全文获取类型
收费全文 | 2214篇 |
免费 | 73篇 |
国内免费 | 45篇 |
专业分类
测绘学 | 60篇 |
大气科学 | 147篇 |
地球物理 | 430篇 |
地质学 | 826篇 |
海洋学 | 171篇 |
天文学 | 507篇 |
综合类 | 10篇 |
自然地理 | 181篇 |
出版年
2022年 | 10篇 |
2021年 | 49篇 |
2020年 | 43篇 |
2019年 | 46篇 |
2018年 | 58篇 |
2017年 | 54篇 |
2016年 | 68篇 |
2015年 | 64篇 |
2014年 | 66篇 |
2013年 | 136篇 |
2012年 | 83篇 |
2011年 | 108篇 |
2010年 | 106篇 |
2009年 | 140篇 |
2008年 | 111篇 |
2007年 | 126篇 |
2006年 | 108篇 |
2005年 | 74篇 |
2004年 | 100篇 |
2003年 | 74篇 |
2002年 | 72篇 |
2001年 | 61篇 |
2000年 | 50篇 |
1999年 | 54篇 |
1998年 | 43篇 |
1997年 | 28篇 |
1996年 | 17篇 |
1995年 | 27篇 |
1994年 | 26篇 |
1993年 | 13篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 23篇 |
1986年 | 12篇 |
1985年 | 10篇 |
1984年 | 16篇 |
1983年 | 15篇 |
1982年 | 17篇 |
1981年 | 19篇 |
1980年 | 16篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1975年 | 15篇 |
1974年 | 13篇 |
1973年 | 8篇 |
排序方式: 共有2332条查询结果,搜索用时 10 毫秒
991.
992.
The preflare activity of a plage filament is analysed from H observations made with the Multichannel Subtractive Double Pass Spectrograph (MSDP) of the Meudon Solar Tower. The June 22, 1980 event is studied and interpreted in terms of preflare heating of a filament, connected to the rise of emerging flux, and the relative approach of pores of different magnetic polarity, prior to the onset of a two-ribbon flare.The region with enhanced magnetic field, around the filament, begins to brighten slowly 20 min before the triggering of the flare, in the center of H. Filament dark material begins to rise rapidly while the brightest point on one side drifts towards it, 6 min before the onset of the two-ribbon flare. Simultaneously the absorbing material separates from the remaining part of the filament.In the discussion, we suggest that most of the observed features may be the consequence of emergence of new magnetic flux and the related reconnection processes. 相似文献
993.
994.
Isaac G. Roseboom Kevin A. Pimbblet Michael J. Drinkwater Russell D. Cannon Roberto De Propris Alastair C. Edge Daniel J. Eisenstein Robert C. Nichol Ian Smail David A. Wake Joss Bland-Hawthorn Terry J. Bridges Daniel Carson Matthew Colless Warrick J. Couch Scott M. Croom Simon P. Driver Paul C. Hewett Jon Loveday Nic Ross Donald P. Schneider Tom Shanks Robert G. Sharp Peter Weilbacher 《Monthly notices of the Royal Astronomical Society》2006,373(1):349-360
We present a detailed investigation into the recent star formation histories of 5697 luminous red galaxies (LRGs) based on the Hδ (4101 Å), and [O ii ] (3727 Å) lines and the D4000 index. LRGs are luminous ( L > 3 L *) galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study, we utilize LRGs from the recently completed 2dF-SDSS LRG and QSO Survey (2SLAQ). Equivalent widths of the Hδ and [O ii ] lines are measured and used to define three spectral types, those with only strong Hδ absorption (k+a), those with strong [O ii ] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (∼80 per cent); however, significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low-redshift comparison. While the em and em+a fractions are consistent with the low-redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected lifetime of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted level of k+a and em+a activities is not sufficient to reconcile the predicted mass growth for massive early types in a hierarchical merging scenario. 相似文献
995.
996.
