全文获取类型
收费全文 | 53333篇 |
免费 | 997篇 |
国内免费 | 418篇 |
专业分类
测绘学 | 1474篇 |
大气科学 | 4284篇 |
地球物理 | 10434篇 |
地质学 | 18107篇 |
海洋学 | 4336篇 |
天文学 | 12614篇 |
综合类 | 189篇 |
自然地理 | 3310篇 |
出版年
2021年 | 368篇 |
2020年 | 400篇 |
2019年 | 375篇 |
2018年 | 975篇 |
2017年 | 948篇 |
2016年 | 1258篇 |
2015年 | 839篇 |
2014年 | 1394篇 |
2013年 | 2695篇 |
2012年 | 1332篇 |
2011年 | 1705篇 |
2010年 | 1630篇 |
2009年 | 2138篇 |
2008年 | 1930篇 |
2007年 | 1894篇 |
2006年 | 1784篇 |
2005年 | 1648篇 |
2004年 | 1581篇 |
2003年 | 1473篇 |
2002年 | 1425篇 |
2001年 | 1303篇 |
2000年 | 1215篇 |
1999年 | 1170篇 |
1998年 | 1128篇 |
1997年 | 1104篇 |
1996年 | 910篇 |
1995年 | 883篇 |
1994年 | 848篇 |
1993年 | 776篇 |
1992年 | 735篇 |
1991年 | 709篇 |
1990年 | 806篇 |
1989年 | 711篇 |
1988年 | 674篇 |
1987年 | 777篇 |
1986年 | 649篇 |
1985年 | 856篇 |
1984年 | 986篇 |
1983年 | 957篇 |
1982年 | 896篇 |
1981年 | 852篇 |
1980年 | 758篇 |
1979年 | 738篇 |
1978年 | 730篇 |
1977年 | 659篇 |
1976年 | 617篇 |
1975年 | 556篇 |
1974年 | 632篇 |
1973年 | 619篇 |
1972年 | 385篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
The 362.156 GHz absorption spectrum of H2O2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H2O2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ∼10 km altitudes between the Ls=112° period of the Encrenaz et al. upper limit measurement and the current Ls=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H2O2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H2O2. Observation of Mars H2O2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own. 相似文献
23.
24.
25.
Rostov State Pedagogical Institute; All-Union Scientific Research Institute Gradient. Translated from Astrofizika, Vol. 33, No. 3, pp. 379–393, November–December, 1990. 相似文献
26.
Victor R. Bond 《Celestial Mechanics and Dynamical Astronomy》1985,35(1):1-7
A transformation of the differential equations of motion of the two-body problem in the spherical coordinates to oscillator form is derived. It is shown that the independent variable transformation dt/ds=r2 is a transformation which makes the oscillator form possible. 相似文献
27.
28.
The results of a general theoretical investigation of three commonly used types of inductive conductivity sensors, i.e., the single transformer, the double transformer, and the double transformer with an additional loop, are presented. The resulting formulas describe the dependence of the sensor output signal not only on the conductivity of the seawater but also on the parameters of the electrical circuit, among them the permeability of the transformer core(s), which-unlike the other parameters-shifts considerably during oceanographic in situ measurements. A mathematical discussion of these formulas shows that for certain circuit configurations, the sensor output is independent of changes in permeability. Most of these configurations form the basis of existing oceanographical conductivity sensors, among them the "classical" sensors developed by H. Hinkelmann [3], [4], and by N. L. Brown [14], while some others make evident further possibilities for eliminating the unwanted effects of shifting permeability. In the era of microelectronics, the latter might lead to a reassessment, especially of the single transformer-type sensor. 相似文献
29.
Transverse secondary circulations involving surface convergence, observed in a well-mixed estuary in North Wales, are made visible by the collection of surface material along an axial line which extends continuously for many kilometres through the estuary. The circulation and axial convergence, however, are seen only during the flood phase of the tide and no similar behaviour has been observed during the ebb phase.Convergent circulations in the estuary are associated with small but steady transverse density gradients in the cross-section, produced by non-uniform advection of the longitudinal gradient through the channel. A diagnostic model, using measured mean distributions of cross-sectional density, indicates surface transverse velocities (~0.1 ms?1) similar to those observed in the estuary. The model further predicts appreciable transverse divergent currents at a fractional depth of 0.75: a prediction which has been tested in the estuary using a vertical array of accurately resolving current direction indicators. 相似文献
30.