全文获取类型
收费全文 | 79413篇 |
免费 | 1147篇 |
国内免费 | 569篇 |
专业分类
测绘学 | 2017篇 |
大气科学 | 5185篇 |
地球物理 | 16717篇 |
地质学 | 28380篇 |
海洋学 | 6867篇 |
天文学 | 18319篇 |
综合类 | 231篇 |
自然地理 | 3413篇 |
出版年
2022年 | 637篇 |
2021年 | 1050篇 |
2020年 | 1158篇 |
2019年 | 1231篇 |
2018年 | 2667篇 |
2017年 | 2499篇 |
2016年 | 2928篇 |
2015年 | 1549篇 |
2014年 | 2675篇 |
2013年 | 4297篇 |
2012年 | 2991篇 |
2011年 | 3573篇 |
2010年 | 3278篇 |
2009年 | 3995篇 |
2008年 | 3475篇 |
2007年 | 3599篇 |
2006年 | 3255篇 |
2005年 | 2189篇 |
2004年 | 2148篇 |
2003年 | 1968篇 |
2002年 | 1959篇 |
2001年 | 1805篇 |
2000年 | 1612篇 |
1999年 | 1300篇 |
1998年 | 1334篇 |
1997年 | 1350篇 |
1996年 | 1039篇 |
1995年 | 1031篇 |
1994年 | 955篇 |
1993年 | 822篇 |
1992年 | 756篇 |
1991年 | 763篇 |
1990年 | 833篇 |
1989年 | 761篇 |
1988年 | 682篇 |
1987年 | 764篇 |
1986年 | 662篇 |
1985年 | 837篇 |
1984年 | 958篇 |
1983年 | 848篇 |
1982年 | 834篇 |
1981年 | 753篇 |
1980年 | 682篇 |
1979年 | 633篇 |
1978年 | 675篇 |
1977年 | 591篇 |
1976年 | 520篇 |
1975年 | 532篇 |
1974年 | 492篇 |
1973年 | 572篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
M. V. Sazhin V. E. Zharov T. A. Kalinina 《Monthly notices of the Royal Astronomical Society》2001,323(4):952-964
Parallax measurements allow distances to celestial objects to be determined. Together with measurements of their position on the celestial sphere, they give a full three-dimensional picture of the location of the objects relative to the observer. The distortion of the parallax value of a distant source affected by weak microlensing is considered. This means that the weak microlensing leads to distortion of the distance scale. The gravitational deflection causes a retrograde apparent motion of the image, which is revealed as a negative parallax. It is shown that the distortions may significantly change the parallax values when they amount to several microseconds of arc. In particular, at this level many measured values of parallaxes should be negative. 相似文献
42.
V. Garnier D. Ohnenstetter G. Giuliani P. Blanc D. Schwarz 《Mineralogy and Petrology》2002,76(3-4):179-193
Summary ?Mong Hsu rubies of the “trapiche” type are sporadically seen in the gem market. However, they have never been described in
the field. The study of the nature of solid inclusions, the variation of trace-element contents, as well as the cathodoluminescence
behaviour of six “trapiche” rubies permit the conclusion that these rubies crystallised in the same geological environment
(marble-type deposit) as the normal rubies from Mong Hsu: (1) Cr and V are the main chromophorous elements in both ruby types;
they act, together with Ti, as activators or quenchers for cathodoluminescence; (2) calcite, dolomite, rutile, mica, diaspore,
apatite, chlorite, and feldspar are solid inclusions found in both ruby types; (3) the presence of bastn?site in trapiche
ruby and fluorite in non-trapiche ruby indicates the circulation of F-bearing fluids during ruby deposition; (4) the distribution
of trace-element contents in the crystal is similar for both ruby types. In the Cr2O3 vs. Fe2O3 and Cr2O3 vs. Fe2O3/TiO2 diagrams, the population fields of Mong Hsu “trapiche” and non-“trapiche” rubies overlap. They are distinct from those of
rubies and sapphires hosted in basalts from South-east Asia.
Received October 30, 2001; revised version accepted March 25, 2002 相似文献
43.
K.A. MAASCH P.A. MAYEWSKI E.J. ROHLING J.C. STAGER W. KARLEN L.D. MEEKER E.A. MEYERSON 《Geografiska Annaler: Series A, Physical Geography》2005,87(1):7-15
ABSTRACT. Although considerable attention has been paid to the record of temperature change over the last few centuries, the range and rate of change of atmospheric circulation and hydrology remain elusive. Here, eight latitudinally well-distributed (pole-equator-pole), highly resolved (annual to decadal) climate proxy records are presented that demonstrate major changes in these variables over the last 2000 years. A comparison between atmospheric 14 C and these changes in climate demonstrates a first-order relationship between a variable Sun and climate. The relationship is seen on a global scale. 相似文献
44.
45.
46.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
47.
An HF radar called the Coastal Ocean Dynamics Applications Radar (CODAR) is presently being used in several forms to measure ocean surface parameters. The original version was developed by the National Oceanic and Atmospheric Administration (NOAA) and utilizes a four-element receive array. The array consists of four equally spaced elements arranged on a circle with a radius of 0.2151 wavelengths (at 25.4 MHz). It was designed to measure ocean currents using a direction-finding technique based on an extension to a simple two-element interferometer. The problem of determining the bearing of a radiating source can be readily shown to be equivalent to that incurred in spectral estimation. In an attempt to improve upon the processing of existing data, modern nonlinear spectral estimation techniques are applied in a beam-forming bearing estimation procedure and compared against several direction-finding algorithms. Enhancement of bearing estimators via analysis of the eigenstructure of a spatial correlation matrix is included. Antenna response patterns are calculated and used to investigate properties of direction-finding algorithms. Simulated data are used for a comparison of direction finding and beam forming. The asymmetrical bias of each method is investigated to determine its effect on the error in estimating the angle of arrival of a radar target. 相似文献
48.
G. Djurašević P. Rovithis H. Rovithis-Livaniou E. Fragoulopoulou 《Astrophysics and Space Science》2005,296(1-4):311-314
BV light curves of the eclipsing binary UV Leo obtained at the Kryonerion Astronomical Station of the National Observatory of
Athens, Greece, are analyzed. The analysis is based on a Roche configuration with two spots on the secondary surface. The
elements of the two components of the system are calculated and the spot characteristics are given. 相似文献
49.
50.
Summary We present compositions of reheated melt inclusions in clinopyroxene phenocrysts from three mafic xenoliths in Breccia Museo,
Campi Flegrei, Italy. Melt inclusion compositions are remarkably different from the compositions of known contemporary Campi
Flegrei lavas, being significantly enriched in K2O and depleted in Na2O. Some differences are also evident in FeO* (total Fe as FeO) and TiO2 contents. The clinopyroxene phenocrysts could not have crystallised from Campi Flegrei magmas. We suggest that they originated
from a volcanic system genetically very similar to, and possibly linked with, the >14 ka volcanic system of Mt. Somma, another
Campanian volcano ∼ 30 km east from Campi Flegrei, from which Vesuvius subsequently developed. This result indicates a close
relationship (or link) between the two volcanic systems which have until now been considered separate. We speculate that the
link was established prior to eruption of the Neapolitan Yellow Tuff (NYT) (∼ 12 ka). The xenoliths were derived from a volcanic
system older than the host breccias themselves. We suggest that this older volcanism had close similarities with the volcanism
of the older products of Mt. Somma (∼25 ka).
Received March 20, 2000; accepted November 2, 2000 相似文献