全文获取类型
收费全文 | 254580篇 |
免费 | 5176篇 |
国内免费 | 3347篇 |
专业分类
测绘学 | 6847篇 |
大气科学 | 18876篇 |
地球物理 | 53473篇 |
地质学 | 88154篇 |
海洋学 | 21158篇 |
天文学 | 55790篇 |
综合类 | 1005篇 |
自然地理 | 17800篇 |
出版年
2021年 | 2219篇 |
2020年 | 2584篇 |
2019年 | 2835篇 |
2018年 | 3185篇 |
2017年 | 2820篇 |
2016年 | 5539篇 |
2015年 | 4184篇 |
2014年 | 6879篇 |
2013年 | 14190篇 |
2012年 | 6359篇 |
2011年 | 7561篇 |
2010年 | 6570篇 |
2009年 | 9218篇 |
2008年 | 8113篇 |
2007年 | 7488篇 |
2006年 | 9624篇 |
2005年 | 7605篇 |
2004年 | 7561篇 |
2003年 | 7056篇 |
2002年 | 6688篇 |
2001年 | 5982篇 |
2000年 | 5926篇 |
1999年 | 5196篇 |
1998年 | 5216篇 |
1997年 | 5026篇 |
1996年 | 4671篇 |
1995年 | 4423篇 |
1994年 | 4089篇 |
1993年 | 3841篇 |
1992年 | 3624篇 |
1991年 | 3583篇 |
1990年 | 3749篇 |
1989年 | 3508篇 |
1988年 | 3301篇 |
1987年 | 3844篇 |
1986年 | 3404篇 |
1985年 | 4216篇 |
1984年 | 4733篇 |
1983年 | 4400篇 |
1982年 | 4307篇 |
1981年 | 3923篇 |
1980年 | 3634篇 |
1979年 | 3504篇 |
1978年 | 3478篇 |
1977年 | 3275篇 |
1976年 | 3044篇 |
1975年 | 2956篇 |
1974年 | 2914篇 |
1973年 | 3073篇 |
1972年 | 2024篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
291.
D. H. Lumb A. Finoguenov R. Saxton B. Aschenbach P. Gondoin M. Kirsch I. M. Stewart 《Experimental Astronomy》2003,15(2):89-111
We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09
and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described.
We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement
between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation
with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster
emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base,
for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on
the assumed spectral responses of the cameras for on-axis targets.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
292.
Impulsive acoustic waveforms are characterized by a different set of derived quantities than are continuous waveforms. This note presents commonly accepted definitions, units, and symbols used to describe the magnitude of impulsive underwater signals 相似文献
293.
J. A. WOOD 《Meteoritics & planetary science》1996,31(5):641-645
Abstract— A widely held view of nebular evolution is that during the ~0.5 Ma while interstellar material was collapsing onto the disk, the latter grew in mass to the point of gravitational instability. It responded to this by losing axial symmetry, growing spiral arms that had the capacity to tidally redistribute disk mass (inward) and angular momentum (outward) and prevent further increase in the disk/protosun mass ratio. The spiral arms (density waves) rotated differently than the substance of the nebula, and in some parts of the disk, nebular material may have encountered the arms at supersonic velocities. The disk gas, and solid particles entrained in it, would have been heated to some degree when they passed through shock fronts at the leading edges of the spiral arms. The present paper proposes this was the energetic nebular setting or environment that has long been sought, in which the material now in the planets and chondritic meteorites was thermally processed. 相似文献
294.
A major difficulty in remote sensing is handling the many data from sensors aboard aircraft and satellites. In this paper we identify an optimal procedure for sampling remotely sensed data before their storage or on their retrieval. The procedure depends on spatial correlation in the scene and uses kriging to estimate values that have been lost. An example in which data from an airborne multispectral scanner could be diminished to only about one tenth without serious loss of precision illustrates the method. 相似文献
295.
296.
