全文获取类型
| 收费全文 | 1673篇 |
| 免费 | 52篇 |
| 国内免费 | 73篇 |
专业分类
| 测绘学 | 90篇 |
| 大气科学 | 133篇 |
| 地球物理 | 114篇 |
| 地质学 | 33篇 |
| 海洋学 | 57篇 |
| 天文学 | 1300篇 |
| 综合类 | 40篇 |
| 自然地理 | 31篇 |
出版年
| 2024年 | 2篇 |
| 2023年 | 1篇 |
| 2022年 | 13篇 |
| 2021年 | 8篇 |
| 2020年 | 17篇 |
| 2019年 | 17篇 |
| 2018年 | 17篇 |
| 2017年 | 18篇 |
| 2016年 | 19篇 |
| 2015年 | 22篇 |
| 2014年 | 26篇 |
| 2013年 | 37篇 |
| 2012年 | 28篇 |
| 2011年 | 88篇 |
| 2010年 | 83篇 |
| 2009年 | 161篇 |
| 2008年 | 170篇 |
| 2007年 | 173篇 |
| 2006年 | 132篇 |
| 2005年 | 133篇 |
| 2004年 | 120篇 |
| 2003年 | 127篇 |
| 2002年 | 71篇 |
| 2001年 | 73篇 |
| 2000年 | 57篇 |
| 1999年 | 46篇 |
| 1998年 | 53篇 |
| 1997年 | 16篇 |
| 1996年 | 8篇 |
| 1995年 | 17篇 |
| 1994年 | 7篇 |
| 1993年 | 5篇 |
| 1992年 | 3篇 |
| 1991年 | 2篇 |
| 1990年 | 8篇 |
| 1989年 | 1篇 |
| 1988年 | 3篇 |
| 1987年 | 4篇 |
| 1986年 | 1篇 |
| 1985年 | 7篇 |
| 1980年 | 2篇 |
| 1978年 | 1篇 |
| 1954年 | 1篇 |
排序方式： 共有1798条查询结果，搜索用时 15 毫秒
151.
The angular cross-correlation between two galaxy samples separated in redshift is shown to be a useful measure of weak lensing by large-scale structure. Angular correlations in faint galaxies arise as a result of spatial clustering of the galaxies as well as gravitational lensing by dark matter along the line of sight. The lensing contribution to the two-point autocorrelation function is typically small compared with the gravitational clustering. However, the cross-correlation between two galaxy samples is almost unaffected by gravitational clustering provided that their redshift distributions do not overlap. The cross-correlation is then induced by magnification bias resulting from lensing by large-scale structure. We compute the expected amplitude of the cross-correlation for popular theoretical models of structure formation. For two populations with mean redshifts of ≃0.3 and 1, we find a cross-correlation signal of ≃1 per cent on arcmin scales and ≃3 per cent on scales of a few arcsec. The dependence on the cosmological parameters Ω and Λ, the dark matter power spectrum and the bias factor of the foreground galaxy population is explored. 相似文献
152.
The nucleus of Comet C/2001 A2 (LINEAR) split several times during its recent apparition, presenting an unusual opportunity to search for chemical differences in freshly exposed material. We conducted this search using NIRSPEC at the W.M. Keck Observatory on four dates in 2001: 9.5 and 10.5 July and 4.4 and 10.5 August. We detected the R0 and R1 lines of the ν3 vibrational band of CH4 near 3.3 μm on all dates. The R2 line was detected on 4.4 and 10.5 August. When we compare production rates of CH4 to H2O, we find evidence of a significant enhancement in August relative to that found in July. H2CO was securely detected via its ν1 and ν5 bands on 9.5 July. On 10.5 July, H2CO emission was much weaker, and its mixing ratio had dropped by a factor of about four. The mixing ratios for other detected volatile species did not change significantly over the course of the observations. We discuss the implications of this evidence for chemical heterogeneity in the nucleus of Comet C/2001 A2. 相似文献
153.
