全文获取类型
收费全文 | 41977篇 |
免费 | 738篇 |
国内免费 | 356篇 |
专业分类
测绘学 | 1292篇 |
大气科学 | 3061篇 |
地球物理 | 8541篇 |
地质学 | 13873篇 |
海洋学 | 3628篇 |
天文学 | 10336篇 |
综合类 | 116篇 |
自然地理 | 2224篇 |
出版年
2021年 | 328篇 |
2020年 | 362篇 |
2019年 | 451篇 |
2018年 | 973篇 |
2017年 | 899篇 |
2016年 | 1177篇 |
2015年 | 665篇 |
2014年 | 1115篇 |
2013年 | 2025篇 |
2012年 | 1223篇 |
2011年 | 1619篇 |
2010年 | 1473篇 |
2009年 | 2038篇 |
2008年 | 1789篇 |
2007年 | 1813篇 |
2006年 | 1703篇 |
2005年 | 1279篇 |
2004年 | 1306篇 |
2003年 | 1190篇 |
2002年 | 1228篇 |
2001年 | 1088篇 |
2000年 | 1018篇 |
1999年 | 898篇 |
1998年 | 892篇 |
1997年 | 894篇 |
1996年 | 716篇 |
1995年 | 671篇 |
1994年 | 629篇 |
1993年 | 547篇 |
1992年 | 479篇 |
1991年 | 488篇 |
1990年 | 474篇 |
1989年 | 488篇 |
1988年 | 438篇 |
1987年 | 510篇 |
1986年 | 462篇 |
1985年 | 541篇 |
1984年 | 668篇 |
1983年 | 580篇 |
1982年 | 567篇 |
1981年 | 509篇 |
1980年 | 448篇 |
1979年 | 435篇 |
1978年 | 452篇 |
1977年 | 387篇 |
1976年 | 348篇 |
1975年 | 354篇 |
1974年 | 328篇 |
1973年 | 369篇 |
1972年 | 267篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
We have used Pollack et al.'s 1976 calculations of the quasi-equilibrium contraction of Saturn to study the influence of the planet's early high luminosity on the formation of its satellites and rings. Assuming that the condensation of ices ceased at the same time within Jupiter's and Saturn's primordial nebulae, and using limits for the time of cessation derived for Jupiter's system by Pollack and Reynolds (1974) and Cameron and Pollack (1975), we arrive at the following tentative conclusions. Titan is the innermost satellite at whose position a methane-containing ice could condense, a result consistent with the presence of methane in this satellite's atmosphere. Water ice may have been able to condense at the position of all the satellites, a result consistent with the occurrence of low-density satellites close to Saturn. The systematic decrease in the mass of Saturn's regular satellites with decreasing distance from Saturn may have been caused partially by the larger time intervals for the closer satellites between the start of contraction and the first condensation of ices at their positions and between the start of contraction and the time at which Saturn's radius became less than a satellite's orbital radius. Ammonia ices, principally NH4SH, were able to condense at the positions of all but the innermost satellites.Water ice may bave been able to condense in the region of the rings close to the end of the condensation period. We speculate that the rings are unique to Saturn because on the one hand, temperatures within Jupiter's Roche limit never became cool enough for ice particles to form before the end of the condensation period and on the other hand, ice particles formed only very early within Uranus' and Neptune's Roche limits, and were eliminated by gas drag effects that caused them to spiral into the planet before the gas of these planets' nebula was eliminated. Gas drag would also have eliminated any rocky particles initially present inside the Roche limit.We also derive an independent estimate of several million years for the time between the start of the quasi-equilibrium contraction of Saturn and the cessation of condensation. This estimate is based on the density and mass characteristics of Saturn's satellites. Using this value rather than the one found for Jupiter's satellites, we find that the above conclusions about the rings and the condensation of methane-and ammonia-containing ices remain valid. 相似文献
962.
