全文获取类型
收费全文 | 66514篇 |
免费 | 1431篇 |
国内免费 | 498篇 |
专业分类
测绘学 | 1649篇 |
大气科学 | 5328篇 |
地球物理 | 13814篇 |
地质学 | 21604篇 |
海洋学 | 5767篇 |
天文学 | 15377篇 |
综合类 | 139篇 |
自然地理 | 4765篇 |
出版年
2020年 | 477篇 |
2019年 | 506篇 |
2018年 | 944篇 |
2017年 | 928篇 |
2016年 | 1377篇 |
2015年 | 1028篇 |
2014年 | 1425篇 |
2013年 | 3260篇 |
2012年 | 1498篇 |
2011年 | 2287篇 |
2010年 | 1954篇 |
2009年 | 2950篇 |
2008年 | 2687篇 |
2007年 | 2413篇 |
2006年 | 2477篇 |
2005年 | 2161篇 |
2004年 | 2254篇 |
2003年 | 2080篇 |
2002年 | 1974篇 |
2001年 | 1789篇 |
2000年 | 1760篇 |
1999年 | 1511篇 |
1998年 | 1505篇 |
1997年 | 1489篇 |
1996年 | 1278篇 |
1995年 | 1216篇 |
1994年 | 1100篇 |
1993年 | 1000篇 |
1992年 | 952篇 |
1991年 | 803篇 |
1990年 | 1019篇 |
1989年 | 860篇 |
1988年 | 762篇 |
1987年 | 937篇 |
1986年 | 819篇 |
1985年 | 1029篇 |
1984年 | 1195篇 |
1983年 | 1131篇 |
1982年 | 1027篇 |
1981年 | 986篇 |
1980年 | 845篇 |
1979年 | 829篇 |
1978年 | 877篇 |
1977年 | 797篇 |
1976年 | 757篇 |
1975年 | 704篇 |
1974年 | 706篇 |
1973年 | 719篇 |
1972年 | 445篇 |
1971年 | 390篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
251.
252.
S. Baes-Fischlmair W.W. Zeilinger J.-C. Vega-Beltran J.E. Beckman 《Astrophysics and Space Science》2003,284(2):735-738
It is assumed that the two-fold disc-wide symmetry of spirals is caused by density waves, but also the potential of a bar
component may have a significant influence on structural properties. The strength of the bar component appears to be anti-correlated
with the degree of symmetry of star-forming regions in the spiral arms (Rozas et al., 1998). We present new results of R and
Hα surface photometry of a sample of bright barred spirals. A photometric decompositon of the galaxy components is carried
out in order to make a more accurate measurement of the strength of the bar and its interrelation to gas and stars in the
disc.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
253.
254.
The host galaxies of active galactic nuclei 总被引:2,自引:0,他引:2
Guinevere Kauffmann Timothy M. Heckman Christy Tremonti Jarle Brinchmann Stéphane Charlot Simon D. M. White Susan E. Ridgway Jon Brinkmann Masataka Fukugita Patrick B. Hall eljko Ivezi Gordon T. Richards Donald P. Schneider 《Monthly notices of the Royal Astronomical Society》2003,346(4):1055-1077
We examine the properties of the host galaxies of 22 623 narrow-line active galactic nuclei (AGN) with 0.02 < z < 0.3 selected from a complete sample of 122 808 galaxies from the Sloan Digital Sky Survey. We focus on the luminosity of the [O iii ]λ5007 emission line as a tracer of the strength of activity in the nucleus. We study how AGN host properties compare with those of normal galaxies and how they depend on L [O iii ]. We find that AGN of all luminosities reside almost exclusively in massive galaxies and have distributions of sizes, stellar surface mass densities and concentrations that are similar to those of ordinary early-type galaxies in our sample. The host galaxies of low-luminosity AGN have stellar populations similar to normal early types. The hosts of high-luminosity AGN have much younger mean stellar ages. The young stars are not preferentially located near the nucleus of the galaxy, but are spread out over scales of at least several kiloparsecs. A significant fraction of high-luminosity AGN have strong Hδ absorption-line equivalent widths, indicating that they experienced a burst of star formation in the recent past. We have also examined the stellar populations of the host galaxies of a sample of broad-line AGN. We conclude that there is no significant difference in stellar content between type 2 Seyfert hosts and quasars (QSOs) with the same [O iii ] luminosity and redshift. This establishes that a young stellar population is a general property of AGN with high [O iii ] luminosities. 相似文献
255.
256.
B. R. Johnson M. E. Abroe P. Ade J. Bock J. Borrill J. S. Collins P. Ferreira S. Hanany A. H. Jaffe T. Jones A. T. Lee L. Levinson T. Matsumura B. Rabii T. Renbarger P. L. Richards G. F. Smoot R. Stompor H. T. Tran C. D. Winant 《New Astronomy Reviews》2003,47(11-12):1067
We discuss MAXIPOL, a bolometric balloon-borne experiment designed to measure the E-mode polarization anisotropy of the cosmic microwave background radiation (CMB) on angular scales of 10′ to 2°. MAXIPOL is the first CMB experiment to collect data with a polarimeter that utilizes a rotating half-wave plate and fixed wire-grid polarizer. We present the instrument design, elaborate on the polarimeter strategy and show the instrument performance during flight with some time domain data. Our primary dataset was collected during a 26 h turnaround flight that was launched from the National Scientific Ballooning Facility in Ft. Sumner, New Mexico in May 2003. During this flight five regions of the sky were mapped. Data analysis is in progress. 相似文献
257.
258.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1 . The highest energy lines require a column density of N H ∼ 5 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.4 . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143. 相似文献
259.
Introducing a concept of equivalent mass depth of flow, this study describes the phenomenon of non‐point source pollutant (metal) transport for pavement (or overland) flow in analogy with wave propagation in wide open channels. Hysteretic and normal mass rating curves are developed for runoff rate and mass of 12 dissolved and particulate‐bound metal elements (pollutants) using the rainfall‐runoff and water quality data of the 15 × 20 m2 instrumented pavement in Cincinnati, USA. Normal mass rating curves developed for easy computation of pollutant load are found to be of a form similar to Manning's, which is valid for open channel flows. Based on the hysteresis analysis, wave types for dissolution and mixing of particulate‐bound metals are identified. The analysis finds that the second‐order partial‐differential equation normally used for metal transport does not have the efficacy to describe fully the strong non‐linear phenomena such as is described for various metal elements by dynamic waves. In addition, the proportionality concept of the popular SCS‐CN concept is extended for determining the potential maximum metal mass Mp of all the 12 elements transported by a rain storm and related to the antecedent dry period (ADP). For the primary metal zinc element, Mp is found to increase with the ADP and vice versa. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
260.
Lance B. Morrissey Philippe Janvier Simon J. Braddy John P. Bennett Susan B. Marriott Peter R. Tarrant 《Geology Today》2006,22(2):66-67
Armoured jawless fish, or 'ostracoderms', lived 450–360 million years ago, and display unusual morphologies, unlike any modern fish group. Since they left no living descendants, their mode of swimming has, until recently, remained speculative, although this is a crucial question as the first true pectoral fins evolved within the 'ostracoderms'. The discovery of the oldest-known fish trails, from the Early Devonian (400 million year old) 'Lower Old Red Sandstone' of south-east Wales offers new insights into the swimming behaviour of these early fish, notably the osteostracan 'ostracoderms' (or cephalaspids), whose horseshoe-shaped head and paddle-shaped pectoral fins have remained a functional riddle. 相似文献