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   49817篇
  免费   2193篇
  国内免费   3282篇
测绘学   1616篇
大气科学   4073篇
地球物理   9750篇
地质学   20828篇
海洋学   5449篇
天文学   9873篇
综合类   1096篇
自然地理   2607篇
  2023年   366篇
  2022年   1153篇
  2021年   1527篇
  2020年   1461篇
  2019年   1565篇
  2018年   2476篇
  2017年   2287篇
  2016年   2410篇
  2015年   1317篇
  2014年   2206篇
  2013年   2800篇
  2012年   2379篇
  2011年   2751篇
  2010年   2511篇
  2009年   2672篇
  2008年   2425篇
  2007年   2590篇
  2006年   2310篇
  2005年   1269篇
  2004年   1152篇
  2003年   1198篇
  2002年   1181篇
  2001年   1171篇
  2000年   936篇
  1999年   776篇
  1998年   708篇
  1997年   758篇
  1996年   586篇
  1995年   601篇
  1994年   550篇
  1993年   450篇
  1992年   428篇
  1991年   385篇
  1990年   410篇
  1989年   371篇
  1988年   325篇
  1987年   314篇
  1986年   237篇
  1985年   338篇
  1984年   334篇
  1983年   323篇
  1982年   303篇
  1981年   268篇
  1980年   278篇
  1979年   221篇
  1978年   250篇
  1977年   213篇
  1976年   183篇
  1975年   195篇
  1973年   213篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
83.
CCD photometry data of the T Tauri star H 187 are presented. They show that a new eclipse of this star began at the end of 2004. Since only one eclipse of this star has been observed previously with a duration of 3.5 years, our data indicate that the eclipses of this object are periodic with a period P=4.9 years between eclipses. Thus, in terms of the absolute duration of the eclipses and in terms of their relative length as a fraction of the period, H 187 is one of the most exotic objects in the sky. __________ Translated from Astrofizika, Vol. 48, No. 4, pp. 529–534 (November 2005).  相似文献   
84.
Possible orbits for the motion of a region in the gravitational field of the central body of the galaxy NGC 4151 are presented. The region is manifest through its line emission, observed in the red wing of the Hα, Pβ, and other broad lines. We carried out a computer selection of all Keplerian orbits for which the measured radial velocities of the emission-line region could be observed. We used radial-velocity data obtained by us at the Fesenkov Astrophysical Institute, as well as data from the literature. The computation results can be used to determine the mass range for the central body of NGC 4151 that provides the best agreement with the observational data: (61–65)× 106 M . Suitably designed monitoring of active galactic nuclei can be used to verify these results, and to carry out similar analyses for other Seyfert galaxies.  相似文献   
85.
Variations in the cosmic ray density during the initial phase of the Forbush effect during the first hours after the arrival of the interplanetary shock wave have been studied with the use of data on variations in the cosmic ray density with a rigidity of 10 GV obtained by the global survey method by the world network of neutron monitors in 1957?2012. It is found that behavior of this parameter after the arrival of the shock wave demonstrates high variability. A small (~1/5 of total number), though distinct, group of Forbush effects, in which the density of the cosmic ray increases (not decreases) after the arrival of the shock wave, is defined. As a whole, the initial variation in cosmic ray density is correlated with the Forbush effect magnitude and the strength of the associated geomagnetic disturbance.  相似文献   
86.
Dust storms cause significant damage to health, property, and the environment worldwide every year. To help mitigate the damage, dust forecast models simulate and predict upcoming dust events, providing valuable information to scientists, decision makers, and the public. These simulation outputs are in four-dimensions (i.e., latitude, longitude, elevation, and time) and represent spatially heterogeneous dust storm features and their evolution over space and time. This research investigates and proposes an automatic multi-threshold, region-growing-based algorithm to identify critical dust storm features from 3D dust storm simulations. A multi-threshold scheme is defined for the identification of dust storm features with different dust concentrations. Based on the multi-thresholds, dust storm features are iteratively identified by developing a region-growing algorithm that splits a clustered dust storm feature into multiple sub-features. The proposed approach is compared with three commonly used methods in image processing and thunderstorm identification. The proposed approach outperforms the other three methods in sensitivity and quantitative/qualitative accuracy. This research approach may also be slightly adjusted to identify critical 3D features from simulation outputs for other severe weather and geographical phenomena.  相似文献   
87.
88.
The results of studying spread F obtained from the DPS-4 ionosonde data at the observatory of the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (Moscow) are presented. The methodical questions that arise during the study of a spread F phenomenon in the ionosphere are considered; the current results of terrestrial observations are compared with previously published data and the results of sounding onboard an Earth-satellite vehicle. The automated algorithm for estimation of the intensity of frequency spread F, which was developed by the authors and was successfully verified via comparison of the data of the digisonde DPS-4 and the results of manual processing, is described. The algorithm makes it possible to quantify the intensity of spread F in megahertz (the dFs parameter) and in the number of points (0, 1, 2, 3). The strongest spread (3 points) is shown to be most likely around midnight, while the weakest spread (0 points) is highly likely to occur during the daytime. The diurnal distribution of a 1–2 point spread F in the winter indicates the presence of additional maxima at 0300–0600 UT and 1400–1700 UT, which may appear due to the terminator. Despite the large volume of processed data, we can not definitively state that the appearance of spread F depends on the magnetic activity indices Kp, Dst, and AL, although the values of the dFs frequency spread interval strongly increased both at day and night during the magnetic storm of March 17–22, 2015, especially in the phase of storm recovery on March 20–22.  相似文献   
89.
90.
The existence of macroscopic regions with antibaryon excess in the baryon asymmetric Universe with general baryon excess is the possible consequence of practically all models of baryosynthesis. Diffusion of matter and antimatter to the border of antimatter domains defines the minimal scale of the antimatter domains surviving to the present time. A model of diffused antiworld is considered, in which the density within the surviving antimatter domains is too low to form gravitationally bound objects. The possibility to test this model by measurements of cosmic gamma ray fluxes is discussed. The expected gamma ray flux is found to be acceptable for modern cosmic gamma ray detectors and for those planned for the near future.  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