全文获取类型
收费全文 | 4483篇 |
免费 | 572篇 |
国内免费 | 768篇 |
专业分类
测绘学 | 550篇 |
大气科学 | 1100篇 |
地球物理 | 1490篇 |
地质学 | 569篇 |
海洋学 | 421篇 |
天文学 | 1104篇 |
综合类 | 346篇 |
自然地理 | 243篇 |
出版年
2024年 | 27篇 |
2023年 | 79篇 |
2022年 | 121篇 |
2021年 | 156篇 |
2020年 | 170篇 |
2019年 | 188篇 |
2018年 | 144篇 |
2017年 | 158篇 |
2016年 | 150篇 |
2015年 | 230篇 |
2014年 | 257篇 |
2013年 | 266篇 |
2012年 | 251篇 |
2011年 | 311篇 |
2010年 | 254篇 |
2009年 | 321篇 |
2008年 | 271篇 |
2007年 | 299篇 |
2006年 | 249篇 |
2005年 | 253篇 |
2004年 | 203篇 |
2003年 | 199篇 |
2002年 | 166篇 |
2001年 | 126篇 |
2000年 | 142篇 |
1999年 | 138篇 |
1998年 | 109篇 |
1997年 | 88篇 |
1996年 | 71篇 |
1995年 | 86篇 |
1994年 | 112篇 |
1993年 | 51篇 |
1992年 | 41篇 |
1991年 | 26篇 |
1990年 | 24篇 |
1989年 | 24篇 |
1988年 | 17篇 |
1987年 | 15篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 5篇 |
排序方式: 共有5823条查询结果,搜索用时 31 毫秒
11.
采用新光压模型ECOMC和ECOM-9,分别使用5种IGU精密超快速星历进行精密定轨,以最终IGS精密星历作为真值,比较GPS卫星的定轨精度.结果 表明,相较于ECOM-9光压模型,ECOMC光压模型能够提升卫星径向、切向和法向的轨道精度,其中径向提升较为明显;对比5种IGU超快速精密星历的定轨结果,ECOMC光压模型... 相似文献
12.
13.
We present the thermal infrared (5-35 μm) spectrum of 956 Elisa as measured by the Spitzer Infrared Spectrograph (“IRS”; Houck, J.R. et al. [2004]. Astrophys. J. Suppl. 154, 18-24) together with new groundbased lightcurve data and near-IR spectra. From the visible lightcurve photometry, we determine a rotation period of 16.494 ± 0.001 h, identify the rotational phase of the Spitzer observations, and estimate the visible absolute magnitude (HV) at that rotational phase to be 12.58 ± 0.04. From radiometric analysis of the thermal flux spectrum, we find that at the time of observation 956 Elisa had a projected radius of 5.3 ± 0.4 km with a visible albedo pV = 0.142 ± 0.022, significantly lower than that of the prototype V-type asteroid, 4 Vesta. (This corresponds to a radius of 5.2 ± 0.4 km at lightcurve mean.) Analysis with the standard thermal model (STM) results in a sub-solar temperature of 292.3 ± 2.8 K and beaming parameter η = 1.16 ± 0.05. Thermophysical modeling places a lower limit of on the thermal inertia of the asteroid’s surface layer (if the surface is very smooth) but more likely values fall between 30 and depending on the sense of rotation.The emissivity spectrum, calculated by dividing the measured thermal flux spectrum by the modeled thermal continuum, exhibits mineralogically interpretable spectral features within the 9-12 μm reststrahlen band, the 15-16.5 μm Si-O-Si stretching region, and the 16-25 μm reststrahlen region that are consistent with pyroxene of diogenitic composition: extant diogenitic pyroxenes fall within the narrow compositional range Wo2±1En74±2Fs24±1. Spectral deconvolution of the 9-12 μm reststrahlen features indicates that up to ≈20% olivine may also be present, suggesting an olivine-diogenite-like mineralogy. The mid-IR spectrum is inconsistent with non-cumulate eucrite as the major component on the surface of 956 Elisa, although cumulate eucrite material may be present at abundances lower than that of the diogenite component.Analysis of new near-IR spectra of 956 Elisa with the Modified Gaussian Model (MGM; Sunshine, J.M., Pieters, C.M., Pratt, S.F. [1990]. J. Geophys. Res. 95 (May), 6955-6966) results in two pyroxene compositions: 75% magnesian low-Ca pyroxene and 25% high-Ca pyroxene. High-Ca pyroxene is not evident in the mid-IR data, but may belong to a component that is underrepresented in the mid-IR spectrum either because of its spatial distribution on the asteroid or because of its particle size. High-Ca pyroxenes that occur as exsolution lamellae may also be more evident spectrally in the NIR than in the mid-IR. In any case, we find that the mid-IR spectrum of 956 Elisa is dominated by emission from material of diogenite-like composition, which has very rarely been observed among asteroids. 相似文献
14.
