全文获取类型
收费全文 | 1727篇 |
免费 | 63篇 |
国内免费 | 18篇 |
专业分类
测绘学 | 40篇 |
大气科学 | 123篇 |
地球物理 | 497篇 |
地质学 | 567篇 |
海洋学 | 165篇 |
天文学 | 282篇 |
综合类 | 7篇 |
自然地理 | 127篇 |
出版年
2021年 | 21篇 |
2020年 | 23篇 |
2019年 | 24篇 |
2018年 | 42篇 |
2017年 | 32篇 |
2016年 | 56篇 |
2015年 | 36篇 |
2014年 | 42篇 |
2013年 | 83篇 |
2012年 | 45篇 |
2011年 | 62篇 |
2010年 | 78篇 |
2009年 | 75篇 |
2008年 | 66篇 |
2007年 | 83篇 |
2006年 | 61篇 |
2005年 | 55篇 |
2004年 | 51篇 |
2003年 | 52篇 |
2002年 | 49篇 |
2001年 | 26篇 |
2000年 | 33篇 |
1999年 | 28篇 |
1998年 | 27篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 29篇 |
1994年 | 31篇 |
1993年 | 14篇 |
1992年 | 31篇 |
1991年 | 29篇 |
1990年 | 38篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 19篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 12篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1971年 | 11篇 |
排序方式: 共有1808条查询结果,搜索用时 15 毫秒
911.
Paul A. Crowther Raman K. Prinja Max Pettini Charles C. Steidel 《Monthly notices of the Royal Astronomical Society》2006,368(2):895-902
We reconsider the use of the equivalent width of C iv λ1549, EW(C iv ), as an indicator of the oxygen abundance in star-forming galaxies, as proposed by Heckman et al. for nearby starbursts. We refine the local calibration of EW(C iv ) versus log (O/H) by using a restricted wavelength window which minimizes blending with interstellar absorption lines. When applied to the stellar component only of the complex C iv λ1549 features in two high-redshift galaxies with good quality spectra, MS 1512−cB58 ( z = 2.7268) and Q1307−BM1163 ( z = 1.4105) , the local calibration gives values of the oxygen abundance which are in good agreement with other metallicity determinations based on nebular emission and interstellar absorption lines. Our main conclusion is that for this method to give reliable results at high redshifts, it should only be used on data of sufficiently high spectral resolution ( R ≳ 1000) for stellar and interstellar C iv components to be clearly separated. Oxygen abundances will be systematically overestimated if the local calibration is applied to spectra of high-redshift galaxies obtained with the low resolving powers ( R ≃ 200–300) of many current wide-field surveys. It will also be necessary to understand better the causes of the scatter in the local relation, before we can be confident of inferences from it at high z . 相似文献
912.
Charles S. Cockell Matt Balme John C. Bridges Alfonso Davila Susanne P. Schwenzer 《Icarus》2012,217(1):184-193
Investigations of Mars as a potential location for life often make the assumption that where there are habitats, they will contain organisms. However, the observation of the ubiquitous distribution of life in habitable environments on the Earth does not imply the presence of life in martian habitats. Although uninhabited habitats are extremely rare on the Earth, a lack of a productive photosynthetic biosphere on Mars to generate organic carbon and oxygen, thus providing a rapidly available redox couple for energy acquisition by life and/or a lack of connectivity between habitats potentially increases the scope and abundance of uninhabited habitats for much of the geological history of the planet. Uninhabited habitats could have existed on Mars from the Noachian to the present-day in impact hydrothermal systems, megaflood systems, lacustrine environments, transient melted permafrost, gullies and local regions of volcanic activity; and there may be evidence for them in martian meteorites. Uninhabited habitats would provide control habitats to investigate the role of biology in planetary-scale geochemical processes on the Earth and they would provide new constraints on the habitability of Mars. Future robotic craft and samples returned from Mars will be able to directly show if uninhabited habitats exist or existed on Mars. 相似文献
913.
Juan C. Martínez-Oliveros Charles Lindsey Stuart D. Bale Säm Krucker 《Solar physics》2012,279(1):153-171
Low-frequency solar and interplanetary radio bursts are generated at frequencies below the ionospheric plasma cutoff and must therefore be measured in space, with deployable antenna systems. The problem of measuring both the general direction and polarization of an electromagnetic source is commonly solved by iterative fitting methods such as linear regression that deal simultaneously with both directional and polarization parameters. We have developed a scheme that separates the problem of deriving the source direction from that of determining the polarization, avoiding iteration in a multi-dimensional manifold. The crux of the method is to first determine the source direction independently of concerns as to its polarization. Once the source direction is known, its direct characterization in terms of Stokes vectors, in a single iteration if desired, is relatively simple. This study applies the source-direction determination to radio signatures of flares received by STEREO. We studied two previously analyzed radio type III bursts and found that the results of the eigenvalue decomposition technique are consistent with those obtained previously by Reiner et al. (Solar Phys. 259, 255, 2009). For the type III burst observed on 7 December 2007, the difference in travel times from the derived source location to STEREO A and B is the same as the difference in the onset times of the burst profiles measured by the two spacecraft. This is consistent with emission originating from a single, relatively compact source. For the second event of 29 January 2008, the relative timing does not agree, suggesting emission from two sources separated by 0.1 AU, or perhaps from an elongated region encompassing the apparent source locations. 相似文献
914.