T. P. Armstrong S. M. Krimigis D. Hovestadt B. Klecker G. Gloeckler 《Solar physics》1976,49(2):395-407
Comprehensive measurements of the temporal variations of the Z 6 charge composition in the 4 July 1974 solar event made with experiments aboard IMP-7 and 8 show that large variations in the charge composition occur in three-hour-averaged intensities. Hourly averaged fluxes show variations as large as factors of 3 to 4 from one hour to the next and 3 hour averages show nearly a factor of 10 peak to minimum over the event. The precision of the measurements are limited by counting statistics of the Fe-group channel. Iron to oxygen variations are established by both rate channel and pulse height analysis techniques. Comparison of measurements from IMP-7 and 8, separated by about 70 R
E shows that, while significant differences in composition and intensity exist for brief periods, the gross compositional variations are reproduced well at both spacecraft. These observations provide particularly stringent conditions for theories of the acceleration, release, and propagation of solar energetic particles. 相似文献
963.
It is shown that in high-temperature stars in which high speed mass loss is occurring, the rapidly recombining plasma in the stellar envelope can act as an amplifying medium. Model calculations for laser action in Heii 4686, using the collisional-radiative model, are presented. Menzel's hypothesis of laser action in distended stellar atmospheres is shown to be fully substantiated. The relevance of these results in resolving the problem of intensity anomalies in the spectra of Wolf-Rayet stars is pointed out. 相似文献
964.
D. E. Blackwell P. A. Ibbetson C. A. Murray N. M. Pratt 《Astrophysics and Space Science》1976,43(2):469-473
A quantitative definition of limiting magnitude for stellar photographs is proposed, together with a method of measurement employing a Racine prism. Limiting magnitudes inU, B andV for the Isaac Newton telescope at Herstmonceux derived from a total of 72 prime focus photographs are presented and the results correlated with the size of the photographed image. The limiting magnitude for an extrapolated value of the seeing of 1 isB=22m.0, in good agreement with a value obtained by scaling the limiting magnitudes of the 200 inch telescope. 相似文献
965.
The flow law determined experimentally for solid CO2 establishes that a hypothesis of glacial flow of CO2 at the Martian poles is not physically unrealistic. Compression experiments carried out under 1 atm pressure and constant strain rate demonstrate that the strength of CO2 near its sublimation point is considerably less than the strength of water ice near its melting point. The data fit a power law “creep” equation of the form , where ? is compressive strain rate (sec?1), σ is compressive stress (bars), R is the gas constant in calories per mole, and T is absolute temperature. The exponent of σ of 3.9 contrasts with a value near 3.1 for water ice, and indicates that the strain rate is somewhat more sensitive to stress for CO2 than for water. Likewise, the low activation energy for creep, 12 200 cal mole?1, illustrates that CO2 is not highly sensitive to temperature and is thus likely to flow over a broad range of temperatures below its melting point. Strength values for CO2 are of the order of one-tenth to one-third the strength of ice under equivalent conditions.A plausible glacial model for the Martian polar caps can be constructed and is helpful in explaining the unique character of the polar regions. CO2-rich layers deposited near the pole would have flowed outward laterally to relieve high internal shear stresses. The topography of the polar caps, the uniform layering of the layered deposits, and the general extent of the polar “sediments” could all be explained using this model. Flow of CO2 rather than water ice greatly reduces the problems with Martian glaciation. Nevertheless, problems do remain, in particular the large amounts of CO2 necessary, the need to increase vapor pressure and temperature with depth in the polar deposits, and the lack of good observational evidence of flor features. Within the limits of the present knowledge of surface conditions of Mars, CO2 glaciation appears to be a realistic alternate working hypothesis for the origin of the polar features. 相似文献
966.
A.J. Kliore G. Fjeldbo B.L. Seidel D.N. Sweetnam T.T. Sesplaukis P.M. Woiceshyn S.I. Rasool 《Icarus》1975,24(4):407-410
The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 104 elcm?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na+ and 495 K if N2+ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 103 elcm?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N2? and 131 K for Na+. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 1011?1012 cm3, corresponding to a surface pressure of less than 10?8 to 10?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation. 相似文献
967.
