全文获取类型
收费全文 | 1941篇 |
免费 | 60篇 |
国内免费 | 23篇 |
专业分类
测绘学 | 59篇 |
大气科学 | 161篇 |
地球物理 | 473篇 |
地质学 | 712篇 |
海洋学 | 164篇 |
天文学 | 310篇 |
综合类 | 12篇 |
自然地理 | 133篇 |
出版年
2021年 | 24篇 |
2020年 | 21篇 |
2019年 | 24篇 |
2018年 | 54篇 |
2017年 | 35篇 |
2016年 | 55篇 |
2015年 | 31篇 |
2014年 | 50篇 |
2013年 | 99篇 |
2012年 | 57篇 |
2011年 | 106篇 |
2010年 | 75篇 |
2009年 | 85篇 |
2008年 | 86篇 |
2007年 | 70篇 |
2006年 | 58篇 |
2005年 | 63篇 |
2004年 | 45篇 |
2003年 | 40篇 |
2002年 | 57篇 |
2001年 | 36篇 |
2000年 | 51篇 |
1999年 | 54篇 |
1998年 | 32篇 |
1997年 | 48篇 |
1996年 | 43篇 |
1995年 | 34篇 |
1994年 | 30篇 |
1993年 | 28篇 |
1992年 | 20篇 |
1991年 | 22篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 13篇 |
1987年 | 34篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 29篇 |
1983年 | 27篇 |
1982年 | 35篇 |
1981年 | 21篇 |
1980年 | 29篇 |
1979年 | 20篇 |
1978年 | 19篇 |
1977年 | 21篇 |
1976年 | 21篇 |
1975年 | 25篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1971年 | 12篇 |
排序方式: 共有2024条查询结果,搜索用时 296 毫秒
181.
Green L.M. López fuentes M.C. Mandrini C.H. Démoulin P. Van Driel-Gesztelyi L. Culhane J.L. 《Solar physics》2002,208(1):43-68
Coronal mass ejections (CMEs) are thought to be the way by which the solar corona expels accumulated magnetic helicity which is injected into the corona via several methods. DeVore (2000) suggests that a significant quantity is injected by the action of differential rotation, however Démoulin et al. (2002b), based on the study of a simple bipolar active region, show that this may not be the case. This paper studies the magnetic helicity evolution in an active region (NOAA 8100) in which the main photospheric polarities rotate around each other during five Carrington rotations. As a result of this changing orientation of the bipole, the helicity injection by differential rotation is not a monotonic function of time. Instead, it experiences a maximum and even a change of sign. In this particular active region, both differential rotation and localized shearing motions are actually depleting the coronal helicity instead of building it. During this period of five solar rotations, a high number of CMEs (35 observed, 65 estimated) erupted from the active region and the helicity carried away has been calculated, assuming that each can be modeled by a twisted flux rope. It is found that the helicity injected by differential rotation (–7×1042 Mx2) into the active region cannot provide the amount of helicity ejected via CMEs, which is a factor 5 to 46 larger and of the opposite sign. Instead, it is proposed that the ejected helicity is provided by the twist in the sub-photospheric part of the magnetic flux tube forming the active region. 相似文献
182.
Biomass Burning in Southern Africa: Individual Particle Characterization of Atmospheric Aerosols and Savanna Fire Samples 总被引:2,自引:0,他引:2
Xiande Liu Piet Van Espen Freddy Adams Jan Cafmeyer Willy Maenhaut 《Journal of Atmospheric Chemistry》2000,36(2):135-155
Ambient atmospheric aerosols and savanna fireparticulate emission samples from southern Africa werecharacterised in terms of particle classes and theirnumber abundance by electron probe X-ray microanalysis(EPXMA). About ten particle classes were identifiedfor each sample. The major classes werealuminosilicates and sea salts for ambient coarse(2–10 m equivalent aerodynamic diameter (EAD))samples, and K-S and S-only particles for ambient fine(<2 m EAD) samples. The K-S particles are oneof the major products of biomass burning. The EPXMAresults were found to be consistent with the resultsfrom bulk analyses on a sample by sample basis. Forsavanna fire fine samples, quantitative EPXMA revealedthat many particles had a composition of simple saltssuch as KCl. Some particles had a deviatingcomposition in the sense that more ionic species wereinvolved in sustaining the balance between cations andanions, and they were composite or mixed salts.Because of extensive processing during the atmospherictransport, the composition of the K-S particles in theambient samples was different from K2SO4,and such particles were enriched with S. The finepyrogenic KCl particles and the fine sea-saltparticles were much depleted in chlorine. 相似文献
183.
H. Lemmens M. Czank G. Van Tendeloo S. Amelinckx 《Physics and Chemistry of Minerals》2000,27(6):386-397
Using minimum exposure techniques, it is feasible to perform high resolution electron microscopy on the α-cristobalite phase
of (Si0.9 Ge0.1)O2, which is extremely radiation sensitive. Such images reveal atomic scale information of twins and tridymite-like stacking
faults on (1 1 1)β planes, as well as of domain boundaries resulting from the β→α transition. Polytype structures are formed in certain cases.
Morphological features suggest that the phase transformation cristobalite → tridymite proceeds by means of a zonal dislocation
mediated synchro-shear process on (1 1 1)β planes; the geometry of this process is analyzed.
Received: 13 June 1999 / Accepted: 30 October 1999 相似文献
184.
