全文获取类型
收费全文 | 59900篇 |
免费 | 829篇 |
国内免费 | 430篇 |
专业分类
测绘学 | 1261篇 |
大气科学 | 3920篇 |
地球物理 | 11665篇 |
地质学 | 21528篇 |
海洋学 | 5653篇 |
天文学 | 13798篇 |
综合类 | 109篇 |
自然地理 | 3225篇 |
出版年
2022年 | 385篇 |
2021年 | 721篇 |
2020年 | 708篇 |
2019年 | 814篇 |
2018年 | 1688篇 |
2017年 | 1525篇 |
2016年 | 1796篇 |
2015年 | 907篇 |
2014年 | 1637篇 |
2013年 | 3108篇 |
2012年 | 1873篇 |
2011年 | 2506篇 |
2010年 | 2303篇 |
2009年 | 2963篇 |
2008年 | 2493篇 |
2007年 | 2705篇 |
2006年 | 2440篇 |
2005年 | 1746篇 |
2004年 | 1698篇 |
2003年 | 1657篇 |
2002年 | 1631篇 |
2001年 | 1461篇 |
2000年 | 1300篇 |
1999年 | 1075篇 |
1998年 | 1073篇 |
1997年 | 1092篇 |
1996年 | 879篇 |
1995年 | 861篇 |
1994年 | 807篇 |
1993年 | 713篇 |
1992年 | 699篇 |
1991年 | 653篇 |
1990年 | 663篇 |
1989年 | 664篇 |
1988年 | 607篇 |
1987年 | 709篇 |
1986年 | 637篇 |
1985年 | 785篇 |
1984年 | 819篇 |
1983年 | 759篇 |
1982年 | 682篇 |
1981年 | 718篇 |
1980年 | 610篇 |
1979年 | 605篇 |
1978年 | 576篇 |
1977年 | 554篇 |
1976年 | 497篇 |
1975年 | 496篇 |
1974年 | 488篇 |
1973年 | 530篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Surface waters of Alsea Bay, an unpolluted estuary on the Oregon coast, were analysed for nitrous oxide, nitrate and nitrite on a weekly or biweekly basis during the summer of 1979. The estuary was found to be a variable source of N2O to the atmosphere. Large and rapid increases in the concentrations of N2O, NO3?, and NO2? occurred at the beginning of the sampling period and are attributed to the influx of nutrient-rich upwelling water into the estuary with the tide. The subsequent decline in concentrations of nitrate, nitrite and nitrous oxide over the remainder of the summer is attributed to a decrease in upwelling intensity, a decline in nitrification rates and to assimilatory nitrate reduction. Measurements of nitrous oxide at six stations along the Alsea River were also made in September and October before and after the onset of the rainy season. Samples taken after flood conditions were established were systematically 50% higher than pre-flood samples. The data suggest that soil runoff results in elevated concentrations of N2O in rivers. 相似文献
102.
103.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
104.
Farmland on the Downs between Lewes and Brighton suffered severe erosion during the autumn of 1982. The erosion was widespread and affected a variety of topographic situations, but it was confined to areas of arable land and recently-sown grass leys. Erosion on the scale recorded during the autumn of 1982 has never previously been recorded from this area. Three major sites of erosion are described and explanations for the erosion are sought through an analysis of rainfall conditions experienced during autumn 1982 and in recent changes in agricultural land use on the Downs. It is concluded that, whilst total rainfall and the intensity of rainstorms were both unusually high, similar events are likely to recur several times a century. Evidence is presented which suggests that the ploughing up of permanent pasture, the removal of field boundaries and the increased adoption of autumn-sown cereals have all contributed to the onset of severe erosion. It appears that a major re-activation of erosion on the Downs may be commencing which threatens the long-term viability of farming in the area. 相似文献
105.
The existence of prolonged periods of abnormally low solar activity (such as the Maunder minimum) is explained within the framework of Leighton's model of a solar cycle with a hypothetical internal magnetic field of the Sun taken into account. 相似文献
106.
107.
We performed polarization observations of giant radio pulses from the millisecond pulsar B1937+21. The observations were carried out in July 2002 with the 64-m Kalyazin radio telescope at a frequency of 600 MHz in two polarization channels with left-and right-hand circular polarizations (RCP and LCP). We used the S2 data acquisition system with a time resolution of 125 ns. The duration of an observing session was 20 min. We detected twelve giant radio pulses with peak flux densities higher than 1000 Jy; five and seven of these pulses appeared in the RCP and LCP channels, respectively. We found no event that exceeded the established detection threshold simultaneously in the two polarization channels. Thus, we may conclude that the detected giant pulses have a high degree of circular polarization, with the frequency of occurrence of RCP and LCP pulses being the same. 相似文献
108.
Recently, exact spatially-homogeneous solutions have been found for a conformally invariant massless scalar fieldS(t) coupled to a Robertson-Walker geometry. We investigate extending these solutions to inhomogeneous scalar fieldsS(t, x
l
). For an isotropic scalar fieldS(t, r) we find a solution satisfying three of the five field equations. If we use REDUCE, we show that the remaining equations forceS=S(t). 相似文献
109.
P. A. M. Van Hoof M. E. Foord R. F. Heeter J. E. Bailey H.-K. Chung M. E. Cuneo W. H. Goldstein V. Jonauskas F. P. Keenan R. Kisielius D. A. Liedahl C. Ramsbottom S. J. Rose P. T. Springer R. S. Thoe 《Astrophysics and Space Science》2005,298(1-2):147-153
In experiments at the high-power Z-facility at Sandia National Laboratory in Albuquerque, New Mexico, we have been able to
produce a low density photoionized laboratory plasma of Fe mixed with NaF. The conditions in the experiment allow a meaningful
comparison with X-ray emission from astrophysical sources. The charge state distributions of Fe, Na and F are determined in
this plasma using high resolution X-ray spectroscopy. Independent measurements of the density and radiation flux indicate
unprecedented values for the ionization parameter ξ = 20–25 erg cm s−1 under nearly steady-state conditions. First comparisons of the measured charge state distributions with X-ray photoionization
models show reasonable agreement, although many questions remain. 相似文献