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91.
Summary. Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review
we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial r?le in the
cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity
can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve
many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields,
outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor
galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and
the physical processes that are at work in these objects. The question on how they are related and may evolve from one class
of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion
of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe,
we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in
the universe. We discuss what do QSO absorption lines and known distant galaxies tell us already? We illustrate the importance
of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator
and discuss the possibility to detect nearly metal-free galaxies at high redshift from Ly emission.
Received 19 August 1999 / Published online: 15 February 2000 相似文献
92.
Using sewage sludge as a sealing layer to remediate sulphidic mine tailings: a pilot-scale experiment, northern Sweden 总被引:1,自引:0,他引:1
At the Kristineberg mine, northern Sweden, sulphidic mine tailings were remediated in an 8-year pilot-scale experiment using sewage sludge to evaluate its applicability as a sealing layer in a composite dry cover. Sediment, leachate water, and pore gas geochemistry were collected in the aim of determining if the sludge was an effective barrier material to mitigate acid rock drainage (ARD) formation. The sludge was an effective barrier to oxygen influx as it formed both a physical obstruction and functioned as an organic reactive barrier to prevent oxygen to the underlying tailings. Sulphide oxidation and consequential ARD formation did not occur. Sludge-borne trace elements accumulated in a reductive, alkaline environment in the underlying tailings, resulting in an effluent drainage geochemistry of Cd, Cu, Pb and Zn below 10 μg/L, high alkalinity (810 mg/L) and low sulphate (38 mg/L). In contrast, the uncovered reference tailings received a 0.35-m deep oxidation front and typical ARD, with dissolved concentrations of Cd, Zn and sulphate, 20.8 μg/L, 16,100 μg/L and 1,390 mg/L, respectively. Organic matter degradation in the sludge may be a limiting factor to the function of the sealing layer over time as 85 % loss of the organic fraction occurred over the 8-year experimental period due to aerobic and anaerobic degradation. Though the cover may function in the short to medium term (100 years), it is unlikely to meet the demands of a long-term remedial solution. 相似文献
93.
94.
Yngve Öhman 《Solar physics》1985,96(1):209-212
It is suggested, that effects of total reflection and refraction may appear sometimes in various objects on the Sun in connection with grazing incidence towards layers formed by magnetic fields and perhaps also by electrostatic double layers or shock waves. It seems possible that such effects may influence sometimes the contrast of bright structures as well as dark structures at selective wavelengths as well as in white light. 相似文献
95.
Kâmuran Avcioglu M. Türker Özkan H. Hüseyin Mentese 《Astrophysics and Space Science》1990,164(2):263-276
We investigate the near-ultraviolet high-resolution LWR spectra of the stars Cas, And, Tau, Gem, Cru, Boo, and Peg, obtained with the aid of the International Ultraviolet Explorer Satellite. We have given here a list of the strongest and most prevalent emission lines in the near-ultraviolet spectra of Boo, KlIIIp, and Peg, M2.5II-III which have the same luminosity class and different spectral type. The near-ultraviolet continuum flux measurements and integrated emission line fluxes of these stars for the 2500–3200 Å region are presented in order to compare the variations in the appearance of the near-ultraviolet flux distribution with the temperature structure of their chromospheres for K and M giant stars. We also discuss differences between observed and calculated fluxes found from the Planck function. 相似文献
96.
97.
This paper reports the results of a study of the N-S asymmetry in the flare index using the results of Knoka (1985) combined with our results for the solar cycles 17 to the current cycle 22. By comparing the time-variation of the asymmetry curve with the solar activity variation of the 11-year cycle, we have found that the flare index asymmetry curve is not in phase with the solar cycle and that the asymmetry peaks during solar minimum. A periodic behaviour in the N-S asymmetry appears: the activity in one hemisphere is more important during the ascending part of the cycle whereas during the descending part the activity becomes more important in the other hemisphere. The dominance of flare activity in the southern hemisphere continues during cycle 22 and, according to our findings, this dominance will increase gradually during the following cycle 23. 相似文献
98.
Bertaux Jean-Loup QuÉmerais Eric Lallement Rosine KyrÖlÄ Erkki Summanen Tuula 《Solar physics》1997,170(2):365-370
In a recently published paper, Scherer and Fahr (1995) claimed that the departures of sky L emission measured by Prognoz 5 and 6 from an optically thin model can be attributed entirely to deficiencies of the optically thin approximation, and are not due to variations of solar wind ionization rate with latitude, as advocated since many years by our research group. They base their claim on the result of their new sophisticated model of L radiation transport.It is shown here that their new model, in the simple case of isotropic solar wind, predicts a distribution of intensity in a simple geometry which is completely contradicted by the observations: they find a minimum of intensity near the upwind direction, where a maximum has been observed consistently by all L instruments. Therefore, their conclusion based on an erroneous model must be rejected. 相似文献
99.
100.