M. C. Santos P. Vaníček W. E. Featherstone R. Kingdon A. Ellmann B. -A. Martin M. Kuhn R. Tenzer 《Journal of Geodesy》2006,80(12):691-704
Following our earlier definition of the rigorous orthometric height [J Geod 79(1-3):82–92 (2005)] we present the derivation and calculation of the differences between this and the Helmert orthometric height, which is embedded in the vertical datums used in numerous countries. By way of comparison, we also consider Mader and Niethammer’s refinements to the Helmert orthometric height. For a profile across the Canadian Rocky Mountains (maximum height of ~2,800 m), the rigorous correction to Helmert’s height reaches ~13 cm, whereas the Mader and Niethammer corrections only reach ~3 cm. The discrepancy is due mostly to the rigorous correction’s consideration of the geoid-generated gravity disturbance. We also point out that several of the terms derived here are the same as those used in regional gravimetric geoid models, thus simplifying their implementation. This will enable those who currently use Helmert orthometric heights to upgrade them to a more rigorous height system based on the Earth’s gravity field and one that is more compatible with a regional geoid model. 相似文献
185.
Effect of common point selection on coordinate transformation parameter determination 总被引:2,自引:0,他引:2
The use of satellite positioning techniques commonly requires a transformation from a Conventional Terrestrial coordinate
system to a Geodetic coordinate system, or vice versa. For such a transformation, the main problem is the determination of
transformation parameters between these coordinate systems. The transformation parameters are estimated by a least-squares
process using “common” points, i.e., those points whose coordinates are known in both systems. Therefore, the precision of
so estimated transformation parameters is closely related to certain characteristics of the common points. In this contribution,
we have formulated some theoretical relations between the transformation parameters and the number and the distribution of
common points, and corroborated the theoretical results numerically, using a simulated geodetic network. 相似文献
186.
Explicit formula for the geoid-quasigeoid separation 总被引:1,自引:0,他引:1
The explicit formula for the geoid-to-quasigeoid correction is derived in this paper. On comparing the geoidal height and
height anomaly, this correction is found to be a function of the mean value of gravity disturbance along the plumbline within
the topography. To evaluate the mean gravity disturbance, the gravity field of the Earth is decomposed into components generated
by masses within the geoid, topography and atmosphere. Newton’s integration is then used for the computation of topography-and
atmosphere-generated components of the mean gravity, while the combined solution for the downward continuation of gravity
anomalies and Stokes’ boundary-value problem is utilized in computing the component of mean gravity disturbance generated
by mass irregularities within the geoid. On application of this explicit formulism a theoretical accuracy of a few millimetres
can be achieved in evaluation of the geoid-to-quasigeoid correction. However, the real accuracy could be lower due to deficiencies
within the numerical methods and to errors within the input data (digital terrain and density models and gravity observations). 相似文献
187.
The Inclusion of Stakeholder Knowledge and Perspectives in Integrated Assessment of Climate Change 总被引:1,自引:0,他引:1
Over the past few decades Integrated Assessment (IA) has emerged as an approach to link knowledge and action in a way that
is suitable to accommodate uncertainties, complexities and value diversities of global environmental risks. Responding to
the complex nature of the climate problem and to the changing role of climate change in the international climate policy process,
the scientific community has started to include stakeholder knowledge and perspectives in their assessments. Participatory
Integrated Assessment (PIA) is in its early stage of development. Methodology varies strongly across PIA projects. This paper
analyzes four recent IA projects of climate change that included knowledge or perspectives from stakeholders in one-way or
another. Approaches and methods used turn out to differ in whether stakeholders are involved actively or passively, whether
the approach is bottom-up or top-down, and whether the different functions in the IA process are open or closed to stakeholder
input. Also, differences can be seen in the degree to which boundaries are pre-set that limit the roles and domains of competencies
attributed to each scientific or non-scientific participant (so-called boundary work). The paper discusses pros and cons of
the various approaches identified, and outlines heuristics and considerations to assist those who plan, design or fund new
IA processes with stakeholder input on what approaches best to choose in view of the objectives for stakeholder involvement,
in view of the role that the IA plays in the overall risk management process and in view of considerations regarding boundary
work. 相似文献
188.
189.
Luminescence dating of old (>70 ka) Chinese loess: A comparison of single-aliquot OSL and IRSL techniques 总被引:1,自引:0,他引:1
J.P. Buylaert D. Vandenberghe A.S. Murray S. Huot F. De Corte P. Van den Haute 《Quaternary Geochronology》2007,2(1-4):9-14
The applicability of two different approaches in the luminescence dating of old (>70 ka) Chinese loess is investigated. Both SAR-OSL ages obtained on 63–90 μm quartz grains and SAR-IRSL ages obtained on 4–11 μm polymineral grains, for samples collected from two sites in the Chinese Loess Plateau (Luochuan and Dongchuan) are presented. The characteristics of the luminescence signals stimulated by blue and infrared light are investigated in terms of dose response and dose recovery, and as a function of age. Additionally, anomalous fading measurements from the 410 nm IRSL emission in polymineral fine-grains are reported. An average value of g2days amounting to 3% per decade was measured and seems to be independent of site location and age. For the samples from Luochuan, independent age control (pedostratigraphy and palaeomagnetism) is available. At both sites, the SAR-OSL ages are always lower than the SAR-IRSL ages after they have been corrected for anomalous fading. It seems that the quartz-based SAR-OSL ages are accurate for the younger ages, but that they underestimate the true age of deposition for loess that was deposited about 60–70 ka ago. The fading-corrected SAR-IRSL ages are in better agreement with the pedostratigraphic age control (75 and 130 ka) and allow dating beyond the quartz OSL range. Based on our results, we suggest that conventional SAR-OSL and SAR-IRSL protocols at these sites should be restricted to samples of ages not exceeding 40–50 and 100–120 ka, respectively. 相似文献
190.
The Mesoarchean emergence of modern-style subduction 总被引:6,自引:4,他引:6
Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated.In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process. 相似文献