首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   283741篇
  免费   10693篇
  国内免费   10822篇
测绘学   9181篇
大气科学   25281篇
地球物理   60666篇
地质学   102438篇
海洋学   24740篇
天文学   56642篇
综合类   4484篇
自然地理   21824篇
  2022年   2568篇
  2021年   3708篇
  2020年   3783篇
  2019年   4340篇
  2018年   4840篇
  2017年   4403篇
  2016年   7262篇
  2015年   5718篇
  2014年   8715篇
  2013年   15922篇
  2012年   8083篇
  2011年   9337篇
  2010年   8581篇
  2009年   11042篇
  2008年   9831篇
  2007年   9088篇
  2006年   10794篇
  2005年   8715篇
  2004年   8409篇
  2003年   7877篇
  2002年   7459篇
  2001年   6820篇
  2000年   6918篇
  1999年   6624篇
  1998年   6343篇
  1997年   6085篇
  1996年   5636篇
  1995年   5274篇
  1994年   4832篇
  1993年   4492篇
  1992年   4134篇
  1991年   3996篇
  1990年   4057篇
  1989年   3797篇
  1988年   3544篇
  1987年   3971篇
  1986年   3534篇
  1985年   4290篇
  1984年   4809篇
  1983年   4460篇
  1982年   4364篇
  1981年   3962篇
  1980年   3660篇
  1979年   3535篇
  1978年   3485篇
  1977年   3289篇
  1976年   2962篇
  1975年   2901篇
  1974年   2866篇
  1973年   3059篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Photoionization by stars cannot be the only cause for the diffuseionized gas component in the distant halos of galaxies.Quasistationary localized magnetic reconnection can efficientlycontribute to the ionization of the extraplanar halo gas. The gas isheated and re-ionized in numerous current filaments fast enough tocompensate recombination. The necessary magnetic energy is provided bythe disk activity.  相似文献   
952.
D.L. Mickey 《Solar physics》2004,220(1):21-27
I describe a method for quickly and accurately determining the plate parallelism in a tunable Fabry-Pérot interferometer. The method takes advantage of the fact that our Fabry-Pérot is installed in a spectropolarimeter: we measure profiles of solar or telluric absorption lines in three differently polarized telescope subapertures and use the residual polarization profiles to determine the plate parallelism error. An example of the error as a function of time during the observing day is also shown; it changes in a consistent way with telescope hour angle but the drift is enough to require frequent adjustment.  相似文献   
953.
An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented  相似文献   
954.
The structure of beam noise measured at the output of a vertical array in a range dependent ocean basin was investigated using the modified wide-angle parabolic equation (PE). Noise sources were distributed throughout the basin, and the field due to each noise source at an array located in the midbasin was calculated. The response of the array to the superposition of the noise sources was found by beamforming. An efficient and direct approach that superimposes the noise sources on the PE field as the field is marched toward the array was developed. Downslope calculations of the midbasin vertical directionality were made between 50 and 400 Hz with this technique. Use of a geoacoustic model shows that the bottom behaves as a low-pass filter  相似文献   
955.
A four-year record from an inverted echo sounder deployed near Palmyra Island at 6°N in the central Pacific Ocean is compared with a simultaneous record of subsurface pressure from this island lagoon. A factor m, converting round-trip acoustic travel time to surface dynamic height relative to a deep pressure level, was estimated from the ratio of the spectra of the two records in the energetic synoptic oscillation band. Year-to-year variation in m was not statistically significant. For the overall record, m was found to be -70±8 dynamic m/s, where the error bounds represent a 90% confidence interval. This is consistent with first-baroclinic-mode excitation  相似文献   
956.
957.
958.
Astronomy Letters - The acceleration of anomalous cosmic rays (ACRs) at the heliospheric termination shock and their influence on the shock structure and location are analyzed in terms of a...  相似文献   
959.
960.
A. Gerdes 《地学学报》2001,13(4):305-312
Recent studies have shown that melts and residues may not equilibrate during anatexis, and uncertainty exists about the scale on which magmas can be homogenized. This study of elemental and isotopic homogeneity of the South Bohemian Weinsberg granites (˜ 5000 km2) identifies three voluminous, relatively homogeneous magma batches. Each batch has different 87Sr/86Srinit (0.7080, 0.7093 and 0.7106), but all equilibrated at ˜ 327–329 Ma, very similar to the time of monazite crystallization. The data cannot entirely prove melt/residue equilibration during anatexis. However, elemental and isotopic compositions imply magma generation by partial melting of heterogeneous South Bohemian crust and chemical differentiation subsequent to Sr-isotope equilibration. Assuming relatively rapid ascent and solidification rates, magma homogenization must have occurred mostly just after partial melting, during melt segregation and accumulation in the deeper crust with slow prograde heating. Models of rapid crustal heating and instantaneous melt extraction are incompatible with the data.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号