968.
E. P. Mazets S. V. Golenetskii V. N. Il'inskii Yu. A. Gur'yan T. V. Kharitonova 《Astrophysics and Space Science》1975,33(2):347-357
Diffuse cosmic background and atmospheric gamma-radiation in the range 28 keV-4.1 MeV were studied with a scintillation spectrometer on board of the Kosmos 461 satellite. Separation of the cosmic and atmospheric components was made possible through a reliable determination of the geomagnetic dependences of albedo gamma-radiation: The spectrum of diffuse background in the energy range covered cannot be fitted with a common law. At energies below 400 keV the spectrum follows a power-law $$I = (5.6 \pm 0.5) \times 10^{ - 3} E^{ - (2.80 \pm 0.05)} cm^{ - 2} s^{ - 1} sr^{ - 1} MeV^{ - 1} .$$ Starting from 400 keV, this power-law breaks down; the spectrum revealing a clearly pronounced shoulder. Extrapolation of the power-law spectrum to higher energies shows that the gamma-ray component responsible for the change in the shape of the spectrum is quite strong, becoming predominant in the diffuse background in the range 1–100 MeV. The intensity of excess radiation is maximum in the region of 700–800 keV reaching ~1.8×10?2 cm?2s?1sr?1 MeV?1. The shape of the high energy component spectrum of the diffuse background constructed using the data of Kosmos 461 and SAS-2 is in agreement with the hypotheses of the cosmological origin of the radiation. 相似文献
969.
Activity diagrams in the system KAlSi3O8-NaAlSi3O8-Al2SiO5-SiO2-H2O-HClhave been calculated in terms of aK+/aH+ and aN+/aH+ from existingexperimental data. They show the effect of temperature, pressure,and aH2O on the stability fields of the alkali feldspars, micas,and aluminium silicate. These activity diagrams are useful in revealing the bufferingcapacity of mineral assemblages and the chemical potential gradientsestablished by changes in T, P, aH2O, and mineral assemblage.An analysis of mineral paragenesis in terms of these diagramssuggests that mosaic equilibrium, allowing limited metasomatismand internal buffering of chemical potentials, best describemetamorphic systems. Thus the dehydration reaction: muscovite+quartz=K-feldspar+Al2SiO5+H2O which is most important in closed systems, probably fails todescribe in detail the mechanism of natural muscovite decomposition.Rather the decomposition of muscovite is more likely representedby ionic reactions. The replacement of muscovite by feldspar: muscovite+6 SiO2+2 K+=3 K-feldspar+2 H+ muscovite+6 SiO2+3 Na+=3 Albite+K++2 H+ is favored at high temperature and low pressure, and may accountfor the crystallization of some feldspars in metamorphic rocks.The reaction involving aluminium silicate replacement of muscovite: 2 muscovite+2 H+=3 Al2SiO5+3 SiO2+3 H2O+2 K+ is favored at high temperature and pressure and low aH2O, andcould contribute to the development of the aluminium silicates.It is concluded that both activity diagrams and AKNa projectionsshould be used together to more completely evaluate mineralparagenesis in terms of mosaic equilibria. 相似文献
970.
P. Kaufmann P. Iacomo Jr. E. H. Koppe P. Marques Dos Santos R. E. Schaal J. R. Blakey 《Solar physics》1975,45(1):189-197
The active region McMath 10433 was the source of several flares and radio outbursts during the early part of July 1974. This region was tracked continuously, for several periods during the month at 22.2 GHz using a telescope with a 4 beam. Comparison with the results obtained simultaneously with a normal 7 GHz solar patrol instrument indicate that there is important burst activity occurring at levels below the detection limit of normal solar patrol instruments. The time-development morphology of these bursts is similar to those normally observed and has enabled the simple events to be re-interpreted. A completely new type of event - the fast absorption - has also been recognized. The correlation of the microwave events with SPA events observed on VLF propagation is also discussed.Spending a sabbatical year at CRAAM, São Paulo, Brazil. 相似文献