全文获取类型
收费全文 | 64067篇 |
免费 | 1007篇 |
国内免费 | 587篇 |
专业分类
测绘学 | 1646篇 |
大气科学 | 4323篇 |
地球物理 | 12795篇 |
地质学 | 22326篇 |
海洋学 | 5567篇 |
天文学 | 15578篇 |
综合类 | 144篇 |
自然地理 | 3282篇 |
出版年
2021年 | 555篇 |
2020年 | 640篇 |
2019年 | 678篇 |
2018年 | 1518篇 |
2017年 | 1417篇 |
2016年 | 1758篇 |
2015年 | 978篇 |
2014年 | 1686篇 |
2013年 | 3181篇 |
2012年 | 1900篇 |
2011年 | 2520篇 |
2010年 | 2281篇 |
2009年 | 3124篇 |
2008年 | 2676篇 |
2007年 | 2708篇 |
2006年 | 2537篇 |
2005年 | 1899篇 |
2004年 | 1939篇 |
2003年 | 1781篇 |
2002年 | 1698篇 |
2001年 | 1504篇 |
2000年 | 1480篇 |
1999年 | 1280篇 |
1998年 | 1285篇 |
1997年 | 1243篇 |
1996年 | 1017篇 |
1995年 | 1028篇 |
1994年 | 963篇 |
1993年 | 794篇 |
1992年 | 761篇 |
1991年 | 719篇 |
1990年 | 847篇 |
1989年 | 721篇 |
1988年 | 676篇 |
1987年 | 812篇 |
1986年 | 691篇 |
1985年 | 868篇 |
1984年 | 978篇 |
1983年 | 927篇 |
1982年 | 845篇 |
1981年 | 815篇 |
1980年 | 731篇 |
1979年 | 658篇 |
1978年 | 649篇 |
1977年 | 622篇 |
1976年 | 575篇 |
1975年 | 579篇 |
1974年 | 545篇 |
1973年 | 573篇 |
1972年 | 364篇 |
排序方式: 共有10000条查询结果,搜索用时 421 毫秒
21.
A spectacular change in the lower corona on the south-west limb has been found in solar images taken by the Yohkoh soft X-ray telescope. The event is characterized by a large topological change in magnetic field and a large intensity decrease observed after the X1. 1/1B flare on 9 November, 1991. A coronal mass ejection (CME) was observed by the Mark III K-coronameter (MK3) at the HAO/Mauna Loa Observatory. Both the MK3 (white-light) and soft X-ray observations showed that one leg of this CME was located above the flare site. An interplanetary shock associated with this event was observed by Pioneer Venus Orbiter, and, possibly, by IMP-8.Also Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, U.S.A. 相似文献
22.
23.
24.
We examine a siphon-like mechanism for moving mass from the chromosphere to a gravitational well at the top of a magnetic loop to form a prominence. The calculations assume no apriori flow velocity at the loop base. Instead heating in the loop legs drives the flow. The prominence formation process requires two steps. First, the background heating rate must be reduced to on the order of 1 % of the initial heating rate required to maintain the coronal loop. This forms an initial condensation at the top of the loop. Second, the heating must take place only in the loop legs in order to produce a pressure differential which drives mass up into the well at the top of the loop. The heating rate in the loop must be increased once the prominence has begun to form or full prominence densities can not be achieved in a reasonable time. We conclude that this heating driven siphon-like mechanism is feasible for producing and maintaining prominences. 相似文献
25.
In recent times it has been emphasized that the present kinematical structures of asteroid families should be evolved with respect to the original post-impact situations, according to numerical simulations performed taking into account also the previously neglected Yarkovsky effect. In this paper we show that also a “classical” approach based on an analysis of the current kinematical properties of families leads to conclude that the distributions of proper eccentricities and semimajor axes of family members exhibit evidence of an evolution. The importance of this approach is that it yields a fully independent and quantitative estimate of an evolutionary spreading of the proper elements. In particular, we find that the original post-impact families had to be on the average about twice more compact than the families we observe now, when considering family members down to about 5 km in size. This result can be used in future analyses to derive estimates of the ages of different families, and to better constrain the typical values of the ejection velocities of the fragments in family-forming events. 相似文献
26.
