全文获取类型
收费全文 | 2910篇 |
免费 | 95篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 73篇 |
大气科学 | 225篇 |
地球物理 | 583篇 |
地质学 | 837篇 |
海洋学 | 306篇 |
天文学 | 707篇 |
综合类 | 2篇 |
自然地理 | 286篇 |
出版年
2021年 | 22篇 |
2020年 | 29篇 |
2019年 | 42篇 |
2018年 | 55篇 |
2017年 | 41篇 |
2016年 | 64篇 |
2015年 | 51篇 |
2014年 | 66篇 |
2013年 | 157篇 |
2012年 | 72篇 |
2011年 | 127篇 |
2010年 | 91篇 |
2009年 | 162篇 |
2008年 | 113篇 |
2007年 | 121篇 |
2006年 | 107篇 |
2005年 | 88篇 |
2004年 | 86篇 |
2003年 | 85篇 |
2002年 | 81篇 |
2001年 | 77篇 |
2000年 | 81篇 |
1999年 | 63篇 |
1998年 | 70篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 41篇 |
1994年 | 46篇 |
1993年 | 37篇 |
1992年 | 33篇 |
1991年 | 36篇 |
1990年 | 32篇 |
1989年 | 35篇 |
1988年 | 24篇 |
1987年 | 43篇 |
1986年 | 29篇 |
1985年 | 51篇 |
1984年 | 53篇 |
1983年 | 53篇 |
1982年 | 47篇 |
1981年 | 47篇 |
1980年 | 44篇 |
1979年 | 33篇 |
1978年 | 28篇 |
1977年 | 38篇 |
1976年 | 30篇 |
1975年 | 34篇 |
1974年 | 14篇 |
1973年 | 21篇 |
1972年 | 20篇 |
排序方式: 共有3019条查询结果,搜索用时 15 毫秒
991.
X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized. 相似文献
992.
We explore a systematic error that arises in feature-tracking measurements of time-average rotation. It stems from the flows of features across latitudes, and as these flows vary with the solar activity cycle, the error has a pattern of variation which mocks the torsional oscillation. We develop a series expansion for this error and evaluate the leading terms for the example case of cycle 21. It grows with the time lag; for a 30 day lag it is 1%, depending on how the correlations are done and interpreted. We conclude that the mock pattern cannot, however, account for the magnetic-rotation torsional oscillations pattern found in recent analyses of magnetograms from Kitt Peak and Mount Wilson. For the 1-day time lag in the Kitt Peak study, the error is negligible, and for the 30-day time lag in the Mount Wilson study, it represents at most about 30% of the signal. 相似文献
993.
994.
The gravitational attraction of any polygonally shaped vertical prism with inclined top and bottom faces 总被引:2,自引:0,他引:2
D. A. Smith 《Journal of Geodesy》2000,74(5):414-420
A closed formula for computing the gravitational attraction of a general vertical prism with N+2 faces (N faces are vertical planes, the other two are the inclined top and bottom planes) in Cartesian coordinates is presented. In
addition, the special case of a triangular prism is discussed. Algebraic differences and overlooked singularity conditions
of a previously published formula of this computation (which was only for the triangular special case) were identified and
are also presented.
Received: 22 March 1999 / Accepted: 2 February 2000 相似文献
995.
Summary Two numerical models are used to investigate aspects of thunderstorm dynamics and thunderstorm initiation in the northern
Alpine foreland. The first, an isentropic model of airflow over and around the Alps, is used to investigate flow patterns
favourable for the initiation of deep convection in the region. It is found that a stably-stratified southerly flow towards
the Alps leads to a southwesterly flow in the Alpine foreland, a situation most often found during thunderstorm periods, and
to the formation of a gravity wave in the lee of the Alps. This wave is accompanied by raised isentropes which, in reality,
would lead to a reduction in static stability and convective inhibition as well as an increase in convective available potential
energy. The second model, a cloud model, is used to study the development of an observed squall line over southern Bavaria.
The model is initialized with wind, temperature and moisture profiles from a radiosonde sounding ahead of the squall line
and the squall line is initiated by an array of thermal bubbles. The model simulation is used to interpret the evolution of
the squall line.
Received March 9, 1999/Revised July 10, 1999 相似文献
996.
