全文获取类型
收费全文 | 71502篇 |
免费 | 652篇 |
国内免费 | 382篇 |
专业分类
测绘学 | 1899篇 |
大气科学 | 5403篇 |
地球物理 | 12859篇 |
地质学 | 27072篇 |
海洋学 | 5657篇 |
天文学 | 15591篇 |
综合类 | 245篇 |
自然地理 | 3810篇 |
出版年
2020年 | 341篇 |
2019年 | 339篇 |
2018年 | 3702篇 |
2017年 | 3536篇 |
2016年 | 2590篇 |
2015年 | 808篇 |
2014年 | 1195篇 |
2013年 | 2448篇 |
2012年 | 2180篇 |
2011年 | 4176篇 |
2010年 | 3946篇 |
2009年 | 4441篇 |
2008年 | 3706篇 |
2007年 | 4342篇 |
2006年 | 1721篇 |
2005年 | 2019篇 |
2004年 | 1858篇 |
2003年 | 1846篇 |
2002年 | 1592篇 |
2001年 | 1259篇 |
2000年 | 1192篇 |
1999年 | 1123篇 |
1998年 | 1058篇 |
1997年 | 1063篇 |
1996年 | 846篇 |
1995年 | 839篇 |
1994年 | 795篇 |
1993年 | 738篇 |
1992年 | 709篇 |
1991年 | 681篇 |
1990年 | 777篇 |
1989年 | 674篇 |
1988年 | 649篇 |
1987年 | 741篇 |
1986年 | 621篇 |
1985年 | 825篇 |
1984年 | 948篇 |
1983年 | 913篇 |
1982年 | 863篇 |
1981年 | 829篇 |
1980年 | 740篇 |
1979年 | 698篇 |
1978年 | 693篇 |
1977年 | 627篇 |
1976年 | 600篇 |
1975年 | 518篇 |
1974年 | 597篇 |
1973年 | 587篇 |
1972年 | 363篇 |
1971年 | 335篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We have analyzed magnetograph observations of the 5-min oscillations. We find that most of the oscillatory power is concentrated in space and frequency. Interference effects where these concentrations overlap can explain some of the variations in amplitude of the oscillation.Of the National Bureau of Standards and University of Colorado. 相似文献
992.
N. R. Sheeley Jr. 《Solar physics》1971,20(1):19-25
By photographically averaging time sequences of high-resolution CN 3883 spectroheliograms, the noise level due to the rapidly fluctuating intensity of the solar background has been reduced significantly. Very faint faculae that are lost in the noise on a single frame are easily visible on such an enhanced picture. A comparison between these enhanced spectroheliograms and a photoelectric magnetogram suggests that the brightness-magnetic field correlation extends to much weaker field strengths and fainter faculae than can be detected on a single, high quality CN 3883 spectroheliogram.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. 相似文献
993.
994.
It has previously been suggested that the solar wind might terminate at distances of 5 AU to 20 AU from the Sun, and that the solar wind might be drastically slowed down by charge exchange and photoionization of interstellar hydrogen atoms which approach the Sun. However, recent satellite measurements of resonantly scattered Lyman alpha radiation, together with pulsar dispersion and Faraday rotation measures, imply very small values for the interstellar hydrogen density (0.05 cm−3) and magnetic field strength (3 μG). As a result, the solar wind is not expected to be slowed down by more than about 30% inside the termination distance, which is expected to be about 100 AU. 相似文献
995.
Atoms which escape Titan's atmosphere are unlikely to possess escape velocity from Saturn, and can orbit the planet until lost by ionization or collision with Titan. It is predicted that a toroidal ring of between ~1 and ~103 atoms or molecules cm?3 exists around Saturn at a distance of about 10 times the radius of the visible rings. This torus may be detectable from Earth-orbit and detection of nondetection of it may provide some information about the presence or absence of a Saturnian magnetic field, and the exospheric temperature and atmospheric escape rate of Titan. It is estimated that, if Titan has a large exosphere, ~97% or more of the escaping atoms can be recaptured by Titan, thereby decreasing the effective net atmospheric loss rate by up to two orders of magnitude. With such a reduction in atmospheric loss rates, it becomes more plausible to suggest that satellites previously thought too small to retain an atmosphere may have one. It is suggested that Saturn be examined by Lyman-α and other observations to search for the gaseous torus of Titan. If successful, these could then be extended to other satellites.The effect of a hypothetical Saturnian magnetosphere on the atmosphere of Titan is investigated. It is shown that, if Saturn has a magnetic field comparable to Jupiter's (~10 G at the planetary surface), the magnetospheric plasma can supply Titan with hydrogen at a rate comparable to the loss rates in some of the models of Trafton (1972) and Sagan (1973). A major part of the Saturnian ionospheric escape flux (~ 1027 photoelectrons sec?1) could perhaps be captured by Titan. At the upper limit, this rate of hydrogen input to the satellite could total ~0.1 atm pressure over the lifetime of the solar system, an amount comparable to estimates of the present atmospheric pressure of Titan. 相似文献
996.
