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   174331篇
  免费   19883篇
  国内免费   43150篇
测绘学   5899篇
大气科学   31039篇
地球物理   40781篇
地质学   83297篇
海洋学   28502篇
天文学   33049篇
综合类   4341篇
自然地理   10456篇
  2022年   1100篇
  2021年   2005篇
  2020年   3388篇
  2019年   6973篇
  2018年   9131篇
  2017年   8670篇
  2016年   9510篇
  2015年   6954篇
  2014年   8191篇
  2013年   12218篇
  2012年   8725篇
  2011年   9762篇
  2010年   9226篇
  2009年   10129篇
  2008年   8752篇
  2007年   8467篇
  2006年   7783篇
  2005年   6547篇
  2004年   6823篇
  2003年   6346篇
  2002年   5892篇
  2001年   5261篇
  2000年   4833篇
  1999年   4222篇
  1998年   4410篇
  1997年   4324篇
  1996年   3604篇
  1995年   3481篇
  1994年   3118篇
  1993年   2915篇
  1992年   2686篇
  1991年   2393篇
  1990年   2445篇
  1989年   2121篇
  1988年   1915篇
  1987年   2130篇
  1986年   1846篇
  1985年   2111篇
  1984年   2370篇
  1983年   2201篇
  1982年   2098篇
  1981年   1879篇
  1980年   1708篇
  1979年   1646篇
  1978年   1554篇
  1977年   1390篇
  1976年   1306篇
  1975年   1242篇
  1974年   1292篇
  1973年   1336篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
901.
Radio noise observations at frequencies of 0·700 Mc and 2·200 Mc were made at altitudes between 3000 and 11,000 km from a Blue Scout Jr. high-altitude rocket probe on 30 July 1963. A steady background flux of (7·5−3+6) × 10−19 W m−2)(c/s)−1 at 0·700 Mc and (1·8+1.0−0.5 × 10−19 W m−2 (c/s)−1 at 2·200 Mc was observed. Assuming a galactic origin of the observed fluxes at both frequencies, the averaged sky brightnesses are b(0·700 Mc) = (6−3+5) × 10−20 W m−2 (c/s)−1 sr−1b(2·200 Mc) = (1.4+1.0−0.5 × 10−20 W m−2 (c/s)−1 sr−1 The observed brightness at 2·200 Mc is in reasonable agreement with the results of other observers. The apparent brightness at 0·700 Mc is, however, greater than was expected from previous observations. An alternative source of the 0·700 Mc flux in the terrestrial exosphere, as well as characteristics of several noise bursts observed during the flight, is briefly discussed.  相似文献   
902.
Summary Simultaneous measurements of wind, air temperature, and humidity profiles as well as of evaporation, presented byTakahashi (1958), were compared with relevant formulae suggested by turbulence theory. Satisfactory agreement could be achieved if, near the sea surface, the influence of molecular diffusion and, at higher levels, the effect of thermal stratification on the profiles were taken into account in addition to a suitable reduction of the value of evaporation obtained by means of a floating pan.The chief results of this report have been presented on 27th April 1962 to the Tenth Yearly Meeting of theSocietà Italiana di Geofisica e Meteorologia (Genova: 26–28 April 1962).  相似文献   
903.
904.
905.
906.
A general analysis of ionospheric conditions has been made in the light of possible ionic reactions occurring in the upper atmosphere. Data obtained on various parameters, such as ionic production and recombination, show that precise knowledge of the spectral distribution of solar radiation is needed and that other experimental determinations on dissociative recombinations are required.

The ionic complexity of the ionosphere is underlined by describing how the atomic ions O+ and N+ react with N2, O2 and NO molecules. The behavior of the molecular ions N+2, O+2and NO+depends on a group of simultaneous processes involving charge transfers and ionatom interchanges which are more important than dissociative recombinations. The altitude distribution of ions is exemplified by discussing the relative importance of various loss coefficients in the D-, E- and F-regions. It is seen that molecular nitrogen ions are subject to important charge transfer processes, that nitric oxide ions are always final products destroyed only by dissociative recombination. Additionally, the entire production of atomic oxygen ions is related to the photoionization of molecular nitrogen. Some information is also given on possible anomalies in the ratio of O+2 and NO+ densities in the lower ionosphere. From the lack of sufficient experimental information on ionic processes it is shown that a precise analysis of ionospheric behavior remains highly speculative.  相似文献   

907.
With an appropriate system of higher order approximations the Faraday-effect is calculated for instantaneous ionospheric models the data of which are derived from the results of vertical soundings. Only one parameter which corresponds to the thickness of the outer ionosphere is left open; it is determined by global comparison with simultaneous Faraday-observations of satellite emissions. The thickness parameter shows various forms of variation with latitude; an increase towards north is the rule for quiet daytime conditions. Comparison with local methods shows considerable differences due to the indetermination of the absolute number of rotations.  相似文献   
908.
Zusammenfassung Gesteinsmagnetische Studien haben gezeigt, daß für das Verstandnis des jeweiligen magnetischen Verhaltens von Gesteinen u. a. detaillierte mineralogischoptische und nicht zuletzt chemische Untersuchungen nötig Bind. Zum Studiurim der Verteilung der Elemente im Erzkorn wurde die Röntgenmikroanalyse benutzt.Die Methode der Röntgenmikroanalyse wird beschrieben. Die Ergebnisse der Untersuchungen an Titanomagnetiten tertiärer Effusiva kennzeichnen die Inhomogenität der chemischen Verteilung im Erzkorn und lassen die Klärung von Phänomenen zu, die mit Hilfe der durchschnittlichen chemischen Analyse Bowie magnetothermischen u. a. Untersuchungen nicht möglich war.
Studies on magnetic rocks have demonstrated that detailed informations on composition within ore-particles are needed in order to understand the respective magnetic behaviour. For studying such compositions microanalytic X-ray methods are used.The applied microanalyser is described.Results, gained on this way with tertiary effusiva, let recognize the inhomogenity of composition e.g. within titanomagnetites. They allow clarifying of phenomena, which cannot be understood on another way.
  相似文献   
909.
910.
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