全文获取类型
收费全文 | 676篇 |
免费 | 20篇 |
国内免费 | 6篇 |
专业分类
测绘学 | 20篇 |
大气科学 | 43篇 |
地球物理 | 135篇 |
地质学 | 217篇 |
海洋学 | 38篇 |
天文学 | 140篇 |
自然地理 | 109篇 |
出版年
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 12篇 |
2017年 | 14篇 |
2016年 | 15篇 |
2015年 | 10篇 |
2014年 | 13篇 |
2013年 | 25篇 |
2012年 | 19篇 |
2011年 | 25篇 |
2010年 | 30篇 |
2009年 | 33篇 |
2008年 | 33篇 |
2007年 | 24篇 |
2006年 | 42篇 |
2005年 | 21篇 |
2004年 | 22篇 |
2003年 | 18篇 |
2002年 | 15篇 |
2001年 | 11篇 |
2000年 | 15篇 |
1999年 | 21篇 |
1998年 | 16篇 |
1997年 | 9篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1994年 | 10篇 |
1993年 | 10篇 |
1992年 | 6篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 10篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 8篇 |
1976年 | 11篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1971年 | 7篇 |
1970年 | 4篇 |
排序方式: 共有702条查询结果,搜索用时 15 毫秒
251.
The temperatures of electrons and ions in the post-shock accretion region of a magnetic cataclysmic variable (mCV) will be equal at sufficiently high mass flow rates or for sufficiently weak magnetic fields. At lower mass flow rates or in stronger magnetic fields, efficient cyclotron cooling will cool the electrons faster than the electrons can cool the ions and a two-temperature flow will result. Here we investigate the differences in polarized radiation expected from mCV post-shock accretion columns modeled with one- and two-temperature hydrodynamics. In an mCV model with one accretion region, a magnetic field ?30 MG and a specific mass flow rate of ~0.5 g?cm?2?s?1, along with a relatively generic geometric orientation of the system, we find that in the ultraviolet either a single linear polarization pulse per binary orbit or two pulses per binary orbit can be expected, depending on the accretion column hydrodynamic structure (one- or two-temperature) modeled. Under conditions where the physical flow is two-temperature, one pulse per orbit is predicted from a single accretion region where a one-temperature model predicts two pulses. The intensity light curves show similar pulse behavior but there is very little difference between the circular polarization predictions of one- and two-temperature models. Such discrepancies indicate that it is important to model some aspect of two-temperature flow in indirect imaging procedures, like Stokes imaging, especially at the edges of extended accretion regions, were the specific mass flow is low, and especially for ultraviolet data. 相似文献
252.
Centimeter-Level Positioning of a U. S. Coast Guard Buoy Tender 总被引:1,自引:1,他引:0
David B. Zilkoski J. Don D'Onofrio Rudolf J. Fury Curtis L. Smith Lloyd C. Huff Barry J. Gallagher 《GPS Solutions》1999,3(2):53-65
With the availability of high-accuracy, differential global positioning system (GPS) results in real-time, there is a new
opportunity to use GPS to accurately measure a marine vessel's dynamic draft (settlement and squat) and 3D attitude (roll,
pitch, and heading). The National Geodetic Survey (NGS) and the Coast Survey (CS), offices of the National Ocean Service (NOS),
National Oceanic and Atmospheric Administration (NOAA), propose to transfer this technology to the shipping industry. The
overall goal of this project is to provide the position of a vessel's keel in real time to within 10 cm (about 4 inches) relative
to the bottom of the shipping channel.
In support of this phase of the project, there were three meetings hosted by the Port of Oakland, California and NOS to discuss
the real-time positioning of vessels project. On December 3 and 4, 1996, CS, NGS, Trimble Navigation Ltd., and the U. S. Coast
Guard (USCG) performed GPS tests on a USCG buoy-tender ship. GPS data were used to compute the vessel's dynamic draft and
3D attitude. During the test, five receivers continually collected data; one receiver was located at a base station on the
USCG pier on Yerba Buena Island, and four were on the ship: two on the stern and two on the bow. CS installed a TSS-335B vertical
reference unit (to measure heave, pitch, and roll) in the engine room of the ship.
