全文获取类型
收费全文 | 4028篇 |
免费 | 57篇 |
国内免费 | 55篇 |
专业分类
测绘学 | 99篇 |
大气科学 | 282篇 |
地球物理 | 779篇 |
地质学 | 1613篇 |
海洋学 | 243篇 |
天文学 | 925篇 |
综合类 | 7篇 |
自然地理 | 192篇 |
出版年
2023年 | 18篇 |
2022年 | 30篇 |
2021年 | 31篇 |
2020年 | 27篇 |
2019年 | 39篇 |
2018年 | 127篇 |
2017年 | 126篇 |
2016年 | 171篇 |
2015年 | 80篇 |
2014年 | 172篇 |
2013年 | 218篇 |
2012年 | 154篇 |
2011年 | 183篇 |
2010年 | 188篇 |
2009年 | 220篇 |
2008年 | 175篇 |
2007年 | 170篇 |
2006年 | 200篇 |
2005年 | 131篇 |
2004年 | 127篇 |
2003年 | 108篇 |
2002年 | 80篇 |
2001年 | 74篇 |
2000年 | 74篇 |
1999年 | 79篇 |
1998年 | 77篇 |
1997年 | 87篇 |
1996年 | 75篇 |
1995年 | 46篇 |
1994年 | 31篇 |
1993年 | 57篇 |
1992年 | 34篇 |
1991年 | 42篇 |
1990年 | 27篇 |
1989年 | 34篇 |
1988年 | 36篇 |
1987年 | 42篇 |
1986年 | 52篇 |
1985年 | 41篇 |
1984年 | 45篇 |
1983年 | 43篇 |
1982年 | 46篇 |
1981年 | 34篇 |
1980年 | 31篇 |
1979年 | 35篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1975年 | 23篇 |
1974年 | 20篇 |
1973年 | 23篇 |
排序方式: 共有4140条查询结果,搜索用时 78 毫秒
491.
X.Z. Zhang S.J. Qian L.A. Higgs T.L. Landecker X.J. Wu 《Astrophysics and Space Science》2002,279(4):355-366
Radio observational results at 232 MHz and multifrequency studies of supernova remnant (SNR) HB21 are presented. Its integrated
flux density at 232 MHz is about 390 ± 30 Jy. Both the integrated spectral index and the spatial variations of spectral index
of the remnant were calculated by combining the new map at 232 MHz with previously published maps made at 408, 1420, 2695,
and 4750 MHz. The SNR has an integrated spectral index of about α = -0.43(S
ν ∝ να) between 232 and 4750 MHz. In general the spectral index varies from –0.5 in southeast and west regions of the remnant to
–0.3 in the central region and near the northwest edge. The new data of 232 MHz reveals that there is interaction between
the remnant and the surrounding gas along the east edge of the remnant which causes the spectrum flattening at low frequency,
while the very good agreement between the structure of X-ray emission and the central flat spectrum area suggests that the
existence of thermal emission is the reason of spectrum flattening in the area.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
492.
Z. Tunca V. Keskin S. Evren C. Ibanoglu M. C. Akan 《Astrophysics and Space Science》1993,204(2):297-304
Two-colour photometric observations of the white-dwarf and K-dwarf eclipsing binary V471 Tau were made during the last four years. The resulting light curves and thirteen timings of mid-eclipse are presented. During the observations two flares were obtained on September 23 and December23, 1992. Both events occurred just after the fourth contact. The increment in the brightness was measured as 0
.
m
067. The energy liberated by the flare was computed to be 5.43 × 1032 erg s–1 in the JohnsonB band, which amounts 1.13 times that of the total radiation of the K-dwarf. 相似文献
493.
Abstract— Two drill cores have been taken from the main mass of the Jilin chondrite, Jilin I. One was bored perpendicular to the 2π surface of the first exposure stage, the other parallel to this surface, and both intersected at the presumed 4π center of Jilin's second exposure stage. The probed depths were 65 cm and 105 cm, respectively, with a diameter of 26.5 mm. Fifteen drill core samples, 2 documented samples and 14 strewn field samples have been analysed for 26Al, 60Co and 22Na. The reconstruction of the preatmospheric Jilin via 60Co resulted in slight deviations for the center position and for the radius (now R = 80+10?5 cm) compared to an earlier study in 1985. The 60Co production rate profile is now almost completely reproduced. A constant production rate of 22Na and of 26Al during the 4π irradiation is confirmed also for the center region. The good linear correlation of 26Al and 21Ne yields slightly revised irradiation ages for both stages of t1 = 7 ± 1 Ma and t2 = 0.32+0.06-0.03 Ma. 相似文献
494.
