全文获取类型
收费全文 | 90402篇 |
免费 | 1841篇 |
国内免费 | 1001篇 |
专业分类
测绘学 | 2299篇 |
大气科学 | 7163篇 |
地球物理 | 18753篇 |
地质学 | 29987篇 |
海洋学 | 8014篇 |
天文学 | 20332篇 |
综合类 | 242篇 |
自然地理 | 6454篇 |
出版年
2020年 | 640篇 |
2019年 | 676篇 |
2018年 | 1358篇 |
2017年 | 1325篇 |
2016年 | 1978篇 |
2015年 | 1430篇 |
2014年 | 1972篇 |
2013年 | 4516篇 |
2012年 | 2179篇 |
2011年 | 3202篇 |
2010年 | 2753篇 |
2009年 | 4016篇 |
2008年 | 3640篇 |
2007年 | 3333篇 |
2006年 | 3322篇 |
2005年 | 2945篇 |
2004年 | 2995篇 |
2003年 | 2803篇 |
2002年 | 2669篇 |
2001年 | 2394篇 |
2000年 | 2352篇 |
1999年 | 2086篇 |
1998年 | 2009篇 |
1997年 | 1991篇 |
1996年 | 1752篇 |
1995年 | 1603篇 |
1994年 | 1473篇 |
1993年 | 1350篇 |
1992年 | 1272篇 |
1991年 | 1145篇 |
1990年 | 1295篇 |
1989年 | 1138篇 |
1988年 | 1055篇 |
1987年 | 1234篇 |
1986年 | 1118篇 |
1985年 | 1381篇 |
1984年 | 1573篇 |
1983年 | 1511篇 |
1982年 | 1392篇 |
1981年 | 1304篇 |
1980年 | 1144篇 |
1979年 | 1093篇 |
1978年 | 1150篇 |
1977年 | 1031篇 |
1976年 | 976篇 |
1975年 | 933篇 |
1974年 | 946篇 |
1973年 | 958篇 |
1972年 | 591篇 |
1971年 | 509篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
282.
We compare the systematics of the magnetic field geometry in the `red' low-energy peaked BL Lacs (LBLs) and `blue' high-energy peaked BL Lacs (HBLs) using VLBI polarimetric images. The LBLs are primarily `radio-selected' BL Lacs and the HBLs are primarily `X-ray selected'. In contrast to the LBLs, which show predominantly transverse jet magnetic fields, the HBLs show predominantly longitudinal fields. Thus, while the SED peaks of core-dominated quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic field geometry does not follow an analogous sequence. We briefly investigate possible connections between the observed parsec-scale magnetic field structures and circular polarization measurements in the literature on various spatial scales. 相似文献
283.
M.J. Rees 《Astrophysics and Space Science》2003,285(2):375-388
Fred Hoyle famously drew attention to the significance of apparent coincidences in the energy levels of the carbon and oxygen
nucleus. This paper addresses the possible implications of other coincidences in cosmology.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
284.
285.
Several recent studies have reported quasi-periodicities with a period between 1 and 2 years (to be called here `mid-term
quasi-periodicities') in various heliospheric parameters, like solar wind speed, interplanetary magnetic field, cosmic rays,
and geomagnetic activity. Here we study their long-term occurrence in geomagnetic activity using an extended aa index which
covers the last 15 solar cycles. We confirm their intermittent occurrence and the alternation of their dominant period between
a slightly shorter period of about 1.2–1.4 years and a slightly longer period of about 1.5–1.7 years. We find that the mid-term
quasi-periodicities were strong during two intervals of high solar activity: in the mid-19th century and since 1930. Instead,
contrary to earlier studies, we find that they were consistently weak during low solar activity from 1860s to 1920s. This
implies a long-term connection between the amplitude of mid-term quasi-periodicities and the solar dynamo strength. Since
the rotation speed at the bottom of the solar convection layer (tachocline) has recently been found to vary at a mid-term
periodicity, this suggests that the stronger the solar dynamo is, the more variable the rotation rate of the tachocline is.
We also note that the disappearance of mid-term periodicities may be used as a precursor for long intervals of very weak solar
activity, like great minima. 相似文献
286.
287.