NOS processed the GPS data and computed the vessel's dynamic draft and 3D attitude. The results indicate that the linear equivalent
to the vessel's dynamic draft and 3D attitude were accurate to the 10-cm level using GPS. It was also demonstrated how a ship
can be used to measure local water-level changes and actual water-level values everywhere it travels. ? 1999 John Wiley &
Sons, Inc. 相似文献
253.
254.
Coeval δ13C shifts recorded in buried soils at both piedmont slope and basin floor sites in the northern Chihuahuan Desert indicate a major shift from C4grasses to C3desert-scrub between 7 and 9 ka. The age assignments are based on stratigraphic correlations to charcoal dates and carbon-14 dates of carbonate. This shift is synchronous with a period of cooling in the North Atlantic that may have triggered a period of drought in the south-western United States. Coinciding with this vegetation change, geomorphic evidence in Rio Grande, piedmont, and basin floor eolian environments indicates a major period of erosion. Subsequent gradual enrichment of pedogenic carbonateδ13 C values in younger deposits suggests that C4grasses rebounded in the late Holocene (approximately 4 ka), which is consistent with other evidence of increased moisture regionally. A period of less severe aridity at approximately 2·2 ka is indicated by erosion and subsequent deposition along the alluvial fans and within the basin, and correlates with depleted pedogenic carbonate δ13C values suggesting a decrease in C4grasses. Isotope and packrat midden records should be used together to infer past environmental conditions at different elevations. 相似文献
255.
Population geographers should consider a mixed methods approach to thestudy of migration. This methodological position arises in response to the challenges of contemporary social theory. It is argued that application of structuration theory favours use of a diversity of methods to investigate the recursive relationship between agency and structure. Similarly a realist stance, as advocated by Sayer (1992), leads to a mixing of methods in order to carry out the synthesising tasks expected of geographers. Postmodernism, when interpreted as method, also points the researcher to consider adopting flexible research practices in order to capture the multiplicities of meaning associated with migration and place. These points are illustrated by a case study of migration to and from Hong Kong. 相似文献
256.
Development planners in arid areas face many difficulties in preparing appropriate strategies. Their task is made harder by the dearth of reliable demographic information. This paper presents population projections for the arid area of eastern Jordan covered by the Badia Research and Development Project and evaluates the economic and social implications of these projections over the next 20 years. The paper argues that conventional demographic forecasting methods are extremely problematic when applied to a population such as this. Furthermore, the pastoral economy, which has in the past been the main source of livelihood, cannot hope to sustain the region's future population. High fertility rates will continue to place great stress on the educational and health infrastructure. If appropriate planning responses are not achieved, the demographic regime of the Badia project area and the quality of life of the population may continue to diverge from the patterns found in other parts of Jordan. 相似文献
257.
258.
Keith Richards Mike Batty Kevin Edwards Allan Findlay Giles Foody Lynne Frostick Kelvyn Jones Roger Lee David Livingstone Terry Marsden Judith Petts Chris Philo David Simon Susan Smith David Thomas 《Area》2009,41(3):231-243
We present a summary of the kinds of outputs submitted to the Geography and Environmental Studies sub-panel (H-32) for the 2008 Research Assessment Exercise (RAE), and examine the relationships between the peer assessment of research quality that the RAE process has typified, and alternative modes of assessment based on bibliometrics. This comparison is effected using (in aggregate form) some of the results from the RAE, together with citation data gathered after completion of the RAE assessment, specifically for the purpose of this paper. We conclude that, if it continues to be necessary and desirable to assess, in some measure and however imprecisely, research quality, then peer assessment cannot be replaced by bibliometrics. Bibliometrics permit measurement of something that may be linked to quality but is essentially a different phenomenon – a measure of 'impact', for example. 相似文献
259.