We have solved the coupled momentum and continuity equations for NO+, O2+, and O+ions in the E- and F-regions of the ionosphere. This theoretical model has enabled us to examine the relative importance of various processes that affect molecular ion densities. We find that transport processes are not important during the day; the molecular ions are in chemical equilibrium at all altitudes. At night, however, both diffusion and vertical drifts induced by winds or electric fields are important in determining molecular ion densities below about 200 km. Molecular ion densities are insensitive to the O+ density distribution and so are little affected by decay of the nocturnal F-region or by processes, such as a protonospheric flux, that retard this decay. The O+ density profile, on the other hand, is insensitive to molecular ion densities, although the O+ diffusion equation is formally coupled to molecular ion densities by the polarization electrostatic field. Nitric oxide plays an important role in determining the NO+ to O2+ ratio in the E-region, particularly at night. Nocturnal sources of ionization are required to maintain the E-region through the night. Vertical velocities induced by expansion and contraction of the neutral atmosphere are too small to affect ion densities at any altitude. 相似文献
997.
From riometer records for the sudden impulse event of 4 February 1969, it is shown that ionospheric absorption accompanying a sudden impulse has the same type of latitude and longitude variations found for sudden commencement events. In addition, an examination of magnetograms at College, Alaska shows that some positive sudden impulses may trigger negative bays around local midnight, similar to the recent results for sudden commencements. 相似文献
998.
R. Born 《Solar physics》1974,38(1):127-131
An analysis of 63 young active regions shows that they originate exclusively on the borders of the chromospheric cells. In most cases they do not produce pores or spots, but always arch filament systems and chromospheric faculae. The arch filament systems evolve along the cell border, they are rooted on the network and not in the interior of the chromospheric cells. The sub-photospheric magnetic flux seems to reach the photosphere in units of 1019 Mx. About 3 hr after the first arch filaments appear pores evolve on the cell border. The pores and spots are probably formed by local aggregations of the emerging flux tubes.Mitteilungen aus dem Fraunhofer-Institut Nr. 132. 相似文献
999.
Additional studies of the ion composition results obtained from the OGO-6 satellite support earlier observations of irregularities in the distribution of H+ and He+ within the light ion trough near L = 4, which has been associated with the plasmapause. These irregularities are in the form of sub-troughs superimposed upon the major mid latitude decrease of the light ions. In the sub-troughs, ionization depletions and recoveries of as much as an order of magnitude are observed within a few degrees of latitude, usually exhibited in a pattern which changes significantly with longitude as the Earth rotates beneath the relatively fixed satellite orbit. The location and properties exhibited by these sub-troughs appear to be consistent with the concept of a plasmasphere distortion in the form of “plasmatails” resulting from the combined effects of magnetospheric convection plus corotation. Like the light ion trough, the “plasmatail” irregularity in H+ may be obscured on the day side by the dominant topside distribution of O+. Consequently, these light ion irregularities are seen as an important factor for studies of plasmapause-trough relationships. 相似文献
1000.
R. J. Dodd 《Astrophysics and Space Science》1976,44(1):85-99
Star counts, from measurements made by the GALAXY measuring machine, in unit B magnitude intervals were made in nine regions, each of 1.75 square degrees distributed about (l, b)= (140°, 0°). The mean density function and the mean variation of interstellar absorption with distance for the various areas were determined using a combination of theoretical and observed cumulative Wolf diagrams. For the same regions multi-colour (UBV) photographic photometry was used to identify early type stars. The distribution of these stars shows three main peaks which correspond well in distance with the Perseus, - and -spiral arms found by Verschuur from radio measurements of neutral hydrogen. The pattern made by the stellar arms in the (l, r) plane shows a discontinuity nearl=140°. From a consideration of both the star counts and the distribution of the reddened early type stars it is suggested that the cause of the discontinuity is a cloud with 0
.
m
7 total obscuration situated much nearer to the Sun than the Perseus arm and terminating on one side atl=140°. 相似文献