全文获取类型
收费全文 | 268188篇 |
免费 | 4422篇 |
国内免费 | 3418篇 |
专业分类
测绘学 | 7052篇 |
大气科学 | 19721篇 |
地球物理 | 55974篇 |
地质学 | 93674篇 |
海洋学 | 22393篇 |
天文学 | 57648篇 |
综合类 | 1065篇 |
自然地理 | 18501篇 |
出版年
2021年 | 2298篇 |
2020年 | 2650篇 |
2019年 | 2897篇 |
2018年 | 4526篇 |
2017年 | 4233篇 |
2016年 | 6280篇 |
2015年 | 4283篇 |
2014年 | 6992篇 |
2013年 | 14317篇 |
2012年 | 6925篇 |
2011年 | 8771篇 |
2010年 | 7855篇 |
2009年 | 10391篇 |
2008年 | 9082篇 |
2007年 | 8797篇 |
2006年 | 9790篇 |
2005年 | 7999篇 |
2004年 | 7857篇 |
2003年 | 7350篇 |
2002年 | 6878篇 |
2001年 | 6075篇 |
2000年 | 6037篇 |
1999年 | 5258篇 |
1998年 | 5270篇 |
1997年 | 5063篇 |
1996年 | 4704篇 |
1995年 | 4455篇 |
1994年 | 4135篇 |
1993年 | 3880篇 |
1992年 | 3651篇 |
1991年 | 3603篇 |
1990年 | 3782篇 |
1989年 | 3534篇 |
1988年 | 3315篇 |
1987年 | 3859篇 |
1986年 | 3426篇 |
1985年 | 4247篇 |
1984年 | 4756篇 |
1983年 | 4440篇 |
1982年 | 4330篇 |
1981年 | 3942篇 |
1980年 | 3655篇 |
1979年 | 3523篇 |
1978年 | 3499篇 |
1977年 | 3292篇 |
1976年 | 3060篇 |
1975年 | 2977篇 |
1974年 | 2929篇 |
1973年 | 3097篇 |
1972年 | 2036篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
811.
We have re-examined the prospects of HOCl as an inert reservoir for atmospheric chlorine in the light of new theoretical calculations and available experimental measurements of its photodissociation cross-sections. The theoretical calculations and most recent laboratory studies imply that the broad maxima 3200 Å observed in two other experimental spectra may not belong to HOCl. On the basis of this implication HOCl could have a long lifetime against photodissociation in the stratosphere, and could, thereby, become a reservoir for atmospheric chlorine comparable to ClONO2 or even HCl. In this capacity HOCl could reduce the predicted ozone destruction due to any given level of total chlorine burden. We have also examined the difficulties in laboratory measurements of the HOCl absorption spectrum with particular emphasis on identifying the impurities which may be present in the experimental system. It appears that specialized new experiments are needed to clearly establish the nature and strength of HOCl absorption in the neighbourhood of 3200 Å. Some refinements in the theoretical calculations also seem desirable. In view of the difficulties involved in the laboratory determination of HOCl photodissociation cross-sections, it is suggested that a search for possible stratospheric HOCl by atmospheric spectroscopists would be worthwhile. 相似文献
812.
It is shown that variations in 6300 Å airglow intensities can, under certain assumptions, be simply related to and its time derivative. In deriving the relationship it is not necessary to assume that the concentration of the neutral atmosphere remains constant and so the relationship is useful on occasions when changes in the neutral atmosphere do occur making it difficult to obtain agreement between observed and calculated 6300 Å intensities; An example is given of a night in which a post-midnight enhancement occurred in the airglow and for which the observations could not be reproduced using a neutral atmosphere constant with time. It is shown that the airglow variations can be explained in terms of the variations of f0F2, implying that the airglow is due to recombination and that, during the night, changes occurred in the concentrations of the constituents of the neutral atmosphere. 相似文献
813.
The dissociation of N2 by electron impact and by e.u.v. photo-absorption is studied, and it is shown that the forbidden predissociation of the numerous 1Πu and 1Σu+ valence and Rydberg states of N2 in the 11–24eV energy range is the dominant mechanism for N atom production. By measuring the absolute emission cross sections for the e.u.v. singlet bands of N2 and by using the generalized oscillator strength data of Lassettre (1974), it has been possible to construct a detailed model of the total N2 dissociation cross section which is in good agreement with the measurements of Winters (1966) and Niehaus (1967) and provides some insights into the maximum possible yield from dissociative excitation. The total cross section for exciting N2 e.u.v. radiation in the 800Å–1100Å wavelength range has been measured and found to have a value of 3.4 ×10?17 cm2 at 100 eV under optically thin conditions. Although this result implies that large fluxes of e.u.v. photons should be excited in auroral substorms and in the airglow, they are not observed, and we show that this development is a consequence of radiation entrapment and predissociation. The total cross section for dissociating N2 by electron impact is given for optically thin and thick media. And some questions concerning the energy budget of a magnetospheric storm which are raised by these results, are discussed. 相似文献
814.