A novel localized finite-amplitude hydrodynamic stability analysis is established in a unified treatment for the study of real oceanic and atmospheric processes, which are in general highly nonlinear, and intermittent in space and time. We first re-state the classical definition using the multi-scale energy and vorticity analysis (MS-EVA) developed in Liang and Robinson [Liang, X.S., Robinson, A.R., 2005. Localized multiscale energy and vorticity analysis. I. Fundamentals. Dyn. Atmos. Oceans 38, 195–230], and then manipulate certain global operators to achieve the temporal and spatial localization. The key of the spatial localization is transfer-transport separation, which is made precise with the concept of perfect transfer, while relaxation of marginalization leads to the localization of time. In doing so the information of transfer lost in the averages is retrieved and an easy-to-use instability metric is obtained. The resulting metric is field-like (Eulerian), conceptually generalizing the classical formalism, a bulk notion over the whole system. In this framework, an instability has a structure, which is of particular use for open flow processes. We check the structure of baroclinic instability with the benchmark Eady model solution, and the Iceland–Faeroe Frontal (IFF) intrusion, a highly localized and nonlinear process occurring frequently in the region between Iceland and Faeroe Islands. A clear isolated baroclinic instability is identified around the intrusion, which is further found to be characterized by the transition from a spatially growing mode to a temporally growing mode. We also check the consistency of the MS-EVA dynamics with the barotropic Kuo model. An observation is that a local perturbation burst does not necessarily imply an instability: the perturbation energy could be transported from other processes occurring elsewhere. We find that our analysis yields a Kuo theorem-consistent mean–eddy interaction, which is not seen in a conventional Reynolds stress framework. Using the techniques of marginalization and localization, this work sets up an example for the generalization of certain geophysical fluid dynamics theories for more generic purposes. 相似文献
260.
Trace and minor elements in sphalerite: A LA-ICPMS study 总被引:18,自引:0,他引:18
Nigel J. Cook Cristiana L. Ciobanu Allan Pring William Skinner Leonid Danyushevsky Frank Melcher 《Geochimica et cosmochimica acta》2009,73(16):4761-4791
Sphalerite is an important host mineral for a wide range of minor and trace elements. We have used laser-ablation inductively coupled mass spectroscopy (LA-ICPMS) techniques to investigate the distribution of Ag, As, Bi, Cd, Co, Cu, Fe, Ga, Ge, In, Mn, Mo, Ni, Pb, Sb, Se, Sn and Tl in samples from 26 ore deposits, including specimens with wt.% levels of Mn, Cd, In, Sn and Hg. This technique provides accurate trace element data, confirming that Cd, Co, Ga, Ge, In, Mn, Sn, As and Tl are present in solid solution. The concentrations of most elements vary over several orders of magnitude between deposits and in some cases between single samples from a given deposit. Sphalerite is characterized by a specific range of Cd (typically 0.2-1.0 wt.%) in each deposit. Higher Cd concentrations are rare; spot analyses on samples from skarn at Baisoara (Romania) show up to 13.2 wt.% (Cd2+ ↔ Zn2+ substitution). The LA-ICPMS technique also allows for identification of other elements, notably Pb, Sb and Bi, mostly as micro-inclusions of minerals carrying those elements, and not as solid solution. Silver may occur both as solid solution and as micro-inclusions. Sphalerite can also incorporate minor amounts of As and Se, and possibly Au (e.g., Magura epithermal Au, Romania). Manganese enrichment (up to ∼4 wt.%) does not appear to enhance incorporation of other elements. Sphalerite from Toyoha (Japan) features superimposed zoning. Indium-sphalerite (up to 6.7 wt.% In) coexists with Sn-sphalerite (up to 2.3 wt.%). Indium concentration correlates with Cu, corroborating coupled (Cu+In3+) ↔ 2Zn2+ substitution. Tin, however, correlates with Ag, suggesting (2Ag+Sn4+) ↔ 3Zn2+ coupled substitution. Germanium-bearing sphalerite from Tres Marias (Mexico) contains several hundred ppm Ge, correlating with Fe. We see no evidence of coupled substitution for incorporation of Ge. Accordingly, we postulate that Ge may be present as Ge2+ rather than Ge4+. Trace element concentrations in different deposit types vary because fractionation of a given element into sphalerite is influenced by crystallization temperature, metal source and the amount of sphalerite in the ore. Epithermal and some skarn deposits have higher concentrations of most elements in solid solution. The presence of discrete minerals containing In, Ga, Ge, etc. also contribute to the observed variance in measured concentrations within sphalerite. 相似文献