全文获取类型
收费全文 | 43098篇 |
免费 | 644篇 |
国内免费 | 334篇 |
专业分类
测绘学 | 992篇 |
大气科学 | 3080篇 |
地球物理 | 8353篇 |
地质学 | 14841篇 |
海洋学 | 3954篇 |
天文学 | 10044篇 |
综合类 | 80篇 |
自然地理 | 2732篇 |
出版年
2021年 | 425篇 |
2020年 | 429篇 |
2019年 | 470篇 |
2018年 | 975篇 |
2017年 | 866篇 |
2016年 | 1137篇 |
2015年 | 648篇 |
2014年 | 1045篇 |
2013年 | 2176篇 |
2012年 | 1176篇 |
2011年 | 1704篇 |
2010年 | 1542篇 |
2009年 | 2061篇 |
2008年 | 1729篇 |
2007年 | 1794篇 |
2006年 | 1658篇 |
2005年 | 1378篇 |
2004年 | 1336篇 |
2003年 | 1307篇 |
2002年 | 1263篇 |
2001年 | 1099篇 |
2000年 | 1014篇 |
1999年 | 857篇 |
1998年 | 858篇 |
1997年 | 852篇 |
1996年 | 688篇 |
1995年 | 676篇 |
1994年 | 639篇 |
1993年 | 596篇 |
1992年 | 542篇 |
1991年 | 515篇 |
1990年 | 527篇 |
1989年 | 522篇 |
1988年 | 487篇 |
1987年 | 564篇 |
1986年 | 505篇 |
1985年 | 624篇 |
1984年 | 672篇 |
1983年 | 610篇 |
1982年 | 542篇 |
1981年 | 599篇 |
1980年 | 489篇 |
1979年 | 473篇 |
1978年 | 438篇 |
1977年 | 441篇 |
1976年 | 394篇 |
1975年 | 383篇 |
1974年 | 386篇 |
1973年 | 389篇 |
1971年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
During the period of the IQSY, January 1964 through December 1965, the sun remained quiet, accelerating few energetic particles. There were many instances during the IQSY when lowenergy detectors on satellites and space probes registered small intensity increases. However, few of these events were associated with protons of energies exceeding 10 MeV. Moreover, the maximum intensities (E
p > 500 keV) were typically 1–8/cm2sec ster. Most of these events were below the threshold of riometer detection.The largest solar cosmic ray event observed in 1964 by polar-based riometers was that of March 16. This event was observed by 30 and 50 Mc/s riometers at McMurdo Sound, Antarctica, and Shepherd Bay, N.W.T., Canada.The largest event in 1965 occurred on February 5 and was the largest during the IQSY. It was associated with a class 2 flare at about 1750 UT, February 5. The propagation time between the sun and Earth was about one hour. This event was well observed by satellites, space probes, and riometers.This paper discusses primarily the 5 February 1965 event. Some discussion is also given to the 16 March 1964 event, other small events during the IQSY, and the recent event in March 1966. 相似文献
132.
Water Resources - The basic indicators of regional water use have been identified and assessed as a component of a single social–natural–technogenic system. These indicators have been... 相似文献
133.
A. Bollhöfer A. Eisenhauer N. Frank D. Pech A. Mangini 《International Journal of Earth Sciences》1996,85(3):577-585
Thorium- and uranium isotopes were measured in a diagenetic manganese nodule from the Peru basin applying alpha- and thermal
ionization mass spectrometry (TIMS). Alpha-counting of 62 samples was carried out with a depth resolution of 0.4 mm to gain
a high-resolution230Thexcess profile. In addition, 17 samples were measured with TIMS to obtain precise isotope concentrations and isotope ratios. We
got values of 0.06–0.59 ppb (230Th), 0.43–1.40 ppm (232Th), 0.09–0.49 ppb (234U) and 1.66–8.24 ppm (238U). The uranium activity ratio in the uppermost samples (1–6 mm) and in two further sections in the nodule at 12.5±1.0 mm
and 27.3–33.5 mm comes close to the present ocean water value of 1.144±0.004. In two other sections of the nodule, this ratio
is significantly higher, probably reflecting incorporation of diagenetic uranium. The upper 25 mm section of the Mn nodule
shows a relatively smooth exponential decrease in the230Thexcess concentration (TIMS). The slope of the best fit yields a growth rate of 110 mm/Ma up to 24.5 mm depth. The section from 25
to 30.3 mm depth shows constant230Thexcess concentrations probably due to growth rates even faster than those in the top section of the nodule. From 33 to 50 mm depth,
the growth rate is approximately 60 mm/Ma. Two layers in the nodule with distinct laminations (11–15 and 28–33 mm depth) probably
formed during the transition from isotopic stage 8 to 7 and in stage 5e, respectively. The Mn/Fe ratio shows higher values
during interglacials 5 and 7, and lower ones during glacials 4 and 6. A comparison of our data with data from adjacent sediment
cores suggests (a) a variable supply of hydrothermal Mn to sediments and Mn nodules of the Peru basin or (b) suboxic conditions
at the water sediment interface during periods with lower Mn/Fe ratios. 相似文献
134.
