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161.
We study dynamics of quiescent prominences using several data sets taken with the Solar Optical Telescope (SOT) on Hinode. We find a number of processes occurring at different stages of prominence evolution that are common for all of our chosen cases and, having universal character, can be related to fundamental plasma instabilities. We combine the observational evidence and theory to identify these instabilities. Here we discuss three examples: i) prominence cavity formation and its evolution, associated with a screw-pinch instability; ii) development of a regular series of plumes and spikes typical to the Rayleigh?–?Taylor (RT) instability; and iii) the appearance of growing ripples at the prominence/corona interface, often followed by a sudden collimated mass upflow, attributed to the Kelvin?–?Helmholtz (KH) instability. The conditions for transition from a linear (rippling mode) to nonlinear stage of the KH instability, known to have an explosive character, are specified. Given excellent Hinode data, all three aspects of prominence dynamics allow quantitative analysis. 相似文献
162.
TitaniQ under pressure: the effect of pressure and temperature on the solubility of Ti in quartz 总被引:5,自引:2,他引:3
Jay B. Thomas E. Bruce Watson Frank S. Spear Philip T. Shemella Saroj K. Nayak Antonio Lanzirotti 《Contributions to Mineralogy and Petrology》2010,160(5):743-759
Quartz and rutile were synthesized from silica-saturated aqueous fluids between 5 and 20 kbar and from 700 to 940°C in a piston-cylinder
apparatus to explore the potential pressure effect on Ti solubility in quartz. A systematic decrease in Ti-in-quartz solubility
occurs between 5 and 20 kbar. Titanium K-edge X-ray absorption near-edge structure (XANES) measurements demonstrate that Ti4+ substitutes for Si4+ on fourfold tetrahedral sites in quartz at all conditions studied. Molecular dynamic simulations support XANES measurements
and demonstrate that Ti incorporation onto fourfold sites is favored over interstitial solubility mechanisms. To account for
the P–T dependence of Ti-in-quartz solubility, a least-squares method was used to fit Ti concentrations in quartz from all experiments
to the simple expression
RTlnX\textTiO 2 \textquartz = - 60952 + 1.520 ·T(K) - 1741 ·P(kbar) + RTlna\textTiO 2 RT\ln X_{{{\text{TiO}}_{ 2} }}^{\text{quartz}} = - 60952 + 1.520 \cdot T(K) - 1741 \cdot P(kbar) + RT\ln a_{{{\text{TiO}}_{ 2} }} 相似文献
163.
Hafnium and neodymium isotopes in surface waters of the eastern Atlantic Ocean: Implications for sources and inputs of trace metals to the ocean 总被引:1,自引:0,他引:1
We present hafnium (Hf) and neodymium (Nd) isotopic compositions and concentrations in surface waters of the eastern Atlantic Ocean between the coast of Spain and South-Africa. These data are complemented by Hf and Nd isotopic and concentration data, as well as rare earth element (REE) concentrations, in Saharan dust.Hafnium concentrations range between a maximum of 0.52 pmol/kg in the area of the Canary Islands and a minimum value of 0.08 pmol/kg in the southern Angola Basin. Neodymium concentrations also show a local maximum in the area of the Canary Islands (26 pmol/kg) but are even higher between ∼20°N and ∼4°N reaching maximum concentrations of 35 pmol/kg. These elevated concentrations provide evidence of inputs from weathering of the Canary Islands and from the partial dissolution of dust from the Sahara/Sahel region. The inputs from ocean island weathering are also reflected in radiogenic Hf and Nd isotopes.The Hf isotopic compositions of dust samples themselves are highly variable, ranging between εHf = −20 and −0.6. The combined Hf and Nd isotopic compositions of dust plot close to the “terrestrial array” during periods of appreciable dust load in the atmosphere. During low atmospheric dust loading combined Hf and Nd isotopic compositions similar to seawater are observed. Most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples, which in turn is linked to sorting during atmospheric transport to the eastern Atlantic Ocean and possibly presorting by sedimentary redistribution on the continent. In addition, increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic Hf isotopic compositions observed.While the Nd isotopic composition in the surface ocean reflects the Nd isotopic composition of the Saharan dust adjacent to the Sahara/Sahel region, the release of Hf from that dust appears to be incongruent and results in surface ocean Hf isotopic compositions which are ∼10 εHf more radiogenic than the bulk dust. Radiogenic Hf appears to be released from clays and possibly from trace apatite. Rare earth element patterns of dust samples indicate the presence of apatite but provide no evidence for ferromanganese grain coatings, suggesting that such coatings are insignificant in the release of Hf and Nd from Saharan dust to the surface ocean.The Nd isotopic composition of the surface waters becomes less radiogenic south of the equator, most likely reflecting the release of Nd from Congo river sediments. The release of Hf from Saharan dust and the Congo river sediments, however, does not produce distinct Hf isotopic signatures in the surface ocean, implying that the mobile fraction of Hf integrated over large continental areas is isotopically uniform. The Hf isotopic uniformity in the surface ocean means that the limited variability in deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization. 相似文献
164.