A New Approach for Studying Hubble Diagrams of Quasars 总被引:2,自引:0,他引:2
In this paper, a new approach for studying Hubble diagrams of quasars is introduced. The purpose is to reduce the well-known
very large scatter in the diagram. We believe that the scatter is mainly caused by the wide spread of luminosity distribution
of quasars. Based on a large number of quasars having been discovered, we introduce a new quantity, the collective apparent
magnitude of certain amount of quasars in a corresponding redshift interval, and make a plot of the collective apparent magnitude
versus redshift with the data published by Hewitt and Burbidge (1993). The quantity is contributed by individual apparent
luminosities as well as the luminosity distribution of quasars. Scatter is expected to be largely reduced in the corresponding
diagram, and this is indeed true as shown by the figures. By discussing several possible effects and from the figures, we
find that the assumption that redshifts of quasars are distance indicators is confirmed, and the standard cosmological model
is supported.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
495.
Z. G. Zhang S. Svanberg P. Palmeri P. Quinet E. Biémont 《Monthly notices of the Royal Astronomical Society》2002,334(1):1-10
The lifetimes of three short-lived levels belonging to the 4f9 6p configuration and of two long-lived levels of the 4f9 5d configuration of Dy iii have been measured for the first time using time-resolved laser-induced fluorescence techniques. They are in good agreement with theoretical values calculated within the framework of a pseudo-relativistic configuration interaction approximation. Using the experimental lifetimes and the theoretical branching fractions, a first set of transition probabilities of astrophysical interest has been obtained for Dy iii . 相似文献
496.
497.
The author puts forward the proposal in this paper that all the terrestrial planets (Venus, the Earth, and Mars) as well as
the Moon deviate from hydrostatic equilibrium to some degree. The Earth's level of deviation of these four celestial bodies
is minimum, and that of Mars is maximum. Moreover, the author estimates Martian nonhydrostatic components of the principal
moments-of-inertia using five models for the interior of Mars. Comparison with other terrestrial planets shows that setting
the range of mean moment-of-inertia ratio, I/MR2, in 0.345 ~ 0.355for Mars is reasonable.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
498.
Estimation of evapotranspiration from a crop field is of great importance for detecting crop water status and proper irrigation scheduling. The Penman–Monteith equation is widely viewed as the best method to estimate evapotranspiration but it requires canopy resistance, which is very difficult to determine in practice. This paper presents a simple method simplified from the Penman–Monteith equation for estimating canopy temperature (Tc). The proposed method is a biophysically‐sound extended version of that proposed by Todorovic. The estimated canopy temperature is used to calculate sensible heat flux, and then latent heat flux is calculated as the residual of the surface energy balance. An eddy covariance (EC) system and an infrared thermometer (IRT) were installed in an irrigated winter wheat field on the North China Plain in 2004 and 2005, to measure Tc, and sensible and latent heat fluxes were used to test the modified Todorovic model (MTD). The results indicate that the original Todorovic model (TD) severely underestimates Tc and sensible heat flux, and hence severely overestimates the latent heat flux. However, the MTD model has good capability for estimating Tc, and gives acceptable results for latent heat flux at both half‐hourly and daily scales. The MTD model results also agreed well with the evapotranspiration calculated from the measured Tc. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
499.
Harm Jan Pierik Jelle I. M. Moree Karianne M. van der Werf Lonneke Roelofs Marcio Boechat Albernaz Antoine Wilbers Bert van der Valk Marieke van Dinter Wim Z. Hoek Tjalling de Haas Maarten G. Kleinhans 《Sedimentology》2023,70(1):179-213
In the transformation from tidal systems to freshwater coastal landscapes, plants act as eco-engineering species that reduce hydrodynamics and trap sediment, but nature and timing of the mechanisms of land creation along estuaries remains unclear. This article focuses on the Old Rhine estuary (The Netherlands) to show the importance of vegetation in coastal landscape evolution, predominantly regarding tidal basin filling and overbank morphology. This estuary hosted the main outflow channel of the river Rhine between ca 6500 to 2000 cal bp , and was constrained by peat during most of its existence. This study reconstructs its geological evolution, by correlating newly integrated geological data and new field records to varying conditions. Numerical modelling was performed to test the inferred mechanisms. It was found that floodbasin vegetation and resulting organic accumulation strongly accelerated back-barrier infill, by minimizing tidal influence. After tidal and wave transport had already sufficiently filled the back-barrier basin, reed rapidly expanded from its edges under brackish conditions, as shown by diatom analysis and datings. Reed growth provided a positive infilling feedback by reducing tidal flow and tidal prism, accelerating basin infilling. New radiocarbon dates show that large-scale crevassing along the Old Rhine River – driven by tidal backwater effect – only started as nutrient-rich river water transformed the floodbasin into an Alder carr in a next phase of estuary evolution. Such less dense vegetation promotes crevassing as sediments are more easily transported into the floodbasin. As river discharge increased and estuary mouth infilling progressed, crevasse activity diminished around 3800 to 3000 cal bp , likely due to a reduced tidal backwater effect. The insights from this data-rich Holocene study showcase the dominant role that vegetation may have in the long-term evolution of coastal wetlands. It provides clues for effective use of vegetation in vulnerable wetland landscapes to steer sedimentation patterns to strategically adapt to rising water levels. 相似文献
500.
Geomagnetism and Aeronomy - The results of a comparison of three different plasmochemical models of the ionospheric D region during C- and X-class X-ray flares are presented. Four-, five- and... 相似文献