全文获取类型
收费全文 | 77356篇 |
免费 | 923篇 |
国内免费 | 527篇 |
专业分类
测绘学 | 2116篇 |
大气科学 | 5268篇 |
地球物理 | 14754篇 |
地质学 | 29631篇 |
海洋学 | 6404篇 |
天文学 | 16994篇 |
综合类 | 276篇 |
自然地理 | 3363篇 |
出版年
2022年 | 400篇 |
2021年 | 660篇 |
2020年 | 689篇 |
2019年 | 829篇 |
2018年 | 4011篇 |
2017年 | 3678篇 |
2016年 | 3101篇 |
2015年 | 1062篇 |
2014年 | 1755篇 |
2013年 | 3074篇 |
2012年 | 2785篇 |
2011年 | 4586篇 |
2010年 | 4159篇 |
2009年 | 4933篇 |
2008年 | 4133篇 |
2007年 | 4698篇 |
2006年 | 2530篇 |
2005年 | 2067篇 |
2004年 | 1991篇 |
2003年 | 1966篇 |
2002年 | 1880篇 |
2001年 | 1502篇 |
2000年 | 1367篇 |
1999年 | 1147篇 |
1998年 | 1149篇 |
1997年 | 1157篇 |
1996年 | 917篇 |
1995年 | 884篇 |
1994年 | 826篇 |
1993年 | 694篇 |
1992年 | 643篇 |
1991年 | 645篇 |
1990年 | 635篇 |
1989年 | 649篇 |
1988年 | 571篇 |
1987年 | 663篇 |
1986年 | 596篇 |
1985年 | 709篇 |
1984年 | 817篇 |
1983年 | 745篇 |
1982年 | 719篇 |
1981年 | 662篇 |
1980年 | 594篇 |
1979年 | 579篇 |
1978年 | 580篇 |
1977年 | 506篇 |
1976年 | 458篇 |
1975年 | 458篇 |
1974年 | 440篇 |
1973年 | 509篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
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
32.
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
33.
We consider the evolution of certain low-mass binaries, incorporating models of (a) internal evolution, (b) tidal friction,
(c) dynamo activity driven by an elementary α,Ω dynamo, (d) stellar wind driven by the activity, and (e) magnetic braking
as a consequence of wind and poloidal dynamo-generated magnetic field. In some circumstances the stellar wind is found to
remove mass on a nuclear timescale, as is necessary to explain some observed systems.
We can hope that various uncertainties in the model may be clarified by a careful comparison of the models with such observed
quantities as rotation periods. These are modified by processes (a), (b) and (e). Assuming that stellar evolution is slow,
rotation rate should in some circumstances represent a balance between magnetic braking trying to slow the star down and tidal
friction trying to spin it up. Preliminary attempts are promising, but indicate that some fine tuning is necessary.
When there is a third body present, in an orbit which is inclined but not necessarily of short period, the eccentricity of
a close binary can be strongly modified by ‘Kozai cycles’. We show that this may complicate attempts to account for spin rates
of stars in close binaries. 相似文献
34.
35.
36.
Cygnus A was the first hyper-active galaxy discovered, and it
remains by far the closest of the ultra-luminous radio galaxies. As
such, Cygnus A has played a fundamental role in the study of virtually
all aspects of extreme activity in galaxies. We present a review of jet
theory for powering the double-lobed radio emitting structures in
powerful radio galaxies, followed by a review of observations of Cygnus
A in the radio, optical, and X-ray relevant to testing various aspects
of jet theory. Issues addressed include: jet structure from pc- to
kpc-scales, jet stability, confinement, composition, and velocity, the
double shock structure for the jet terminus and the origin of multiple
radio hotspots, the nature of the filamentary structure in the radio
lobes, and the hydrodynamic evolution of the radio lobes within a dense
cluster atmosphere, including an analysis of pressure balance between
the various gaseous components. Also discussed are relativistic
particle acceleration and loss mechanisms in Cygnus A, as well as
magnetic field strengths and geometries both within the radio source,
and in the intracluster medium. We subsequently review the
classification, cluster membership, and the emission components of the
Cygnus A galaxy. The origin of the activity is discussed.
Concentrating on the nuclear regions of the galaxy, we review the
evidence for an obscured QSO, also given the constraints on the
orientation of the radio source axis with respect to the sky plane. We
present an overview of models of central engines in AGN and observations
of Cygnus A which may be relevant to testing such models. We conclude
with a brief section concerning the question of whether Cygnus A is
representative of powerful high redshift radio galaxies.
Received October 10, 1995 相似文献
37.
38.
39.
Michail I. PETAEV 《Meteoritics & planetary science》1996,31(6):807-815
Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them. 相似文献
40.
We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during
1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal
that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles.
The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease
of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles
of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed. 相似文献