The precautionary principle is a mandate to tread cautiously when managing novel threats to the environment or human health. A major obstacle when applying the principle at the international level is disagreement about how precautionary efforts should be constrained to ensure that policy costs are proportional to the attained level of protection. Proportionality is an unresolved question when preliminary evidence precludes decision-makers from assigning probabilities over future events. The paper suggests practical analytical tools for communicating ex ante trade-offs when probabilities are unavailable. The tools could be used to facilitate discussion and compromise when implementing precautionary decisions in international settings where cooperation is important. The approach is demonstrated in an application to climate policy that uses the integrated assessment model DICE (Nordhaus, 2008). The paper also situates the task of precautionary decision-making within the broader context of implementing a precautionary response at the international level. 相似文献
915.
916.
Revital Bookman Charles T. Driscoll Steven W. Effler Daniel R. Engstrom 《Journal of Paleolimnology》2010,43(3):449-462
Geochemical analysis of 210Pb-dated sediment cores from Otisco Lake (New York) combined with analysis of recent land cover change in the watershed revealed
the history of land use change, lake management, and industrial pollution since European settlement in the northeastern US.
Clearance of forestland for agriculture characterized the early settlement era that was marked in the Otisco Lake sediments
by a decline in organic carbon (OC) and OC:N ratios, which indicate a change in the sources of organic matter to the lake.
Agricultural land use reached its greatest areal extent in the Otisco watershed around 1900, followed by field abandonment
and reforestation over the last century. In the 1920s sediment accumulation rates began to increase coincident with residential
development along the lake shore. Nitrogen and organic carbon, which are transported from the watershed, show increased fluxes
to the lake in response to this change. Deposition of inorganic carbon increased markedly over a short period from the 1940s
to the 1980s, which is consistent with enhanced mobilization of watershed calcium by increased acidic deposition, followed
by alkalinization of the lake waters and calcite precipitation. An increase in copper content in the sediments reflects application,
since 1942, of the algicide copper sulfate to the lake waters to control algal blooms. This CuSO4 marker confirmed the accuracy of the 210Pb chronology. Atmospheric mercury (Hg) fluxes to the lake were affected by increased sediment transport from the watershed.
The maximum Hg peak in the sediment record, however, was dated to the early 1970s and coincides with maximum Hg emissions
to the atmosphere in the Great Lakes region. 相似文献
917.
Knowing our own history? Geography department archives in the UK 总被引:1,自引:0,他引:1
The paper presents an analysis of the returns to a questionnaire survey on the state of department archives within UK departments of geography. The results of the survey are discussed in relation to recent work in geography which has examined the archive as a site for knowledge's making but seldom in its own terms as a resource for the history of geography, and studies within the archival sciences which have considered the archive as something more than a 'storehouse' for collective memory. The paper reveals that the archival record for the history of British geography is at best uneven, and in many departments non-existent, although information on departmental history is held, often as memory, by individual geographers. The paper considers the survey's implications for the future histories of British geography and addresses the nature of the UK geography department archive as resource and responsibility. 相似文献
918.
Groundwater remediation technologies are designed, installed, and operated based on the conceptual models of contaminant hydrogeology that are accepted at that time. However, conceptual models of remediation can change as new research, new technologies, and new performance data become available. Over the past few years, results from multiple-site remediation performance studies have shown that achieving drinking water standards (i.e., Maximum Contaminant Levels, MCLs) at contaminated groundwater sites is very difficult. Recent groundwater research has shown that the process of matrix diffusion is one key constraint. New developments, such as mass discharge, orders of magnitude (OoMs), and SMART objectives are now being discussed more frequently by the groundwater remediation community. In this paper, the authors provide their perspectives on the existing "reach MCLs" approach that has historically guided groundwater remediation projects, and advocate a new approach built around the concepts of OoMs and mass discharge. 相似文献
919.
920.
Rachel Mandelbaum † Glenn van de Ven † Charles R. Keeton 《Monthly notices of the Royal Astronomical Society》2009,398(2):635-657
Many current and future astronomical surveys will rely on samples of strong gravitational lens systems to draw conclusions about galaxy mass distributions. We use a new strong lensing pipeline (presented in Paper I of this series) to explore selection biases that may cause the population of strong lensing systems to differ from the general galaxy population. Our focus is on point-source lensing by early-type galaxies with two mass components (stellar and dark matter) that have a variety of density profiles and shapes motivated by observational and theoretical studies of galaxy properties. We seek not only to quantify but also to understand the physics behind selection biases related to: galaxy mass, orientation and shape; dark matter profile parameters such as inner slope and concentration; and adiabatic contraction. We study how all of these properties affect the lensing Einstein radius, total cross-section, quad/double ratio and image separation distribution, with a flexible treatment of magnification bias to mimic different survey strategies. We present our results for two families of density profiles: cusped and deprojected Sérsic models. While we use fixed lens and source redshifts for most of the analysis, we show that the results are applicable to other redshift combinations, and we also explore the physics of how our results change for very different redshifts. We find significant (factors of several) selection biases with mass; orientation, for a given galaxy shape at fixed mass; cusped dark matter profile inner slope and concentration; concentration of the stellar and dark matter deprojected Sérsic models. Interestingly, the intrinsic shape of a galaxy does not strongly influence its lensing cross-section when we average over viewing angles. Our results are an important first step towards understanding how strong lens systems relate to the general galaxy population. 相似文献