Observations by the EISCAT Svalbard radar in summer have revealed electron density enhancements in the magnetic noon sector under conditions of IMF Bz southward. The features were identified as possible candidates for polar-cap patches drifting anti-Sunward with the plasma flow. Supporting measurements by the EISCAT mainland radar, the CUTLASS radar and DMSP satellites, in a multi-instrument study, suggested that the origin of the structures lay upstream at lower latitudes, with the modulation in density being attributed to variability in soft-particle precipitation in the cusp region. It is proposed that the variations in precipitation may be linked to changes in the location of the reconnection site at the magnetopause, which in turn results in changes in the energy distribution of the precipitating particles. 相似文献
997.
弹塑性有限元模拟分析影响基坑位移的因素 总被引:1,自引:0,他引:1
分析影响基坑支护破坏因素对于防止基坑事故的发生,优化基坑支护设计有重要意义。本文用弹塑性有限元方法模拟基坑破坏过程,分析了影响基坑支护破坏的因素。分析结果表明,土体和桩墙的弹性模量都对基坑的位移有一定影响,但土体弹性模量的影响比桩墙弹性模量的影响更大。根据这些结果,推论出传统基坑设计方法与弹性地基梁法的不足之处。另外,分析结果还表明,基坑坑顶水平位移与土体弹性模量的关系无固定规律,而坑底最大位移与弹性模量呈非线性的关系。由此推论出基坑坑顶水平位移监测方法的不可靠性。 相似文献
998.
E. P. Hamerlynck T. E. Huxman R. S. Nowak S. Redar M. E. Loik D. N. Jordan S. F. Zitzer J. S. Coleman J. R. Seemann S. D. Smith 《Journal of Arid Environments》2000,44(4):425
Of all terrestrial ecosystems, the productivity of deserts has been suggested to be the most responsive to increasing atmospheric CO2. The extent to which this prediction holds will depend in part on plant responses to elevated CO2under the highly variable conditions characteristic of arid regions. The photosynthetic responses ofLarrea tridentata , an evergreen shrub, to a step-increase in atmospheric CO2(to 550 μmolmol−1) were examined in the field using Free-Air CO2Enrichment (FACE) under seasonally varying moisture conditions. Elevated CO2substantially increased net assimilation rate (Anet) in Larrea during both moist and dry periods of the potential growing season, while stomatal conductance (gs) did not differ between elevated and ambient CO2treatments. Seasonal and diurnal gas exchange dynamics in elevated CO2mirrored patterns in ambient CO2, indicating that elevated CO2did not extend photosynthetic activity longer into the dry season or during more stressful times of the day. Net assimilation vs. internal CO2(A/Ci) responses showed no evidence of photosynthetic down-regulation during the dry season. In contrast, after significant autumn rains, Amax(the CO2saturated rate of photosynthesis) and CE (carboxylation efficiency) were lower in Larrea under elevated CO2. In situ chlorophyll fluorescence estimation ofLarrea Photosystem II efficiency (Fv/Fm) responded more to water limitation than to elevated CO2. These findings suggest that predictions regarding desert plant responses to elevated CO2should account for seasonal patterns of photosynthetic regulatory responses, which may vary across species and plant functional types. 相似文献
999.
Mounting evidence suggests that the P-glycoprotein (pgp) efflux pump may be a modulator of bioavailability and a mode of excretion for xenobiotics. Immunohistochemistry was utilized to examine the distribution and inducibility of a pgp like protein in catfish. Immunoreactivity to the MDR C-219 monoclonal antibody was noted primarily in bile canaliculi or bile preductules of the liver, discrete areas of the extratubular region of the kidney and the columnar epithelia of the intestine. Regional differences in pgp content were noted in the intestine with the distal region containing greater pgp levels than the proximal intestine. Dietary administration of vincristine, a prototypic pgp inducer and beta-naphthoflavone an Ah agonist resulted in induction of the C-219 immunoreactivity in the liver and the distal intestine. These results are consistent in location and inducibility with pgp like proteins and support a possible relationship to xenobiotic absorption and/or excretion in the catfish. 相似文献
1000.
G. Handler T. Arentoft R. R. Shobbrook M. A. Wood L. A. Crause P. Crake F. Podmore A. Habanyama T. Oswalt P. V. Birch G. Lowe C. Sterken P. Meintjes J. Brink C. F. Claver R. Medupe J. A. Guzik T. E. Beach P. Martinez E. M. Leibowitz P. A. Ibbetson T. Smith B. N. Ashoka N. E. Raj D. W. Kurtz L. A. Balona D. O'donoghue J. E. S. Costa § M. Breger 《Monthly notices of the Royal Astronomical Society》2000,318(2):511-525