全文获取类型
收费全文 | 90654篇 |
免费 | 1554篇 |
国内免费 | 1007篇 |
专业分类
测绘学 | 2311篇 |
大气科学 | 7152篇 |
地球物理 | 18762篇 |
地质学 | 29953篇 |
海洋学 | 8028篇 |
天文学 | 20317篇 |
综合类 | 240篇 |
自然地理 | 6452篇 |
出版年
2020年 | 645篇 |
2019年 | 680篇 |
2018年 | 1368篇 |
2017年 | 1333篇 |
2016年 | 1991篇 |
2015年 | 1438篇 |
2014年 | 1987篇 |
2013年 | 4549篇 |
2012年 | 2193篇 |
2011年 | 3203篇 |
2010年 | 2751篇 |
2009年 | 4024篇 |
2008年 | 3637篇 |
2007年 | 3340篇 |
2006年 | 3317篇 |
2005年 | 2947篇 |
2004年 | 2995篇 |
2003年 | 2791篇 |
2002年 | 2665篇 |
2001年 | 2390篇 |
2000年 | 2348篇 |
1999年 | 2086篇 |
1998年 | 2005篇 |
1997年 | 1986篇 |
1996年 | 1748篇 |
1995年 | 1598篇 |
1994年 | 1472篇 |
1993年 | 1350篇 |
1992年 | 1266篇 |
1991年 | 1151篇 |
1990年 | 1296篇 |
1989年 | 1143篇 |
1988年 | 1052篇 |
1987年 | 1231篇 |
1986年 | 1115篇 |
1985年 | 1382篇 |
1984年 | 1571篇 |
1983年 | 1509篇 |
1982年 | 1387篇 |
1981年 | 1297篇 |
1980年 | 1141篇 |
1979年 | 1088篇 |
1978年 | 1146篇 |
1977年 | 1028篇 |
1976年 | 974篇 |
1975年 | 928篇 |
1974年 | 943篇 |
1973年 | 954篇 |
1972年 | 591篇 |
1971年 | 504篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
261.
The north-south incidence has been studied of 31 white-light flares observed since 1859 and of 1669 events meeting the criteria for major flares of Dodson and Hedeman (1971) for the period 1955–1974. The asymmetry in favor of the northern hemisphere increases strikingly with the importance of the events. Similarly, magnetically complex sunspot groups (Mt. Wilson classes, and) display a more pronounced asymmetry in favor of the north than non-complex groups for 1962–1970. Contrary to the flare asymmetry, the spottedness asymmetry is independent of the size of sunspots. 相似文献
262.
263.
264.
A one-dimensional model for thinning of the plasma sheet is developed on the basis of launching a fast mode MHD rarefaction wave propagating in the tailward direction along the plasma sheet. Behind the rarefaction wave the pressure is reduced, leading to thinning of the plasma sheet and also to an Earthward plasma flow with a speed on the order of the sound speed a0. The plasma sheet thickness is reduced by a factor of 2 if an Earthward plasma flow speed of 0.8a0 is induced. The predictions of the model are in reasonable agreement with observations. 相似文献
265.
The resonant interaction between the whistler mode waves and the energetic electrons near the plasmapause boundary has been studied in the presence of field aligned currents which seem to exist during substorm activity. It is shown that the electrons which carry the current along the direction of the magnetic field enhance the whistler mode growth considerably if the streaming velocity is small compared to the phase velocity of the wave. It is likely that this is one of the mechanisms explaining the intense VLF emissions observed near the plasmapause during substorm activity. 相似文献
266.
A non-steady, two-dimensional, compressible mathematical model of a fluid with constant viscosity and thermal conductivity is given in order to represent a thin atmospheric layer located at 60 km height, as a function of a known initial vertical distribution of temperature, chemical concentrations of minor components and assumed free boundary conditions.The purpose of this paper is to determine the influence of convective laminar processes in the vertical distribution of minor constituents in a thermal interface. Only three minor components are considered, atomic oxygen, molecular oxygen and ozone. Numerical solutions of the stated timedependent convection problem for different Rayleigh numbers show a marked influence of the established flow pattern upon the distribution of minor constituents. In all cases, an enhancement of mean concentration gradients in relatively small regions adjacent to the warmer and colder boundaries was observed. 相似文献
267.
D.J. Southwood 《Planetary and Space Science》1977,25(6):549-554
It is shown that the field line resonance phenomenon that occurs for quasi-transverse signals in a non-uniform cold plasma generally occurs for disturbances with a compressional magnetic component in a hot plasma like the ring current. The equations describing such localised signals are derived and discussed in various limits. It is pointed out that recent observations of low frequency compressional oscillations on spacecraft may be an example of such localised signals. 相似文献
268.
Intensive partial reflection drift observations were made at Adelaide (35°S) for a seven day period in June 1973. The results have been analysed to isolate the prevailing motion and oscillations of various time scales: planetary, 24 hr, 12 hr and gravity waves. Each is discussed in turn with particular emphasis on the variability of energy from day to day and as a function of height. Evidence is presented for the local generation of planetary waves, the presence of the evanescent S?21 mode in the 24 hr oscillation, the influence of the S42 mode in the 12 hr oscillation and a definite polarization of gravity waves. The energies of all the forms of motion are shown to decay exponentially with increasing height and the deposition of energy and momentum in the upper mesosphere and lower thermosphere is discussed. 相似文献
269.
270.
C. Bartolini A. Bonifazi F. D'antona F. Fusi Pecci L. Oculi A. Piccioni R. Serra 《Astrophysics and Space Science》1982,83(1-2):287-310
The evolutionary status of hydrogen-poor stars is analyzed.Photoelectric observations of six objects are reported as a first step of a long-term project devoted to search for variability of a large sample of hydrogen-poor stars.The observed stars show phenomena of microvariability with an amplitude of the order of 0m. 1 or less.Two extreme helium stars have been examined: a period in the range of 0d.162-0d.164 has been found for BD+ 1002179, andP=0d.1079962 for BD+1303324.The mass-losing O subdwarf (sdO) BD+370 443 presents short-term fluctuations with a time-scale of several minutes and long-term variations on a scale of months. The standard sdO BD+750325 is probably non-variable, although light variations of very small amplitude (m0.03) with a time-scale of about 1 hr might be present. The high gravity sdO BD+2504655, which is very close to the white dwarf stage, also presents variability on a time-scale of about 13 minutes, and might be an analog of the recently discovered pulsating sdO, or hot white dwarf, PG 1159-035.The variability of the intermediate helium star HD 37776 is finally confirmed. 相似文献