REGIONAL DEMOGRAPHIC STRUCTURE OF THE UNITED STATES 总被引:1,自引:0,他引:1
Richard L. Morrill 《The Professional geographer》1990,42(1):38-53
This paper analyzes the surprisingly great variation in demographic character across the states, utilizing data on fertility, mortality, age, sex, mobility, household character, abortion, race and ethnicity. A fairly simple regional pattern is revealed that is less related to levels of economic development than to long-standing historical cultural differences. 相似文献
297.
Kunde Yang Yuanliang Ma Chao Sun Miller J.H. Potty G.R. 《Oceanic Engineering, IEEE Journal of》2004,29(4):964-972
This paper discusses the results of geoacoustic inversion carried out using explosive charge data from the Asian Seas International Acoustic Experiment (ASIAEX) East China Sea (ECS) Experiment. A multifrequency incoherent matched-field inversion processor and a genetic algorithm (GA) are used for the inversion. A multistep matched field inversion approach is presented, which makes use of the varying sensitivities of wave fields at various frequencies to reduce the inversion problem into a sequence of smaller inversions with fewer unknowns to estimate at each stage. Different parameters are estimated using data at different frequencies according to their sensitivities. Inversion results for different areas in the ECS region are summarized and compared with core data. 相似文献
298.
We report multi-frequency and multi-epoch radio continuum observations with multi-spatial resolution for the low-luminosity active galactic nucleus (LLAGN) NGC 266. In the centimetre regime, we find diffuse components with Very Large Array (VLA) observations, and a variable compact core with a rising spectrum with Very Long Baseline Array (VLBA) observations. Although the spectral index of the rising spectrum is consistent with the prediction of the simple advection-dominated accretion flow (ADAF) model, the observed radio power is slightly high compared with that of the model prediction. A spectral break at centimetre-to-millimetre wavelengths is inferred from the upper limits of flux densities from Nobeyama Millimetre Array (NMA) and James Clerk Maxwell Telescope (JCMT) data at millimetre and submillimetre wavelengths, respectively. More complicated considerations are required for the theoretical model to interpret such observed radio properties. 相似文献
299.
Groundbased radio observations indicate that Jupiter's ammonia is globally depleted from 0.6 bars to at least 4-6 bars relative to the deep abundance of ∼3 times solar, a fact that has so far defied explanation. The observations also indicate that (i) the depletion is greater in belts than zones, and (ii) the greatest depletion occurs within Jupiter's local 5-μm hot spots, which have recently been detected at radio wavelengths. Here, we first show that both the global depletion and its belt-zone variation can be explained by a simple model for the interaction of moist convection with Jupiter's cloud-layer circulation. If the global depletion is dynamical in origin, then important endmember models for the belt-zone circulation can be ruled out. Next, we show that the radio observations of Jupiter's 5-μm hot spots imply that the equatorial wave inferred to cause hot spots induces vertical parcel oscillation of a factor of ∼2 in pressure near the 2-bar level, which places important constraints on hot-spot dynamics. Finally, using spatially resolved radio maps, we demonstrate that low-latitude features exceeding ∼4000 km diameter, such as the equatorial plumes and large vortices, are also depleted in ammonia from 0.6 bars to at least 2 bars relative to the deep abundance of 3 times solar. If any low-latitude features exist that contain 3-times-solar ammonia up to the 0.6-bar ammonia condensation level, they must have diameters less than ∼4000 km. 相似文献
300.
Using photoclinometry, topographic profiles across europan ridges have been produced. These profiles allow the identification of bulges in the terrain adjacent to the ridges. The bulges are assumed to have been produced by flexure of the elastic lithosphere due to the load of the ridges, which lie along cracks in the crust. The distance from the crack to these “fore-bulges” depends on the thickness of the elastic plate being flexed. Based on a survey of ridges in Galileo images with resolution <300 m/pixel, the thickness of the elastic lithosphere has been determined by this method at a wide variety of sites along the leading and trailing hemispheres of Europa. The average thickness is about 200 m. The elastic lithosphere underneath smooth dilational bands tends to be thicker than plains morphology, an effect that is pronounced at Thynia Linea and Astypalaea Linea. Among the ridges investigated here, more recent loading correlates with a thicker elastic lithosphere, which may either reflect an intrinsically thicker layer, or less viscous relaxation over the shorter time period. 相似文献