Abstract— We have analyzed several types of data associated with the well‐documented fall of the Neuschwanstein meteorites on April 6, 2002 (a total of three meteorites have been recovered). This includes ground‐based photographic and radiometer data as well as infrasound and seismic data from this very significant bolide event (Spurný et al. 2002, 2003). We have also used these data to model the entry of Neuschwanstein, including the expected dynamics, energetics, panchromatic luminosity, and associated fragmentation effects. In addition, we have calculated the differential efficiency of acoustical waves for Neuschwanstein and used these values to compare against the efficiency calculated using available ground‐based infrasound data. This new numerical technique has allowed the source height to be determined independent of ray tracing solutions. We have also carried out theoretical ray tracing for a moving point source (not strictly a cylindrical line emission) and for an infinite speed line source. In addition, we have determined the ray turning heights as a function of the source height for both initially upward and downward propagating rays, independent of the explicit ray tracing (detailed propagation path) programs. These results all agree on the origins of the acoustic emission and explicit source heights for Neuschwanstein for the strongest infrasonic signals. Calculated source energies using more than four different independent approaches agree that Neuschwanstein was certainly <500 kg in initial mass, given the initial velocity of 20.95 km/s, resulting in an initial source energy ≤0.0157‐0.0276 kt TNT equivalent (4.185 times 1012 J). Local source energies at the calculated infrasonic/seismic source altitudes are up to two orders of magnitude smaller than this initial source energy. 相似文献
27.
28.
The orbits of (69230) Hermes and 2002 SY50 are similar and the Earth approaches both of them twice: at the end of October the local orbital minimum distances are smaller
than 0.007 AU, and at the end of April the distances are smaller than 0.04 AU. This gives us opportunities to observe the
meteors associated with these asteroids. Using the geocentric parameters of the orbital close encounters (the theoretical
radiants) and our D
N
distance function (Valsecchi et al. Mon. Not. R. Astron. Soc. 304 (1999) 743), we searched for meteoroids originated by Hermes and 2002 SY50. A search among 1830 good quality photographic meteors gave negative results: we found no meteor dynamically similar to Hermes
or 2002 SY50. In a second search, done in a set of 62150 radio meteors, we applied two methods (M1, M2) and in both cases we found two
streams; the streams found with the M1 method had 43 and 30 members, those found with the M2 method had 39 and 14 members.
However, these results do not look convincing, due to the small number of common members in the corresponding streams. We
therefore conclude that amongst the IAU meteors used in our search there are no compact streams associated with Hermes and
2002 SY50. 相似文献
29.
30.
The 362.156 GHz absorption spectrum of H2O2 in the Mars atmosphere was observed on September 4 of 2003, employing the James Clerk Maxwell Telescope (JCMT) sub-millimeter facility on Mauna Kea, Hawaii. Radiative transfer analysis of this line absorption yields an average volume mixing ratio of 18±0.4 ppbv within the lower (0-30 km) Mars atmosphere, in general accordance with standard photochemical models (e.g., Nair et al., 1994, Icarus 111, 124-150). Our derived H2O2 abundance is roughly three times greater than the upper limit retrieved by Encrenaz et al. (2002, Astron. Astrophys. 396, 1037-1044) from infrared spectroscopy, although part of this discrepancy may result from the different solar longitudes (Ls) of observation. Aphelion-to-perihelion thermal forcing of the global Mars hygropause generates substantial (>200%) increases in HOx abundances above ∼10 km altitudes between the Ls=112° period of the Encrenaz et al. upper limit measurement and the current Ls=250° period of detection (Clancy and Nair, 1996, J. Geophys. Res. 101, 12785-12590). The observed H2O2 line absorption weakens arguments for non-standard homogeneous (Encrenaz et al., 2002, Astron. Astrophys. 396, 1037-1044) or heterogeneous (Krasnopolsky, 2003a, J. Geophys. Res. 108; 2003b, Icarus 165, 315-325) chemistry, which have been advocated partly on the basis of infrared (8 μm) non-detections for Mars H2O2. Observation of Mars H2O2 also represents the first measurement of a key catalytic specie in a planetary atmosphere other than our own. 相似文献