The coupled H+ and O+ time-dependent continuity and momentum equations are solved within a region of the L = 3 magnetic flux tube lying between (and including) the F2-layers of conjugate hemispheres. The method of solution is an extended and modified version of the Murphy et al. (1976) method. The model is used to study the coupling between the F2-layers of conjugate hemispheres during magnetically quiet periods.The results of the calculations strongly indicate that the protonosphere acts as a reservoir, with variable H+ content, which prevents direct coupling between the F2-layers of conjugate hemispheres. However there is generally a significant interhemispheric flow of plasma. This flow is caused by conditions in the summer and winter topside ionospheres and it maintains continuity in the plasma concentration within the protonosphere. There are times when the direction of flow is from the winter hemisphere to the summer hemisphere. It is suggested that maintenance of the winter F2-layer at night is not assisted directly by the F2-layer of the conjugate summer hemisphere.It is shown that during the first few days of protonosphere replenishment after a magnetic storm there is an upflow of H+ in the topside ionosphere at all times in the summer hemisphere. There is also an upflow of H+ during the daytime in both hemispheres. A comparison with the results obtained when the interhemispheric H+ flux is held permanently at zero shows that both F2-layers are little affected by the interhemispheric H+ flux. Nevertheless both F2-layers are affected by the H+ tube content of the protonosphere. When the H+ flux at 1000 km in one hemisphere is much greater than the H+ flux at 1000 km in the conjugate hemisphere, there is a corresponding signature in the interhemispheric H+ flux.The results suggest that there is insufficient time between magnetic storms for complete replenishment of the protonosphere to occur. 相似文献
815.
D. D. Barbosa 《Solar physics》1978,56(1):55-66
We have computed the surface Green's function for linear force-free magnetic fields, where × B = B and is a constant, for application to low coronal levels of the solar atmosphere. Boundary conditions are imposed on the normal component of B on two parallel planes which delineate the force-free volume. This procedure ensures that the magnetic field energy remains bounded, and that the field lines have a smooth behavior. A simple bipolar source distribution is treated and representative field line tracings are shown. 相似文献
816.
T. A. Heppenheimer 《Celestial Mechanics and Dynamical Astronomy》1976,14(2):175-200
We consider a restricted six-body problem, consisting of Jupiter, the four Galilean satellites, and an orbiter. The Galilean satellites' orbits are circular and coplanar; Io, Europa, and Ganymede are in exact resonance; their mean longitudes obey the Laplace relation. We seek periodic orbits which avoid close approaches to any satellite; such orbits are of interest for mission planning. They are approximated as equilibrium points of sets of variational equations associated with time-averaged disturbing functions. Stability of the solutions is also determined. The orbits of greatest interest are:Planar: twice Callisto's period, eccentricity0.6Planar: four times Callisto's period, eccentricity0.75Slightly inclined: twice Callisto's period, eccentricity arbitraryPlanar: 4/5 or 5/4 Europa's period. 相似文献
817.
Max K. Wallis 《Celestial Mechanics and Dynamical Astronomy》1976,13(1):65-74
Gas streaming through the solar system experiences both destructive and scattering processes, the latter primarily in collisional interactions with the solar wind protons. The scattering interactions can be important in filling the downstream wake. They may effectively increase the velocity dispersion and also cause discrete orbit changes.The downstream intensity moment is here evaluated analytically for particles suffering a single, discrete collision, and compared with the moment from a thermal velocity dispersion (both in the absence of a central force field). The elastic scattering collisions of protons in H-gas lead to a contribution to theL backscatter from the wake equivalent to an initial thermal velocity of about 1 km s–1, giving an intensity for cool gas of the order of 10R. This exceeds the contribution due to focussing in the solar gravitational field if the radiation pressure is not less than 0.8 of the gravitational attraction. 相似文献
818.
Solar flares and magnetospheric substorms are discussed in the context of a general theory of impulsive flux transfer events (IFTE). IFTE theory, derived from laboratory observations in the Double Inverse Pinch Device (DIPD), provides a quantitative extension of neutral sheet theories to include nonsteady field line reconnection. Current flow along the reconnection line increases with magnetic flux storage. When flux build-up exceeds the level corresponding to a critical limit on the current, instabilities induce a sudden transition in the mode of conduction. The resulting IFTE, indifferent to the specific modes and instabilities involved, is the more energetic, the lower the initial resistivity. It is the more violent, the greater the resulting resistivity increase and the faster its growth. Violent events can develop very large voltage transients along the reconnection line. Persistent build-up promoting conditions produce relaxation oscillations in the quantity of flux and energy stored (build-up-IFTE cycles). It is difficult to avoid the conclusion: flares and substorms are examples of IFTE. 相似文献
819.
J. R. Blakey 《Solar physics》1976,46(1):241-245
Observations of McMath region 10433 at 22 GHz using a telescope with a 4 beam during July 1974 revealed the existence events or microbursts with intensities below the sensitivity limit of normal solar patrol instruments. Many of these events were simply the high frequency counterpart of more intense bursts observed at lower frequencies. This note considers the small number of events which suggest that the gyro-synchrotron mechanism alone is incapable of explaining the observations and indicates that a thermal mechanism is needed to explain the high frequency event.On leave of absence from Department of Physics, University of Surrey, Guildford, U.K. 相似文献
820.
James F. Vedder 《Earth, Moon, and Planets》1976,15(1-2):31-49
Microcraters were formed in heated soda-lime glass by the normal incidence of spheres of plastic or fused silica with diameters between 0.8 and 4.5m and velocities between 2.5 and 10 km s–1. The morphology of the craters in targets at temperatures up to 800°C is little different from those formed in unheated glass. Spallation still occurs to the same extent and above the same velocity threshold, but the spalls sag and sharp edges become dull in a few seconds at temperatures above the softening point. There is a small increase in the flow of glass from the central pit into a narrow lip at the higher temperatures, but this lip is often removed by spallation, especially at the higher velocities of impact. There is no evidence of a splashed lip with strings of melt overlying the spalled area. The results in conjunction with other evidence suggest that most lunar craters of micrometer size with a smooth central pit, splashed lip, and a spallation zone are the result of primary impacts. 相似文献