全文获取类型
收费全文 | 8652篇 |
免费 | 1609篇 |
国内免费 | 2356篇 |
专业分类
测绘学 | 779篇 |
大气科学 | 3567篇 |
地球物理 | 1769篇 |
地质学 | 2144篇 |
海洋学 | 1135篇 |
天文学 | 1777篇 |
综合类 | 494篇 |
自然地理 | 952篇 |
出版年
2024年 | 55篇 |
2023年 | 127篇 |
2022年 | 235篇 |
2021年 | 330篇 |
2020年 | 359篇 |
2019年 | 404篇 |
2018年 | 313篇 |
2017年 | 333篇 |
2016年 | 328篇 |
2015年 | 434篇 |
2014年 | 546篇 |
2013年 | 558篇 |
2012年 | 561篇 |
2011年 | 556篇 |
2010年 | 472篇 |
2009年 | 631篇 |
2008年 | 583篇 |
2007年 | 620篇 |
2006年 | 600篇 |
2005年 | 559篇 |
2004年 | 463篇 |
2003年 | 453篇 |
2002年 | 377篇 |
2001年 | 331篇 |
2000年 | 384篇 |
1999年 | 313篇 |
1998年 | 288篇 |
1997年 | 216篇 |
1996年 | 200篇 |
1995年 | 184篇 |
1994年 | 185篇 |
1993年 | 120篇 |
1992年 | 96篇 |
1991年 | 89篇 |
1990年 | 63篇 |
1989年 | 55篇 |
1988年 | 55篇 |
1987年 | 36篇 |
1986年 | 22篇 |
1985年 | 24篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 1篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 1篇 |
1954年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
1.
Kaizad F. Patel Ivan J. Fernandez Sarah J. Nelson Stephen A. Norton Cheryl J. Spencer 《水文研究》2021,35(5):e14147
The Bear Brook Watershed in Maine (BBWM) is a long-term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture. 相似文献
2.
Neill Reid 《Monthly notices of the Royal Astronomical Society》2003,342(3):837-850
We present high-resolution echelle spectroscopy of 20 stars in 16 systems catalogued as members of the TW Hydrae association, and 16 stars identified as possible new members. We have calibrated the range of coronal and chromospheric activity expected for such young stars as a function of spectral type by combining our observations with literature data for field and open cluster stars. We also compute space motions for TWA members and candidate members with proper motion measurements, using two techniques to estimate distances to stars lacking direct trigonometric parallax measurements. The mean space motion of the four TWA members with known parallaxes is ( U , V , W : −10.0, −17.8, −4.6) km s−1 . 14 of the candidates have properties inconsistent with cluster membership; the remaining two are potential new members, although further observations are required to confirm this possibility. 相似文献
3.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure. 相似文献
5.
6.
太阳发电机理论主要解释太阳磁场的起源和演化,是太阳物理中最基本、最重要的问题之一.对它的研究不仅具有重要的科学价值,而且对空间天气的预测和空间技术的发展有一定的影响.随着日震学的发展,该理论也得到了很大改进。从相关观测入手,综述了发电机理论需解释的观测事实,以及为发电机理论提供约束的太阳内部动力学结构;介绍了基本理论和近来的主流模型,并指出尚待解决的问题和进一步的努力方向。 相似文献
7.
8.
本文利用宽波段能谱结构讨论了四个PG类星体的红外辐射机制.结果表明,在产生红外连续辐射的可能机制中,非热的、具有幂律特征的辐射和被核光度加热的“核尘埃”的热再辐射是红外辐射的主要来源.对于高光度的活动星系核,它的主星系的红外辐射及恒星形成区的红外辐射是可以忽略的. 相似文献
9.
10.
根据印度洋西北部鸢乌贼(Sym plectoteuthis oualaniensis)作业渔场的现场调查数据.分析了作业水深、作业时段、摄食等级和月相等因子对手钓上钩率的影响。统计表明.晚上、午夜和凌晨3个不同时段和不同作业水深对手钓上钩率均有显著性影响。3个时段中,其摄食等级呈先低后高再低的现象。午夜和凌展,上钩率的变化趋势与摄食等级的变化趋势呈显著的负相关。在月相的影响下.离朔日三四天之前,手钓产量呈上升趋势;离望日五六天之前。手钓产量呈明显的下降趋势。 相似文献