(3):351-366
Abstract— Using an H‐plot analysis, we identify 234 currently known near‐Earth objects that are accessible for rendezvous with a “best case” delta‐V of less than 7 km/s. We provide a preliminary compositional interpretation and assessment of these potential targets by summarizing the taxonomic properties for 44 objects. Results for one‐half (22) of this sample are based on new spectroscopic measurements presented here. Our approach provides an easy‐to‐update method for giving guidelines to both observers and mission analysts for focusing on objects for which actual mission opportunities are most likely to be found. Observing prospects are presented for categorizing the taxonomic properties of the most accessible targets that are not yet measured. 相似文献
120.
We discuss the difficulties encountered when the Heisenberg-Kolmogoroff model for turbulence is applied to the large-scale turbulence in: (A) molecular clouds (specifically the velocity vs size relationship) and (B) stars (specifically, the estimate of convective fluxes).A new model for large-scale turbulence is, therefore, needed. 相似文献