Simon P. Driver Warrick J. Couch Steven Phillipps & Rodney Smith 《Monthly notices of the Royal Astronomical Society》1998,301(2):357-368
We present detailed simulations of long-exposure CCD images. The simulations are used to explore the validity of the statistical method for reconstructing the luminosity distribution of galaxies within a rich cluster, i.e., by the subtraction of field number-counts from those of a sight-line through the cluster. In particular, we use the simulations to establish the reliability of our observational data to be presented in Paper III. Based on our intended CCD field-of-view (6.5 × 6.5 arcmin2 ) and a 1σ detection limit of 26 mag arcsec−2 , we conclude that the luminosity distribution can be robustly determined over a wide range of absolute magnitude (−23 < M R < −16) provided: (a) the cluster has an Abell richness 1.5 or greater; (b) the redshift of the cluster lies in the range 0.1 < z < 0.3; (c) the seeing is better than FWHM 1.25 arcsec, and (d) the photometric zero-points are accurate to within Δ m = ± 0.12. If these conditions are not met, then the recovered luminosity distribution is unreliable. Finally, although the method clearly has limitations, within these limitations the technique represents an extremely promising probe of galaxy evolution and environmental dependences. 相似文献
997.
Yiannis Tsapras Rachel A. Street Keith Horne Alan Penny Fraser Clarke Hans Deeg Francisco Garzon Simon Kemp Maria Rosa Zapatero Osorio Alejandro Oscoz Abad Santiago Madruga Sanchez Carlos Eiroa Alcione Mora Antxon Alberdi rew Cameron John K. Davies Roger Ferlet Carol Grady Allan W. Harris Javier Palacios reas Quirrenbach Heike Rauer Jean Schneider Dolf de Winter Bruno Merin Enrique Solano 《Monthly notices of the Royal Astronomical Society》2001,325(3):1205-1212
In 1998 the EXPORT team monitored microlensing event light curves using a charge-coupled device (CCD) camera on the IAC 0.8-m telescope on Tenerife to evaluate the prospect of using northern telescopes to find microlens anomalies that reveal planets orbiting the lens stars. The high airmass and more limited time available for observations of Galactic bulge sources make a northern site less favourable for microlensing planet searches. However, there are potentially a large number of northern 1-m class telescopes that could devote a few hours per night to monitor ongoing microlensing events. Our IAC observations indicate that accuracies sufficient to detect planets can be achieved despite the higher airmass. 相似文献
998.
Dinah M. Cragg rej M. Sobolev Simon P. Ellingsen J. L. Caswell Peter D. Godfrey Svetlana V. Salii Richard G. Dodson 《Monthly notices of the Royal Astronomical Society》2001,323(4):939-951
We have used the ATNF Mopra antenna and the SEST antenna to search in the directions of several class II methanol maser sources for emission from six methanol transitions in the frequency range 85–115 GHz. The transitions were selected from excitation studies as potential maser candidates. Methanol emission at one or more frequencies was detected from five of the maser sources, as well as from Orion KL. Although the lines are weak, we find evidence of maser origin for three new lines in G345.01+1.79, and possibly one new line in G9.62+0.20.
The observations, together with published maser observations at other frequencies, are compared with methanol maser modelling for G345.01+1.79 and NGC 6334F. We find that the majority of observations in both sources are consistent with a warm dust (175 K) pumping model at hydrogen density ∼106 cm−3 and methanol column density ∼ 5×1017 cm−2 . The substantial differences between the maser spectra in the two sources can be attributed to the geometry of the maser region. 相似文献
The observations, together with published maser observations at other frequencies, are compared with methanol maser modelling for G345.01+1.79 and NGC 6334F. We find that the majority of observations in both sources are consistent with a warm dust (175 K) pumping model at hydrogen density ∼10
999.
The authors give reasons why an extremely simple motion, which Vainshtein recently proposed should self-sustain a magnetic field, may fail to do so. 相似文献
1000.
Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP , and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd. 相似文献