The magnitude and spatial distribution of snow on sea ice are both integral components of the ocean–sea‐ice–atmosphere system. Although there exists a number of algorithms to estimate the snow water equivalent (SWE) on terrestrial surfaces, to date there is no precise method to estimate SWE on sea ice. Physical snow properties and in situ microwave radiometry at 19, 37 and 85 GHz, V and H polarization were collected for a 10‐day period over 20 first‐year sea ice sites. We present and compare the in situ physical, electrical and microwave emission properties of snow over smooth Arctic first‐year sea ice for 19 of the 20 sites sampled. Physical processes creating the observed vertical patterns in the physical and electrical properties are discussed. An algorithm is then developed from the relationship between the SWE and the brightness temperature measured at 37 GHz (55°) H polarization and the air temperature. The multiple regression between these variables is able to account for over 90% of the variability in the measured SWE. This algorithm is validated with a small in situ data set collected during the 1999 field experiment. We then compare our data against the NASA snow thickness algorithm, designed as part of the NASA Earth Enterprise Program. The results indicated a lack of agreement between the NASA algorithm and the algorithm developed here. This lack of agreement is attributed to differences in scale between the Special Sensor Microwave/Imager and surface radiometers and to differences in the Antarctic versus Arctic snow physical and electrical properties. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
288.
Himalayan magmatism and porphyry copper–molybdenum mineralization in the Yulong ore belt, East Tibet
Summary ?The NW–SE-trending Yulong porphyry Cu–Mo ore belt, situated in the Sanjiang0 area of eastern Tibet, is approximately 400 km
long and 35 to 70 km wide. Complex tectonic and magmatic processes during the Himalayan epoch have given rise to favorable
conditions for porphyry-type Cu–Mo mineralization.
Porphyry masses of the Himalayan epoch in the Yulong ore belt are distributed in groups along regional NW–SE striking tectonic
lineaments. They were emplaced mainly into Triassic and Lower Permian sedimentary-volcanic rocks. K–Ar und U–Pb isotopic datings
give an intrusion age range of 57–26 Ma. The porphyries are mainly of biotite monzogranitic and biotite syenogranitic compositions.
Geological and geochemical data indicate that the various porphyritic intrusions in the belt had a common or similar magma
source, are metaluminous to peraluminous, Nb–Y–Ba-depleted, I-type granitoids, and belong to the high-K calc-alkaline series.
Within the Yulong subvolcanic belt a number of porphyry stocks bear typical porphyry type Cu–Mo alteration and mineralization.
The most prominent porphyry Co–Mo deposits include Yulong, Malasongduo, Duoxiasongduo, Mangzong and Zhanaga, of which Yulong
is one of the largest porphyry Cu (Mo) deposits in China with approximately 8 × 106 tons of contained Cu metal. Hydrothermal alteration at Yulong developed around a biotite–monzogranitic porphyry stock that
was emplaced within Upper Triassic limestone, siltstone and mudstone. The earliest alteration was due to the effects of contact
metamorphism of the country rocks and alkali metasomatism (potassic alteration) within and around the porphyry body. The alteration
of this stage was accompanied by a small amount of disseminated and veinlet Cu–Mo sulfide mineralization. Later alteration–mineralization
zones form more or less concentric shells around the potassic zone, around which are distributed a phyllic or quartz–sericite–pyrite
zone, a silicification and argillic zone, and a propylitic zone.
Fluid inclusion data indicate that three types of fluids were involved in the alteration–mineralization processes: (1) early
high temperature (660–420 °C) and high salinity (30–51 wt% NaCl equiv) fluids responsible for the potassic alteration and
the earliest disseminated and/or veinlet Cu–Mo sulfide mineralization; (2) intermediate unmixed fluids corresponding to phyllic
alteration and most Cu–Mo sulfide mineralization, with salinities of 30–50 wt% NaCl equiv and homogenization temperatures
of 460–280 °C; and (3) late low to moderate temperature (300–160 °C) and low salinity (6–13 wt% NaCl equiv) fluids responsible
for argillic and propylitic alteration. Hydrogen and oxygen isotopic studies show that the early hydrothermal fluids are of
magmatic origin and were succeeded by increasing amounts of meteoric-derived convective waters. Sulfur isotopes also indicate
a magmatic source for the sulfur in the early sulfide mineralization, with the increasing addition of sedimentary sulfur outward
from the porphyry stock.
Received August 29, 2001; revised version accepted May 1, 2002
Published online: November 29, 2002 相似文献
289.
L. A. Nolan J. S. Dunlop R. Jimenez A. F. Heavens 《Monthly notices of the Royal Astronomical Society》2003,341(2):464-476
We explore whether the rest-frame near-ultraviolet spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by first testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-ultraviolet spectra of F stars of known (subsolar and supersolar) metallicity. F stars are of particular interest because the rest-frame near-ultraviolet spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turn-off.
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
We find that, in the case of the F stars, where the Hubble Space Telescope ultraviolet spectra have a high signal-to-noise ratio, fitting models in which the metallicity is allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turn-off ages of these stars yielded by model-fitting are well constrained. Encouraged by this we have fitted these same variable-metallicity models to the deep, optical spectra of the z ≃ 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age and metallicity are not so easily constrained for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still ≥3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein–de Sitter universe, and favour a Λ-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3–5 . 相似文献
290.