全文获取类型
收费全文 | 66254篇 |
免费 | 1614篇 |
国内免费 | 1336篇 |
专业分类
测绘学 | 1731篇 |
大气科学 | 5511篇 |
地球物理 | 13760篇 |
地质学 | 23844篇 |
海洋学 | 5583篇 |
天文学 | 14235篇 |
综合类 | 329篇 |
自然地理 | 4211篇 |
出版年
2021年 | 589篇 |
2020年 | 612篇 |
2019年 | 685篇 |
2018年 | 1389篇 |
2017年 | 1327篇 |
2016年 | 1699篇 |
2015年 | 1119篇 |
2014年 | 1659篇 |
2013年 | 3217篇 |
2012年 | 2031篇 |
2011年 | 2716篇 |
2010年 | 2406篇 |
2009年 | 3127篇 |
2008年 | 2708篇 |
2007年 | 2640篇 |
2006年 | 2647篇 |
2005年 | 2034篇 |
2004年 | 1953篇 |
2003年 | 1874篇 |
2002年 | 1861篇 |
2001年 | 1605篇 |
2000年 | 1607篇 |
1999年 | 1295篇 |
1998年 | 1293篇 |
1997年 | 1264篇 |
1996年 | 1108篇 |
1995年 | 1061篇 |
1994年 | 941篇 |
1993年 | 833篇 |
1992年 | 819篇 |
1991年 | 780篇 |
1990年 | 846篇 |
1989年 | 741篇 |
1988年 | 701篇 |
1987年 | 857篇 |
1986年 | 738篇 |
1985年 | 859篇 |
1984年 | 1059篇 |
1983年 | 968篇 |
1982年 | 918篇 |
1981年 | 872篇 |
1980年 | 832篇 |
1979年 | 755篇 |
1978年 | 768篇 |
1977年 | 714篇 |
1976年 | 651篇 |
1975年 | 624篇 |
1974年 | 696篇 |
1973年 | 697篇 |
1972年 | 433篇 |
排序方式: 共有10000条查询结果,搜索用时 781 毫秒
291.
Globorotalia puncticulata and Globorotalia margaritae are critical species that define internationally recognized planktonic foraminiferal biozones in the Pliocene. These biozones are defined from stratotype sections on Sicily and their fauna are commonly considered to have been introduced to the Mediterranean after an influx of Atlantic water that terminated the late Miocene desiccation of the basin. Herein new discoveries of these species in rocks that predate the late Miocene desiccation are described. These data are supported by magneto- and lithostratigraphy that have been integrated at a single continuous section. Not only do these discoveries question the existing foraminiferal biozone stratigraphy, they also suggest new models for the dispersal of planktonic species. It is proposed that Globorotalia puncticulata and perhaps even Globorotalia margaritae evolved in the Mediterranean during earliest Messinian times (during or before chron C3Bn1n) and dispersed into the Atlantic. This suggests that a marine connection remained between the two sea areas until at least chron C3An.1n. Using the existing geomagnetic polarity time scale, these occurrences are some 2 Myr earlier than previously recorded in the Mediterranean. The distribution of G. margaritae and G. puncticulata in Mediterranean sections is likely to reflect palaeoenvironment or the preservation of deposits rather than the absolute age of the sediments. 相似文献
292.
293.
C. Christiansen F. Gertz M. J. C. Laima L. C. Lund-Hansen T. Vang C. Jürgensen 《Environmental Geology》1997,29(1-2):66-77
The yearly nutrient supply from land and atmosphere to the study area in SW Kattegat is 10 900 tons of N and 365 tons of
P. This is only few percent of the supply from adjacent marine areas, as the yearly transport through the study area is 218 000
tons of N and 18 250 tons of P. Yearly net deposition makes up 1340 tons of N (on average 2.5 g m–2 yr–1) and 477 ton of P (on average 0.9 g m–2 yr–1). Shallow-water parts of the study area have no net deposition because of frequent (>35% of the year) resuspension. Resuspension
frequency in deep water is <1% of the year. Resuspension rates, as averages for the study area, are 10–17 times higher than
net deposition rates. Because of resuspension, shallow-water sediments are coarse lag deposits with small amounts of organic
matter (1.1%) and nutrients (0.04% N and 0.02% P). Deep-water sediments, in contrast, are fine grained with high levels of
organic matter (11.7%) and nutrients (0.43% N and 0.15% P). Laboratory studies showed that resuspension changes the diffusive
sediment water fluxes of nutrients, oxygen consumption, and penetration into the sediment. Fluxes of dissolved reactive phosphate
from sediment to water after resuspension were negative in organic-rich sediments (13.2% organic matter) with low porosity
(56) and close to zero in coarse sediments with a low organic matter content (2.3%) and high porosity (73). Fluxes of inorganic
N after resuspension were reduced to 70% and 0–20% in relation to the rates before resuspension, respectively.
Received: 10 July 1995 · Accepted: 19 January 1996 相似文献
294.