We present profiles of the line-of-sight (l.o.s.) ionospheric wind velocities in the southern auroral/polar region of Saturn. Our velocities are derived from the measurement of Doppler shifting of the H3+ν2Q(1,0−) line at 3.953 microns. The data for this study were obtained using the facility high-resolution spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, during the night of February 6, 2003 (UT). The l.o.s. velocity profiles finally derived are consistent with an extended region of the upper atmosphere sub-corotating with the planet: the ion velocities in the inertial reference are only 1/3 of those expected for full planetary corotation. We discuss the results in the light of recent proposals for the kronian magnetosphere, and suggest that, in this region, Saturn's ion winds may be under solar wind control. 相似文献
154.
We present, for the first time, the main sources of sporadic meteors as inferred from meteor-head echoes obtained by a high-power large-aperture radar (HPLAR). Such results have been obtained at the Jicamarca HPLAR (11.95° S, 76.87° W, 1° dip angle). Observations are based on close to 170,000 meteors detected in less than 90 h spread over 14 days, between November 2001 and February 2006. Meteors with solar orbits are observed to come from basically six previously known sources, i.e., North and South Apex, Helion, Anti-Helion, and North and South Toroidal, representing ∼91% of the observations. The other ∼9% represents meteors with observed velocities greater than the Sun's escape velocity at 1 AU, most of them of extra-solar origin. Results are given before and after removing the Earth's velocity and the sources are modeled with two-dimensional Gaussian distributions. In general, our results are in very good agreement with previously known sources reported by Jones and Brown [Jones, J., Brown, P.G., 1993. Mon. Not. R. Astron. Soc. 265, 524-532] using mainly specular meteor radar (SMR) data gathered over many years and different sites. However, we find slightly different locations and widths, that could be explained on the basis of different sensitivities of the two techniques and/or corrections needed to our results. For example, we find that the North and South Apex sources are well defined and composed each of them of two collocated Gaussian distributions, one almost isotropic with ∼10° width and the other very narrow in ecliptic longitude and wide in ecliptic latitude. This is the first time these narrow-width sources are reported. A careful quantitative analysis is needed to be able to compare the strengths of meteor sources as observed with different techniques. We also present speed and initial altitude distributions for selected sources. Using a simple angular sensitivity function of the combined Earth-atmosphere-radar instrument, and an altitude selection criteria, the resulting meteor sources are in better qualitative agreement with the results obtained with SMRs. 相似文献
155.
156.
The backscattered reflectivity of Jupiter's ring has been previously measured over distinct visible and near infrared wavelength bands by a number of ground-based and spaceborne instruments. We present spectra of Jupiter's main ring from 2.21-2.46 μm taken with the NIRSPEC spectrometer at the W.M. Keck observatory. At these wavelengths, scattered light from Jupiter is minimal due to the strong absorption of methane in the planet's atmosphere. We find an overall flat spectral slope over this wavelength interval, except for a possible red slope shortward of 2.25 μm. We extended the spectral coverage of the ring to shorter wavelengths by adding a narrow-band image at 1.64 μm, and show results from 2.27-μm images over phase angles of 1.2°-11.0°. Our images at 1.64 and 2.27 μm reveal that the halo contribution is stronger at the shorter wavelength, possibly due to the redder spectrum of the ring parent bodies as compared with the halo dust component. We find no variation in main ring reflectivity over the 1.2°-11.0° phase angle range at 2.27 μm. We use adaptive optics imaging at the longer wavelength L′ band (3.4-4.1 μm) to determine a 2-σ upper limit of 22 m of vertically-integrated I/F. Our observing campaign also produced an L′ image of Callisto, showing a darker leading hemisphere, and a spectrum of Amalthea over the 2.2-2.5 and 2.85-3.03 μm ranges, showing deep 3-μm absorption. 相似文献
157.
158.
159.
160.