全文获取类型
收费全文 | 55050篇 |
免费 | 1041篇 |
国内免费 | 397篇 |
专业分类
测绘学 | 1556篇 |
大气科学 | 4294篇 |
地球物理 | 10686篇 |
地质学 | 18180篇 |
海洋学 | 4648篇 |
天文学 | 13191篇 |
综合类 | 135篇 |
自然地理 | 3798篇 |
出版年
2021年 | 358篇 |
2020年 | 413篇 |
2019年 | 447篇 |
2018年 | 989篇 |
2017年 | 955篇 |
2016年 | 1262篇 |
2015年 | 854篇 |
2014年 | 1346篇 |
2013年 | 2813篇 |
2012年 | 1340篇 |
2011年 | 1859篇 |
2010年 | 1689篇 |
2009年 | 2274篇 |
2008年 | 2088篇 |
2007年 | 2042篇 |
2006年 | 1921篇 |
2005年 | 1780篇 |
2004年 | 1747篇 |
2003年 | 1588篇 |
2002年 | 1550篇 |
2001年 | 1325篇 |
2000年 | 1283篇 |
1999年 | 1203篇 |
1998年 | 1159篇 |
1997年 | 1128篇 |
1996年 | 915篇 |
1995年 | 908篇 |
1994年 | 875篇 |
1993年 | 805篇 |
1992年 | 789篇 |
1991年 | 739篇 |
1990年 | 824篇 |
1989年 | 720篇 |
1988年 | 696篇 |
1987年 | 790篇 |
1986年 | 671篇 |
1985年 | 884篇 |
1984年 | 1007篇 |
1983年 | 982篇 |
1982年 | 910篇 |
1981年 | 878篇 |
1980年 | 775篇 |
1979年 | 757篇 |
1978年 | 741篇 |
1977年 | 660篇 |
1976年 | 624篇 |
1975年 | 546篇 |
1974年 | 626篇 |
1973年 | 616篇 |
1972年 | 374篇 |
排序方式: 共有10000条查询结果,搜索用时 823 毫秒
281.
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
282.
283.
Oil-weathering processes in ice-free subarctic and Arctic waters include spreading, evaporation, dissolution, dispersion of whole-oil droplets into the water column, photochemical oxidation, water-in-oil emulsification, microbial degradation, adsorption onto suspended particulate material, ingestion by organisms, sinking, and sedimentation. While many of these processes also are important factors in ice-covered waters, the various forms of sea ice (depending on the active state of ice growth, extent of coverage and/or decay) impart drastic, if not controlling, changes to the rates and relative importance of different oil-weathering mechanisms. Flow-through seawater wave-tank experiments in a cold room at −35°C and studies in the Chukchi Sea in late winter provide data on oil fate and effects for a variety of potential oil spill scenarios in the Arctic. Time-series chemical weathering data are presented for Prudhoe Bay crude oil released under and encapsulated in growing first-year columnar ice through spring breakup. 相似文献
284.
I.IN~crIOXLocatedinthecoddlepatofTallmBasin,withanareaof33.76X104klnZ,theTaldirnakan~isinthehinterlandofEurasia.Blockedbythehighmountainsaround,vapourofoceancanhardlyreachthedesert.APartfromthis,asthedeSertisinthesinkingcompensationareaoftheascensionalaircurrentaamstheQinghal-XIZangPlateau,itSprecipitationisrareandtheevaporationcapacityisintensealltheyearround.Allthesecontributetotheformationsofthetypifydrydesertclimate.Theacidityindexofthedesertandthearoundregionisashighasmorethan50,w… 相似文献
285.
286.
287.
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
288.
289.
290.
G.C. Sethia G.J. Bailey R.J. Moffett J.K. Hargreaves 《Planetary and Space Science》1985,33(3):321-332
A modelling study of the electron content of the mid-latitude ionosphere and protonosphere has been carried out for solstice conditions using the mathematical model of Bailey (1983). In the model calculations coupled time-dependent O+, H+ continuity and momentum equations and O+, H+ and electron heat balance equations are solved for a magnetic shell extending over both hemispheres. The inclusion of interhemispheric flow of plasma and of heat balance has enabled us to investigate the role of interhemispheric coupling on the electron content and related shape parameters. The computed results are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.).It has been found that the conjugate photoelectron heating has a major effect on the shape of the daily variation of slant slab thickness (τ) and also on the magnitude of the protonospheric content (Np). Some of the main features of τ are closely related to the sunrise and sunset times in the conjugate ionosphere. Also it is found that night-time increases in total electron content (NT) and F2 region peak electron density (Nmax) in winter are natural consequences of ionization loss at low altitudes causing an enhanced downward flow of plasma from the protonosphere which is coupled to the summer hemisphere. One other important consequence of the coupled protonosphere is that the effects on NT of the neutral air wind are not much different in winter from those in summer. 相似文献