A new method of initial orbit determination 总被引:2,自引:0,他引:2
Up to now we have been dealing with the construction of entirely analytical planetary theories such as VSOP82 (Bretagnon, 1982) and TOP82 (Simon, 1983). These theories take into account the whole of the Newtonian perturbations of nine point masses: the Sun, the Earth-Moon barycentre, the planets Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. They also take into account perturbations due to some minor planets, to the action of the Moon and the relativistic effects. The perturbations of these last three types are in a very simple way under analytical form but they considerably increase the computations when introduced in the numerical integration programs.In the present paper we thus study a solution in which the Newtonian perturbations for the ten point masses are treated through numerical integration, the other perturbations being analytically added. 相似文献
295.
Richard L. Branham Jr. 《Celestial Mechanics and Dynamical Astronomy》1986,39(3):239-247
A method is given, based on the pseudoinverse of the equations of condition, to obtain error estimates for the solution in the normL
1 of an over-determined linear system. The computational labor to obtain the errors, while not trivial, is less than that for various competing methods, particularly if there are many more equations of condition than unknowns. The error estimates for anL
1 solution are substantially larger than those for a least squares solution of the some data. It is suggested that a complete discussion of a linear system include at least bothL
1 and least squares solutions with their respective errors and the condition number of the linear system. 相似文献
296.
L. Zaninetti 《Astrophysics and Space Science》1985,117(2):287-292
We present optical photometric observations of the cataclysmic variables AO Psc and V 1223 Sgr, both of which are believed to contain slowly-rotating white-dwarf pulsars. These objectes belong to the new class of intermediate polars in which the magnetized-white-dwarf rotation period lies in between synchrous and extremely fast rotation periods. The observations reported here were carried out from the Kavalur Observatory using a 1 m reflector. High-time resolution light curves depicting a rich variety of flicker phenomena are presented. Results of a periodic analysis to search for coherent periodic modulations in the light curves are also presented. We have also derived the optical pulse profile of AO Psc and point out its similarities to the X-ray pulse profile. 相似文献
297.
In this paper we discuss the initial phase of chromospheric evaporation during a solar flare observed with instruments on the Solar Maximum Mission on May 21, 1980 at 20:53 UT. Images of the flaring region taken with the Hard X-Ray Imaging Spectrometer in the energy bands from 3.5 to 8 keV and from 16 to 30 keV show that early in the event both the soft and hard X-ray emissions are localized near the footpoints, while they are weaker from the rest of the flaring loop system. This implies that there is no evidence for heating taking place at the top of the loops, but energy is deposited mainly at their base. The spectral analysis of the soft X-ray emission detected with the Bent Crystal Spectrometer evidences an initial phase of the flare, before the impulsive increase in hard X-ray emission, during which most of the thermal plasma at 107 K was moving toward the observer with a mean velocity of about 80 km s-1. At this time the plasma was highly turbulent. In a second phase, in coincidence with the impulsive rise in hard X-ray emission during the major burst, high-velocity (370 km s-1) upward motions were observed. At this time, soft X-rays were still predominantly emitted near the loop footpoints. The energy deposition in the chromosphere by electrons accelerated in the flare region to energies above 25 keV, at the onset of the high-velocity upflows, was of the order of 4 × 1010 erg s-1 cm-2. These observations provide further support for interpreting the plasma upflows as the mechanism responsible for the formation of the soft X-ray flare, identified with chromospheric evaporation. Early in the flare soft X-rays are mainly from evaporating material close to the footpoints, while the magnetically confined coronal region is at lower density. The site where upflows originate is identified with the base of the loop system. Moreover, we can conclude that evaporation occurred in two regimes: an initial slow evaporation, observed as a motion of most of the thermal plasma, followed by a high-speed evaporation lasting as long as the soft X-ray emission of the flare was increasing, that is as long as plasma accumulation was observed in corona. 相似文献
298.
Berkson J. Kloosterman H. Akal T. Berrou J. 《Oceanic Engineering, IEEE Journal of》1985,10(3):299-302
Signals from an explosive source backscattered from the seafloor and received at long range by hydrophones of a towed array are processed to estimate the directional distribution of energy for a given time increment. As assembly of these data shows the time and amplitude of scattering features, and after conversion to distance, the geographic location of the return. A frequency-domain beam-forming procedure is used in which beam levels are averaged over a given band of a broad-band source. The processing is applied to experimental data obtained in the southern Tyrrhenian Sea. The major backscattering occurred at the Baconi Seamounts and the coastal margin of Sardinia. 相似文献
299.
George L. Withbroe 《Solar physics》1985,99(1-2):145-158
EUV observations of two subflares and associated surges have been analyzed. At maximum brightness the emission measures and radiative outputs of the subflares were approximately 20% of the corresponding values for the active region. Multiple EUV surges were observed during and following each subflare, with surge material being ejected in a variety of directions, including toward a coronal bright point located outside of the active region. The total energy of the surges appears to be comparable to that radiated by the subflares, a few times 1028 erg. As reported in previous studies of surges, we find that there was no significant emission from these features in spectral lines formed at temperatures T>106K. The ejection of surges in several different directions and nearly simultaneous flaring of various areas of the active region suggest that the primary site of the subflares was magnetically connected to a variety of different areas in the active region and the surrounding quiet region. 相似文献
300.