全文获取类型
收费全文 | 112621篇 |
免费 | 1828篇 |
国内免费 | 763篇 |
专业分类
测绘学 | 2573篇 |
大气科学 | 7466篇 |
地球物理 | 22067篇 |
地质学 | 40263篇 |
海洋学 | 10107篇 |
天文学 | 26144篇 |
综合类 | 294篇 |
自然地理 | 6298篇 |
出版年
2022年 | 718篇 |
2021年 | 1216篇 |
2020年 | 1324篇 |
2019年 | 1448篇 |
2018年 | 3073篇 |
2017年 | 2830篇 |
2016年 | 3340篇 |
2015年 | 1759篇 |
2014年 | 3272篇 |
2013年 | 5870篇 |
2012年 | 3559篇 |
2011年 | 4688篇 |
2010年 | 4214篇 |
2009年 | 5446篇 |
2008年 | 4719篇 |
2007年 | 4835篇 |
2006年 | 4477篇 |
2005年 | 3284篇 |
2004年 | 3221篇 |
2003年 | 3023篇 |
2002年 | 2990篇 |
2001年 | 2587篇 |
2000年 | 2524篇 |
1999年 | 2046篇 |
1998年 | 2139篇 |
1997年 | 1979篇 |
1996年 | 1717篇 |
1995年 | 1720篇 |
1994年 | 1475篇 |
1993年 | 1378篇 |
1992年 | 1286篇 |
1991年 | 1326篇 |
1990年 | 1336篇 |
1989年 | 1183篇 |
1988年 | 1087篇 |
1987年 | 1292篇 |
1986年 | 1118篇 |
1985年 | 1394篇 |
1984年 | 1587篇 |
1983年 | 1515篇 |
1982年 | 1377篇 |
1981年 | 1335篇 |
1980年 | 1176篇 |
1979年 | 1102篇 |
1978年 | 1083篇 |
1977年 | 953篇 |
1976年 | 951篇 |
1975年 | 926篇 |
1974年 | 896篇 |
1973年 | 990篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
641.
A model of the predawn bulge ionosphere composition and structure is constructed and compared with the ion mass spectrometer measurements from the Pioneer Venus Orbiter during orbits 117 and 120. Particular emphasis is given to the identification of the mass-2 ion which we find unequivocally due to D+ (and not H2+). The atmospheric D/H ratio of 1.4% and 2.5% is obtained at the homopause (~ 130 km) for the two orbits. The H2+ contribution to the mass-2 ion density is less than 10%, and the H2 mixing ratio must be <0.1 ppm at 130 km altitude. The He+ data require a downward He+ flux of ~2 × 107 cm?2 sec?1 in the predawn region which suggest that the light ions also flow across the terminator from day to night along with the observed O+ ion flow. 相似文献
642.
In this paper we prove the existence of ring-type bounded motion in an isolated system consisting of a massive point particle and a homogeneous cube. We study the case of planar motion where the particle moves in a symmetry plane of the cube and we use a rotating frame of reference with its center at the mass center of the cube and its axes coinciding with the symmetry axes of the cube. We prove that, for negative values of the total energy and properly chosen values of the total angular momentum, the relative distance of the bodies has an upper and a lower bound-i.e., the regions of possible motion lie inside an annulus around the cube (motion inside a ring or an island). 相似文献
643.
A regular perturbation analysis is presented for natural convection flow over an uniform flux vertical surface with temperature dependent viscosity. Numerical calculations are presented forP
r=6.7 which show that the first-order correction to the local temperature difference and to the local skin-fraction are negative whereas it is positive for the local Nusselt number. The effects of variable viscosity on the temperature, velocity profiles, the local temperature difference, the local Nusselt number and the local skin fraction are discussed. 相似文献
644.
D.G. King-Hele 《Planetary and Space Science》1985,33(10):1145-1153
The satellite 1967-11G, which had an orbital inclination of 40°, passed through the 14th-order resonance with the Earth's gravitational field in 1974. The changes in its orbital inclination at resonance have been analysed to obtain values for four lumped 14th-order harmonics in the geopotential, with accuracies equivalent to about 5 cm in geoid height. Analysis of the eccentricity was also attempted, but did not yield useful results.As no previous satellite analysed at 14th-order resonance has had an inclination near 40°, the results have proved to be valuable in determining individual 14th-order harmonics in the geopotential. 相似文献
645.
N. Kameswara Rao V. R. Venugopal A. Patnaik 《Journal of Astrophysics and Astronomy》1985,6(3):153-157
VLA observations at 2 and 6 cm have been obtained for six hydrogen-deficient stars R CrB, HD 160641, BD — 9°4395, V348 Sgr,
MV Sgr and Sgrv Upper limits to the massloss rates have been estimated for some of these using the upper limits to the radio flux density.
National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National
Science Foundation, USA 相似文献
646.
647.
648.
649.
The gray crystalline hematite at Meridiani Planum first discovered by the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES) instrument occurs as spherules that have been interpreted as concretions. Analysis of the TES and mini-TES spectra shows that no 390 cm−1 feature is present in the characteristic martian hematite spectrum. Here, we incorporate the mid-IR optical constants of hematite into a simple Fresnel reflectance model to understand the effect of emission angle and crystal morphology on the presence or absence of the 390 cm−1 feature in an IR hematite spectrum. Based on the results we offer two models for the internal structure of the martian hematite spherules. 相似文献
650.
X. Bai T.K. Gaisser A. Karle K. Rawlins G.M. Spiczak Todor Stanev 《Astroparticle Physics》2006,25(6):361-367
The muon flux at the South Pole was measured for five zenith angles, 0°, 15°, 35°, 82.13° and 85.15° with a scintillator muon telescope incorporating ice Cherenkov tank detectors as the absorber. We compare the measurements with other data and with calculations. 相似文献