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   458篇
  免费   8篇
  国内免费   5篇
测绘学   10篇
大气科学   21篇
地球物理   102篇
地质学   84篇
海洋学   51篇
天文学   139篇
自然地理   64篇
  2021年   3篇
  2019年   4篇
  2018年   8篇
  2017年   5篇
  2016年   8篇
  2015年   6篇
  2014年   6篇
  2013年   21篇
  2012年   9篇
  2011年   11篇
  2010年   18篇
  2009年   16篇
  2008年   14篇
  2007年   15篇
  2006年   15篇
  2005年   19篇
  2004年   15篇
  2003年   19篇
  2002年   15篇
  2001年   12篇
  2000年   14篇
  1999年   5篇
  1998年   5篇
  1997年   6篇
  1996年   9篇
  1995年   6篇
  1994年   7篇
  1993年   7篇
  1992年   5篇
  1990年   5篇
  1989年   7篇
  1988年   5篇
  1987年   4篇
  1986年   4篇
  1985年   12篇
  1984年   13篇
  1983年   11篇
  1982年   6篇
  1981年   10篇
  1980年   4篇
  1979年   6篇
  1978年   8篇
  1977年   12篇
  1976年   10篇
  1975年   9篇
  1974年   6篇
  1973年   3篇
  1972年   4篇
  1970年   4篇
  1965年   3篇
排序方式: 共有471条查询结果,搜索用时 15 毫秒
31.
The general features of the slowly varying component at centimeter and millimeter wavelengths are explained by magneto-ionic thermal emission. A model of an active region is constructed in which the electron temperature and density profile is based on recent EUV measurements, and the current-free magnetic field configuration is derived from a longitudinal magnetogram and scalar potential theory. In the model, the contributions of the reflected component of the inward extraordinary wave is important in determining the characteristic features of the radio flux and polarization. Emission by the mechanism of resonance absorption does not appear to be a significant factor in this model.  相似文献   
32.
We report photometry for nine asteroids at wavelengths of 1.25, 1.65, and 2.22 μm. Three C-type objects (88, 129, and 511) seem slightly redder in H?K and J?H color indices than four S-type objects (5, 6, 7, and 116) and an E-type object (64). Eros has an unusually red J?H color index; its infrared spectral reflectance is consistent with an appreciable quantity of metallic iron plus some pyroxene and olivine on its surface. JHK photometry is less diagnostic of composition than either higher resolution spectrophotometry in the infrared or spectrophotometry at shorter wavelengths.  相似文献   
33.
34.
We have observed several emerging flux regions (EFRs) using the Video Spectra-Spectro-Heliograph (VSSHG) at the San Fernando Observatory (SFO). The best studied region, NOAA 7968, was near disk center when it was observed on 5–8 June 1996. This EFR showed no organized upflow between the leader and follower spots over the 4-day period covered by our observations. The main concentrations of magnetic flux in the region (leader and follower) showed a slow separation as flux emerged, but little or no upflow was seen. Two other EFRs were observed for part of a single day each and one region was observed for only one sequence. For all regions observed, no discrete features were seen between the leader and follower polarity sunpots that had upflowing material as the regions grew. In all cases, the downward velocities were smaller in area than the magnetic parts of the regions. At times there were several localized areas of greater-amplitude downflows near sunspots.  相似文献   
35.
36.
The strong 14C increase in the year AD 774/5 detected in one German and two Japanese trees was recently suggested to have been caused by an impact of a comet onto Earth and a deposition of large amounts of 14C into the atmosphere (Liu et al. 2014). The authors supported their claim using a report of a historic Chinese observation of a comet ostensibly colliding with Earth's atmosphere in AD 773 January. We show here that the Chinese text presented by those authors is not an original historic text, but that it is comprised of several different sources. Moreover, the translation presented in Liu et al. is misleading and inaccurate. We give the exact Chinese wordings and our English translations. According to the original sources, the Chinese observed a comet in mid January 773, but they report neither a collision nor a large coma, just a long tail. Also, there is no report in any of the source texts about “dust rain in the daytime” as claimed by Liu et al. (2014), but simply a normal dust storm. Ho (1962) reports sightings of this comet in China on AD 773 Jan 15 and/or 17 and in Japan on AD 773 Jan 20 (Ho 1962). At the relevant historic time, the Chinese held that comets were produced within the Earth's atmosphere, so that it would have been impossible for them to report a “collision” of a comet with Earth's atmosphere. The translation and conclusions made by Liu et al. (2014) are not supported by the historical record. Therefore, postulating a sudden increase in 14C in corals off the Chinese coast precisely in mid January 773 (Liu et al. 2014) is not justified given just the 230Th dating for AD 783 ± 14. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
37.
