全文获取类型
收费全文 | 113884篇 |
免费 | 1893篇 |
国内免费 | 802篇 |
专业分类
测绘学 | 2879篇 |
大气科学 | 8475篇 |
地球物理 | 22531篇 |
地质学 | 41200篇 |
海洋学 | 9805篇 |
天文学 | 24349篇 |
综合类 | 317篇 |
自然地理 | 7023篇 |
出版年
2021年 | 679篇 |
2020年 | 811篇 |
2019年 | 869篇 |
2018年 | 4187篇 |
2017年 | 3960篇 |
2016年 | 3527篇 |
2015年 | 1534篇 |
2014年 | 2195篇 |
2013年 | 4737篇 |
2012年 | 3249篇 |
2011年 | 5669篇 |
2010年 | 5269篇 |
2009年 | 6444篇 |
2008年 | 5438篇 |
2007年 | 5818篇 |
2006年 | 3534篇 |
2005年 | 3424篇 |
2004年 | 3335篇 |
2003年 | 3221篇 |
2002年 | 2941篇 |
2001年 | 2449篇 |
2000年 | 2390篇 |
1999年 | 2007篇 |
1998年 | 2017篇 |
1997年 | 1998篇 |
1996年 | 1736篇 |
1995年 | 1645篇 |
1994年 | 1493篇 |
1993年 | 1362篇 |
1992年 | 1298篇 |
1991年 | 1152篇 |
1990年 | 1352篇 |
1989年 | 1193篇 |
1988年 | 1099篇 |
1987年 | 1294篇 |
1986年 | 1179篇 |
1985年 | 1436篇 |
1984年 | 1624篇 |
1983年 | 1522篇 |
1982年 | 1403篇 |
1981年 | 1404篇 |
1980年 | 1185篇 |
1979年 | 1150篇 |
1978年 | 1165篇 |
1977年 | 1099篇 |
1976年 | 1012篇 |
1975年 | 963篇 |
1974年 | 962篇 |
1973年 | 974篇 |
1972年 | 600篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
New determination of the Earth orientation parameters (EOP), based on optical astrometry observations since the beginning of the century, is now under preparation by the Working group established by Commission 19 of the IAU. The Hipparcos catalog is to define the celestial reference frame in which the new series of EOP are to be described, The novelties of the prepared solution are the higher resolution (5 days) and more parameters estimated from the solution (celestial pole offsets, rheological parameters of the Earth, certain instrumental constants). The mathematical model of the solution is described, and the results based on the observations made with 46 instruments at 29 observatories and a preliminary Hipparcos catalog are presented. 相似文献
42.
43.
44.
Non‐equilibrium concepts lead to a unified explanation of the formation of chondrules and chondrites
Milton BLANDER Arthur D. PELTON In‐Ho JUNG Richard WEBER 《Meteoritics & planetary science》2004,39(12):1897-1910
Abstract— Calculations of the formation of seven types of chondrules in Semarkona from a gas of solar composition were performed with the FACT computer program to predict the chemistries of oxides (including silicates), developed by the authors and their colleagues. The constrained equilibrium theory was used in the calculations with two nucleation constraints suggested by nucleation theory. The first constraint was the blocking of Fe and other metal gaseous atoms from condensing to form solids or liquids because of very high surface free energies and high surface tensions of the solid and liquid metals, respectively. The second constraint was the blocking of the condensation of solids and the formation of metastable liquid oxides (including silicates) well below their liquidus temperatures. Our laboratory experiments suggested subcooling of type IIA chondrule compositions of 400 degrees or more below the liquidus temperature. The blocking of iron leads to a supersaturation of Fe atoms, so that the partial pressure of Fe (pFe) is larger than the partial pressure at equilibrium (pFe(eq)). The supersaturation ratio S = pFe/pFe(eq) becomes larger than 1 and increases rapidly with a decrease in temperature. This drives the reaction Fe + H2O ? H2 + FeO to the right. With S = 100, the activity of FeO in the liquid droplet is 100 times as large as the value at equilibrium. The FeO activities are a function of temperature and provide relative average temperatures of the crystallization of chondrules. Our calculations for the LL3.0 chondrite Semarkona and our study of some non‐equilibrium effects lead to accurate representations of the compositions of chondrules of types IA, IAB, IB, IIA, IIAB, IIB, and CC. Our concepts readily explain both the variety of FeO concentrations in the different chondrule types and the entire process of chondrule formation. Our theory is unified and could possibly explain the formation of chondrules in all chondritic meteorites as well as provide a simple explanation for the complex chemistries of chondrites, and especially for type 3 chondrites. 相似文献
45.
V. Delouille J. De Patoul J. F. Hochedez L. Jacques J. P. Antoine 《Solar physics》2005,228(1-2):301-321
The extreme ultraviolet imaging telescope (EIT) of SOHO offers a unique record of the solar atmosphere for its sampling in
temperature, field of view, resolution, duration, and cadence. To investigate globally and locally its topology and evolution
during the solar cycle, we consider a multi-scale approach, and more precisely we use the wavelet spectrum.
We present three results among the applications of such a procedure. First, we estimate the typical dimension of the supergranules
as seen in the 30.4 nm passband, and we show that the evolution of the characteristic network scale is almost in phase with
the solar cycle. Second, we build pertinent time series that give the evolution of the signal energy present in the corona
at different scales. We propose a method that detects eruptions and post-flaring activity in EUV image sequences. Third, we
introduce a new way to extract active regions in EIT images, with perspectives in, e.g., long-term irradiance analysis. 相似文献
46.
Matthew J. GENGE Monica M. GRADY Robert HUTCHISON 《Meteoritics & planetary science》1996,31(5):627-632
Abstract— Cosmic dust accreted by the Earth can be extensively reprocessed during atmospheric encounters. The textures and compositions of reprocessed material provide important constraints by which the processes affecting extraterrestrial matter in the Earth's atmosphere can be better understood. Here we report results on an unusual Antarctic glassy cosmic spherule that demonstrates strong textural evidence for at least two grazing incidence encounters with the Earth's atmosphere prior to final reentry. The particle consists of a central glassy core with four peripheral glass lobes that transect a silicate particle rim. The texture of the particle confirms previous theoretical speculations that some high velocity, low incidence angle interplanetary particles experience numerous encounters with the Earth's atmosphere and also indicates that micrometeorites demonstrating multiple melting episodes should be interpreted with caution. 相似文献
47.
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
48.
49.
A highly automated moving object detection package 总被引:1,自引:0,他引:1
50.
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