165.
基于MODIS遥感影像和表观热惯量法,以新疆为研究区,建立了适用于干旱半干旱区1 m土体的土壤含水量反演模型。模型根据高表观热惯量,高土壤含水量,低表观热惯量,低土壤含水量这一理论,通过日地表温差和宽波段反照率确定土壤含水量的时空变化。假设通过1 m土体的土壤水通量正比于上下底层土壤含水量的差值,利用水平衡方程建立土壤表面和底层土壤含水量关系方程,并利用中国土壤类型特点确定优化模型。通过验证结果表明,壤土和壤质粘土这两类土壤含水量接近真实值,砂土在区域验证中,模拟与实测差值为2.16%,整个模型模拟精度较好,能够准确地从时空上反演干旱半干旱地区1 m土体的土壤水分情况。 相似文献
166.
167.
Impacts of tectonic and orbital forcing on East African climate: a comparison based on global climate model simulations 总被引:1,自引:1,他引:0
Frank Kaspar Kerstin Prömmel Ulrich Cubasch 《International Journal of Earth Sciences》2010,99(7):1677-1686
A global atmosphere–ocean model has been forced with topographic and orbital scenarios in order to evaluate the relative role
of both factors for the past climate of East Africa. Forcing the model with a significantly reduced topography in Eastern
and Southern Africa leads to a distinct increase in moisture transport from the Indian Ocean into the eastern part of the
continent and increased precipitation in Eastern Africa. Simulations with step-wise reduced height show that this climate
change occurs continuously with the change in topography, i.e., an abrupt change of local climatic features with a critical
height is not found. Simulations of the last interglacial (at 125,000 years before present, i.e., the Eemian interglacial)
and the last glacial inception (at 115,000 years before present) are used as examples for the role of orbital-induced changes
in insolation. Here, changes in meridional temperature gradients lead to modifications in moisture transport of similar order
of magnitude, but with different spatial and seasonal structure. For the Eemian interglacial, a distinct increase in summer
moisture transport from the Atlantic deep into the continent at around 20°N is simulated. 相似文献
168.
169.
The paper deals with the approximation and optimal interpolation of functions defined on the bisphere
\mathbb S2×\mathbb S2\mathbb {S}^{2}\times \mathbb {S}^{2} from scattered data. We demonstrate how the least square approximation to the function can be computed in a stable and efficient
manner. The analysis of this problem is based on Marcinkiewicz–Zygmund inequalities for scattered data which we present here
for the bisphere. The complementary problem of optimal interpolation is also solved by using well-localized kernels for our
setting. Finally, we discuss the application of the developed methods to problems of texture analysis in material science. 相似文献
170.
Yassir A. Abdu Abbasher M. Gismelseed Atta G. Attaelmanan Muawia H. Shaddad Frank C. Hawthorne 《Meteoritics & planetary science》2023,58(5):737-746
The crystal structures of orthopyroxene (En86.3Fs8.6Wo5.1, space group Pbca) and pigeonite (En81.7Fs8.8Wo9.5, space group P21/c) from the Almahata Sitta ureilite (fragment#051) have been refined to R1 indices of 3.10% and 2.53%, respectively, using single-crystal X-ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single-crystal diffraction data. The results indicate a rather disordered intracrystalline Fe2+-Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe-Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si-bearing kamacite, and arise from two different nearest-neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite [(Fe,Ni)3C], schreibersite [(Fe,Ni)3P], and an Fe-oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation. 相似文献
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