For many years, sewage was continuously discharged via inshore outfalls into shallow subtidal habitats around Sydney (New South Wales). In 1990, deepwater ocean outfalls were commissioned and inshore discharge ceased. This study examined the assemblage of organisms found on subtidal cliffs at one inshore outfall to examine differences in, or recovery by, the biota when sewage was no longer discharged. This assemblage was primarily composed of filter-feeding animals (sponges, ascidians, bryozoans, anthozoans) and encrusting algae. Differences and changes in the assemblages at a number of different spatial scales, relative to the assemblages at two reference locations, were examined four times over a 20 month period. Although most taxa showed significant changes over the study period, most of these were unpredictable and varied interactively according to the taxon, plot, depth and location. For most measures, however, the location where sewage had been released lay within the natural variability shown by the reference locations. The implications of these findings are discussed with respect to other studies on the effects of sewage on intertidal and shallow subtidal habitats.  相似文献   
38.
The surface compositions of 110 asteroids are analyzed from statistically representative data sets of polarimetry as a function of phase angle, broad-band radiometry near 10 and 20 μm, and visible and near-infrared spectrophotometry. A comparison of albedos and diameters determined by polarimetry and radiometry shows that a modest upward revision of the radiometric albedo scale is needed and that a single law relating the slope of the polarization-phase curve to geometric albedo may not hold for very dark asteroids. We present reliable adopted albedos and diameters for 56 objects. Roughdi ameters for 52 additional objects are obtained from spectrophotometry using a correlation between albedo and color. Corrections for sampling bias permit investigation of asteroid compositions as a function of diameter, orbit, and other parameters.More than 90% of the minor planets fall into two broad compositional groups, defined by several optical parameters, designated by the symbols C and S. Comparisons with meteorite spectral albedo curves suggest that the two groups are compositionally similar to carbonaceous and stony-metallic meteorites, respectively. C-type asteroids predominate in the belt, especially in the outer half. An unusual distribution of compositions is found between 2.77 and 3.0 AU. Many S-type objects have diameters of 100–200 km; C-type objects are much more common at both larger and smaller sizes. Vesta is unique, being apparently the only differentiated asteroid remaining intact in the belt. The largest C-type objects are compositionally distinct from smaller ones and possibly are metamorphosed. We sketch some implications for meteoritics and for the early history of the solar system and point to the need for further systematic sampling of smaller and fainter objects by these three observational techniques.  相似文献   
39.
There is widespread discontent with the gross domestic product (GDP) as a measure of progress for society. Alternative measures have been proposed, including the genuine progress indicator (GPI). This paper examines the changing institutional arrangements and attitudinal context in the recently developed Wellington region (New Zealand) GPI. We found an alignment between the GPI framework and the Local Government Act. Despite this fit, we identified challenges, including the need to revitalise public participation through deliberative techniques. It remains too early to judge whether the GPI initiative will launch a viable alternative to the GDP for the Wellington region.  相似文献   
40.
The Canada-France-Hawaii Telescope (CFHT) has recently commissioned two facility infrared cameras for astronomical observations in the 1.0 to 2.5 micron region. These cameras use NICMOS3 256×256 pixel arrays fabricated from Hg:Cd:Te. We report here on array performance and discuss two unique features of these arrays, namely corner glow and residual images. We have developed techniques for reducing corner glow and a method of clearing residual image charge. We discuss our techniques and their side effects.  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