Recent models of chemical weathering in alpine glacial meltwaters suggest that sulphide oxidation is a major source of solute in the distributed component of the subglacial hydrological system. This reaction requires O2, and may lower dissolved oxygen levels to below saturation with respect to the atmosphere. This should result in an inverse association between SO72- and dissolved oxygen saturation. However, measurements of O2 saturation in bulk meltwaters draining the Haut Glacier d'Arolla, Switzerland, show that there is a positive association between SO42- and O2 saturation. The O2 content of glacial meltwaters depends on the initial content of snow and icemelt, which may be controlled by the rate of melting, and the kinetic balance between O2 losses (e.g. sulphide oxidation, microbial respiration) and gains (e.g. diffusion of O2 into solution). 相似文献
135.
W. J. Collins D. S. Stevenson C. E. Johnson R. G. Derwent 《Journal of Atmospheric Chemistry》1997,26(3):223-274
A three-dimensional Lagrangian tropospheric chemistry modelis used toinvestigate the impact of human activities on the tropospheric distributionofozone and hydroxyl radicals. The model describes the behaviour of 50 speciesincluding methane, carbon monoxide, oxides of nitrogen, sulphur dioxide andnineorganic compounds emitted from human activities and a range of other sources.Thechemical mechanism involves about 100 chemical reactions of which 16 arephotochemical reactions whose diurnal dependence is treated in full. The modelutilises a five minute chemistry time step and a three hour advection timestepfor the 50,000 air parcels. Meteorological data for the winds, temperatures,clouds and so on are taken from the UK Meteorological Office global model for1994 onwards. The impacts of a 50% reduction in European NOXemissions onglobal ozone concentrations are assessed. Surface ozoneconcentrations decrease in summertime and rise in wintertime, but to differentextents. 相似文献
136.
The convective stability of a simple model chromosphere, consisting of protons, electrons and hydrogen atoms in the ground state, has been studied in the presence of a vertically upward uniform magnetic field to include the effects of FLR, Hall-currents, finite conductivity and ionization. The ionization in the chromosphere is collisional and the recombination is radiative. It is found that the Schwarzchild criterion should necessarily be satisfied for the stability together with the condition thatv > 2v
0, where is kinematic viscosity andv
0 is gyroviscosity. Some special cases have also been investigated. 相似文献
137.
A new Lower Triassic (Induan) Jerus Limestone locality in northwest Pahang,Peninsular Malaysia: Conodont fauna,depositional and tectonic settings 下载免费PDF全文
Ian Metcalfe 《Island Arc》2016,25(2):126-136
Limestones exposed north of Raub, Pahang, Malaysia, and sandwiched between the Bentong‐Raub Suture Zone and the westernmost margin of the Sukhothai Arc terrane, yield a late Dienerian (late Induan) conodont fauna. The co‐occurrence of Neospathodus dieneri Sweet (morphotypes 1, 2 and 3) and Neospathodus pakistanensis Sweet represents the Neospathodus dieneri morphotype 3 sub‐zone of the Neospathodus dieneri Zone. The sampled limestones are interpreted as the northwards extension of the Jerus Limestone which crops out near Cheroh and Jerus villages, significantly extending the known outcrop of the Jerus Limestone northwards. The Jerus Limestone is interpreted as hemipelagic and formed in a foredeep or forearc setting on top of the accretionary complex formed by eastwards subduction of the Palaeo‐Tethys during the Lower to Middle Triassic. 相似文献
138.
139.
Mike Hapgood and Ian Crawford assess the relevance of the BNSC UKSEWG Report for the Royal Astronomical Society. 相似文献
140.
The results of a geoelectrical survey of the Mshentsy area located in Tver oblast near the Moscow–St. Petersburg express road, which is under construction, are presented. In order to obtain information about the geological composition of the area and the processes that occur in it (karst and water flow zones ), we performed our measurements using the electrical resistivity imaging, self potential survey, water electrical conductivity and temperature measurements. Using petrophysical resistivity modeling of the rocks, taking the geoelectric data into account, we completed a lithology differentiation of the section and calculated the hydraulic conductivity values of the rocks. By interpreting this data, we found a wide distribution of fractured carbonates that compose the lower part of the section of the studied territory. The conclusion was drawn that the Mshentsy area belongs to the intense local karst zone. 相似文献