The distribution of the suspended sediment concentration (SSC) in the Bohai Sea, Yellow Sea and East China Sea (BYECS) is studied based on the observed turbidity data and model simulation results. The observed turbidity results show that (i) the highest SSC is found in the coastal areas while in the outer shelf sea areas turbid water is much more difficult to observe, (ii) the surface layer SSC is much lower than the bottom layer SSC and (iii) the winter SSC is higher than the summer SSC. The Regional Ocean Modeling System (ROMS) is used to simulate the SSC distribution in the BYECS. A comparison between the modeled SSC and the observed SSC in the BYECS shows that the modeled SSC can reproduce the principal features of the SSC distribution in the BYECS. The dynamic mechanisms of the sediment erosion and transport processes are studied based on the modeled results. The horizontal distribution of the SSC in the BYECS is mainly determined by the current-wave induced bottom stress and the fine-grain sediment distribution. The current-induced bottom stress is much higher than the wave-induced bottom stress, which means the tidal currents play a more significant role in the sediment resuspension than the wind waves. The vertical mixing strength is studied based on the mixed layer depth and the turbulent kinetic energy distribution in the BYECS. The strong winter time vertical mixing, which is mainly caused by the strong wind stress and surface cooling, leads to high surface layer SSC in winter. High surface layer SSC in summer is restricted in the coastal areas. 相似文献
15.
X1908 + 075 is a highly-absorbed Galactic X-ray source likely made up of a pulsar accreting wind material from a massive companion. We have used near-IR photometric data complemented by follow-up spectroscopy to identify the likely counterpart to this X-ray source and to assign a spectral type O7.5–9.5 If to the primary. Further details can be found in Morel and Grosdidier (2005). 相似文献
16.
The gray crystalline hematite at Meridiani Planum first discovered by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) instrument occurs as spherules that have been interpreted as concretions. Analysis of the TES and mini-TES spectra shows that no 390 cm−1 feature is present in the characteristic martian hematite spectrum. Here, we incorporate the mid-IR optical constants of hematite into a simple Fresnel reflectance model to understand the effect of emission angle and crystal morphology on the presence or absence of the 390 cm−1 feature in an IR hematite spectrum. Based on the results we offer two models for the internal structure of the martian hematite spherules. 相似文献
17.
Xi-Liang Zhang Xiao-Bin Wang Li-Yun Zhang National Astronomical Observatories / Yunnan Observatory Chinese Academy of Sciences Kunming Graduate School of Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2006,6(6):729-732
We measured the light-curve of the asteroid (147) Protogeneia in November 2004, with a CCD detector attached to the 1-meter telescope at the Yunnan Observatory, China. The synodic period and maximum amplitude of (147) at this apparition are 7.852 hours and 0.25 mag, respectively. The value of a/b for (147), from a preliminary estimation, is not less than 1.26:1. 相似文献
18.
Andreas Just 《Astrophysics and Space Science》2003,284(2):727-730
There is a long term dynamical heating of stellar populations with age observed in the age – velocity dispersion – relation
(AVR). This effect allows a determination of the star formation history SFR(t) from local kinematical data of main sequence stars. Using a self-consistent disk model for the vertical structure of the
disk, we find from the kinematics of the stars in the solar neighbourhood that the SFR shows a moderate star burst about 10 Gyr ago followed by a continuous decline to the present day value consistent with the
observed number of OB stars. The gravitational potential of the gas component and of the Dark Matter Halo is included and
the effect of chemical enrichment, finite lifetime of the stars and mass loss of the stellar component are taken into account.
The scale heights for main sequence stars together with the SFR is then used to determine constistently the IMF from the observed local luminosity function. The main new result is that the power law break in the present day mass function
(PDMF) around 1 M
⊙ is entirely due to evolutionary effects of the disk and does not appear in the IMF.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
19.
20.
太阳质子事件耀斑的短期预报 总被引:1,自引:0,他引:1
王家龙 《中国天文和天体物理学报》1999,(4)
回顾产生太阳质子事件耀斑的短期预报,讨论短期预报在近期应做的研究.给出以下结论:(1)在60年代和70年代,质子事件耀斑的预报有相当大的进展;(2)新预报方法的探索和质子流在日冕与行星际的传播问题,是当前改进短期预报的关键;(3)对实际应用的短期预报工作的改进,可能需要从空间天气预报的角度,研究太阳活动区的分类. 相似文献