全文获取类型
收费全文 | 256738篇 |
免费 | 5289篇 |
国内免费 | 3401篇 |
专业分类
测绘学 | 6944篇 |
大气科学 | 19020篇 |
地球物理 | 54051篇 |
地质学 | 88954篇 |
海洋学 | 21349篇 |
天文学 | 55971篇 |
综合类 | 1014篇 |
自然地理 | 18125篇 |
出版年
2021年 | 2249篇 |
2020年 | 2621篇 |
2019年 | 2874篇 |
2018年 | 3268篇 |
2017年 | 2902篇 |
2016年 | 5618篇 |
2015年 | 4246篇 |
2014年 | 6931篇 |
2013年 | 14287篇 |
2012年 | 6440篇 |
2011年 | 7685篇 |
2010年 | 6680篇 |
2009年 | 9306篇 |
2008年 | 8243篇 |
2007年 | 7615篇 |
2006年 | 9730篇 |
2005年 | 7676篇 |
2004年 | 7650篇 |
2003年 | 7137篇 |
2002年 | 6770篇 |
2001年 | 6022篇 |
2000年 | 5958篇 |
1999年 | 5228篇 |
1998年 | 5247篇 |
1997年 | 5045篇 |
1996年 | 4695篇 |
1995年 | 4444篇 |
1994年 | 4113篇 |
1993年 | 3859篇 |
1992年 | 3653篇 |
1991年 | 3600篇 |
1990年 | 3777篇 |
1989年 | 3520篇 |
1988年 | 3312篇 |
1987年 | 3867篇 |
1986年 | 3419篇 |
1985年 | 4242篇 |
1984年 | 4770篇 |
1983年 | 4424篇 |
1982年 | 4318篇 |
1981年 | 3941篇 |
1980年 | 3639篇 |
1979年 | 3527篇 |
1978年 | 3494篇 |
1977年 | 3294篇 |
1976年 | 3057篇 |
1975年 | 2971篇 |
1974年 | 2923篇 |
1973年 | 3086篇 |
1972年 | 2028篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
The Cassini spacecraft, en route to Saturn, passed close to Jupiter while the Galileo spacecraft was completing its 28th and 29th orbits of Jupiter, thus offering a unique opportunity for direct study of the solar wind-Jovian interaction. Here evidence is given of response of the Jovian magnetopause and bow shock positions to changes of the north-south component of the solar wind magnetic field, a phenomenon long known to occur in equivalent circumstances at Earth. The period analyzed starts with the passage over Cassini of an interplanetary shock far upstream of Jupiter. The shock's arrival at Galileo on the dusk-flank of the magnetosphere caused Galileo to exit into the solar wind. Using inter-spacecraft timing based on the time delay established from the shock arrival at each spacecraft, we point out that Galileo's position with respect to the Jovian bow shock appears to correlate with changes in the disturbed north-south reversing field seen behind the shock. We specifically rule out the alternative of changes in the shape of the bow shock with rotations of the interplanetary magnetic field as the cause. 相似文献
53.
54.
55.
56.
57.
青藏高原隆升的非线性动态有限元仿真研究 总被引:7,自引:4,他引:3
根据青藏高原的地质特征建立分析模型,采用3维动态有限元方法,在计算仿真板块速度场的基础上,计算在青藏高原的隆升过程中该地区地壳岩石的等效应力和位移随时间的变化,计算仿真得到的速度场与1998年GPS观测的速度场吻合良好;与过去一贯的假设相反,计算结果反映出地壳应力场不是静态的,而是此起彼伏,不断变化的,应力值最大且变化最剧烈的地区在克什米尔地区、鄂尔多斯地区和鲜水河-小江断裂带,与地震多发区域吻合。 相似文献
58.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
59.
The kinetics of the reactions of C2H radical with ethane (k1), propane (k2), and n-butane (k3) are studied over the temperature range of T = 96-296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis-chemiluminescence technique. The C2H decay profiles in the presence of both the alkane reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k1(T) = (0.51 ± 0.06) × 10−10 exp[(−76 ± 30)K/T] cm3 molecule−1 s−1 (T = 96-800 K), k2(T) = (0.98 ± 0.32) × 10−10exp[(−71 ± 60)K/T] cm3 molecule−1 s−1 (T = 96-361 K), and k3(T) = (1.23 ± 0.26) × 10−10 cm3 molecule−1 s−1 (T = 96-297 K). At T = 296 K, k1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed. 相似文献
60.
Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure. 相似文献