全文获取类型
收费全文 | 30951篇 |
免费 | 462篇 |
国内免费 | 381篇 |
专业分类
测绘学 | 792篇 |
大气科学 | 2813篇 |
地球物理 | 6356篇 |
地质学 | 10778篇 |
海洋学 | 2379篇 |
天文学 | 6760篇 |
综合类 | 71篇 |
自然地理 | 1845篇 |
出版年
2020年 | 179篇 |
2019年 | 196篇 |
2018年 | 491篇 |
2017年 | 483篇 |
2016年 | 691篇 |
2015年 | 449篇 |
2014年 | 675篇 |
2013年 | 1408篇 |
2012年 | 739篇 |
2011年 | 1026篇 |
2010年 | 873篇 |
2009年 | 1242篇 |
2008年 | 1058篇 |
2007年 | 946篇 |
2006年 | 1043篇 |
2005年 | 875篇 |
2004年 | 848篇 |
2003年 | 869篇 |
2002年 | 867篇 |
2001年 | 746篇 |
2000年 | 788篇 |
1999年 | 660篇 |
1998年 | 629篇 |
1997年 | 666篇 |
1996年 | 575篇 |
1995年 | 542篇 |
1994年 | 482篇 |
1993年 | 429篇 |
1992年 | 421篇 |
1991年 | 418篇 |
1990年 | 422篇 |
1989年 | 398篇 |
1988年 | 381篇 |
1987年 | 466篇 |
1986年 | 435篇 |
1985年 | 464篇 |
1984年 | 558篇 |
1983年 | 560篇 |
1982年 | 501篇 |
1981年 | 490篇 |
1980年 | 447篇 |
1979年 | 433篇 |
1978年 | 448篇 |
1977年 | 394篇 |
1976年 | 355篇 |
1975年 | 355篇 |
1974年 | 405篇 |
1973年 | 389篇 |
1972年 | 245篇 |
1971年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism. 相似文献
992.
H. Alawaji 《Geotechnical and Geological Engineering》2004,22(1):151-152
Conference Report
3rd International Conference on Unsaturated Soils (UNSAT 2002) Recife, Brazil, 10–12 March 2002 相似文献993.
994.
This paper presents observations of the MgII h and k lines obtained with the UVSP instrument that flew aboard the SMM satellite. Both spatially averaged and resolved observations are compared with calculated profiles of the lines from standard, plane-parallel solar models. The radiative transfer calculations presented take full account of partial frequency redistribution and wavelength overlap of the h and k lines. A comparison between theoretical and observed wing profiles indicates that current one-dimensional models underestimate the temperature in the middle photosphere. The cores of spatially resolved dark intra-network profiles are well reproduced by the model calculations, while the spatially averaged profiles have in general broader emission peaks, indicating that the additional broadening is due to a contribution of magnetic network profiles. 相似文献
995.
996.
Using the multifrequency radio profiles of pulsar PSR B2319+60, two parameters of inverse Compton scattering model, the initial Lorentz factor and the factor of energy loss of relativistic particles are constrained. 相似文献
997.
We use a KrF laser that generates pulses up to 8 × 1014 watt/cm2 when focused onto a boron nitride target. We measured the line profile of the near UV, n = 0, n = 2 transitions of helium-like boron and we studied the triplet 1s2p3P-1s2s3S at 2825.85 (J = 1), 2821.68 (J = 2) and 2824.57 (J = 0) Å, at various positions from the target surface, in order to correlate details of this profile with the presence of self-generated magnetic fields and/or dynamic turbulence among other processes. We fitted the measured profiles to a 0-D model, that includes, Stark, Doppler, Zeeman and instrumental broadening. The effect of the magnetic field was included using an intermediate field calculations. Preliminary results of these measurements are presented and discussed. 相似文献
998.
We present an outline of our study of the effects of star formation on the different components of the interstellar medium
in the discs of spiral galaxies, both globally and as a function of arm and interarm environment. We are in the process of
obtaining images of 57 spiral galaxies at low inclinations, and analysing them to study the distribution of recent massive
star formation, old stars, young stars, gas and dust. We will dissect the images into arm and interarm regions and compare
and contrast the morphology and scale lengths within these regions inHα, HI, the near infrared, optical and (where available) CO. Modelling will show how the scale lengths are affected by star
formation, how this differs between arms and interarms, and whether the Schmidt Law varies from the global values in the arm
and interarm regions.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
999.
H. Kurokawa 《Solar physics》1982,113(1-2):259-265
From a morphological study of the evolution of six active regions, we found two types of processes for the development of magnetic shear configurations between sunspots: (A) collision of two sunspots of opposite magnetic polarities, and (B) successive emergence of twisted magnetic flux ropes. We conclude that the process (B) might be essential for the production of major flares. 相似文献
1000.
High resolution electric field and particle data, obtained by the S23L1 rocket crossing over a discrete prebreakup arc in January 1979, are studied in coordination with ground observations (Scandinavian Magnetometer Array—SMA, TV and all-sky cameras) in order to clarify the electrodynamics of the arc and its surroundings. Height-integrated conductivities have been calculated from the particle data, including the ionization effects of precipitating protons and assuming a steady state balance between ion production and recombination losses. High resolution optical information of arc location relative to the rocket permitted a check of the validity of this assumption for each flux tube passed by the rocket. Another check was provided by a comparison between calculated (equilibrium values) and observed electron densities along the rocket trajectory. A way to compensate for the finite precipitation time when calculating the electron densities is outlined. The height-integrated HalI-Pedersen conductivity ratio is typically 1.4 within the arc and about 1 at the arc edges, indicative of a relatively softer energy spectrum there. The height-integrated conductivities combined with the DC electric field measurements permitted calculation of the horizontal ionospheric current vectors (J⊥), Birkeland currents (from div J⊥) and energy dissipation through Joule heating (ΣpE2). An eastward current of typically 1 A m?1 was found to be concentrated mainly to the arc region and equatorward of it. A comparison has been made with the equivalent current system deduced from ground based magnetometer data (SMA) showing a generally good agreement with the rocket results. An intense Pedersen current peak (1.2 A m?1) was found at the southern arc edge. This edge constituted a division line between a very intense (> 10 μA m?1) and localized (~ 6 km) downward current sheet to the south, probably carried by upward flowing cold ionospheric electrons and a more extended upward current sheet (> 10 μA m?2) over the arc carried by measured precipitating electrons. Joule and particle heating across the arc were anticorrelated, consistent with the findings of Evans et al. (1977) with a total value of about 100mW m?2. 相似文献